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1
Saines, Paul James. "Structural Studies of Lanthanide Double Perovskites." Thesis, The University of Sydney, 2008. http://hdl.handle.net/2123/3939.
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This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.
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Saines, Paul James. "Structural Studies of Lanthanide Double Perovskites." Faculty of Science. School of Chemistry, 2008. http://hdl.handle.net/2123/3939.
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Doctor of Philosophy(PhD)This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.
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Sher, Falak. "Synthesis and properties of magnetoresistive double perovskites." Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.614834.
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McClure, Eric Thomas. "Synthesis and Characterization of New Visible Light Absorbing, Lead-Free Halide Double Perovskite Semiconductors." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492431322941272.
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Wallace, Thomas Kerr. "The synthesis and characterisation of novel molybdenum double perovskites." Thesis, University of Aberdeen, 2014. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=225337.
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Interest in the molybdenum double perovskites has increased since the discovery of high temperature, low field magnetresistance in Sr2FeMoO6. Molybdenum double perovskites of the form Ba2LnMoO6 (where Ln = Sm, Sm0.5Eu0.5, Eu, Gd, Pr) as well as Sr2ScMoO6 have been synthesized by solid-state reactions up to 1200°C. Laboratory X-ray diffraction, variable temperature neutron and synchrotron X-ray diffraction as well as DC-SQUID magnetometry have been employed, in order to investigate the intricate relationship between spin, lattice and orbital degrees of freedom within these compounds. A variable temperature neutron diffraction experiment has been performed on the molybdenum double perovskite Ba2154SmMoO6 between 353 and 877 K in order to further investigate the crystal structure into the high temperature regime. Variable temperature neutron diffraction studies of Ba2PrMoO6 have been performed to investigate the links between structure, spin and orbital order in this material. A synchrotron X-ray diffraction study of Ba2GdMoO6 is detailed. Ba2GdMoO6 exhibits a ferroelastic phase transition below 220 K; the first time this has been reported in a Mo5+ double perovskite. Furthermore, a study of the geometrically frustrated double perovskite Sr2ScMoO6 has been carried out, in order to further investigate the exotic new valence bond glass phase. The results shed new light on the complex relationship between spin, lattice and orbital degrees of freedom in Ba2LnMoO6 double perovskites.
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Dass, Ronald Ian. "Magnetoresistance and properties of half-metallic, ferromagnetic double perovskites /." Digital version accessible at:, 2000. http://wwwlib.umi.com/cr/utexas/main.
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Erten, Onur. "Electronic and Magnetic Properties of Double Perovskites and Oxide Interfaces." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1376496346.
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Barbosa, Rafael de Lima. "Propriedades dielétricas intrínsecas de perovskitas duplas RE2CoMnO6 (RE = íon terra-rara)." reponame:Repositório Institucional da UFC, 2017. http://www.repositorio.ufc.br/handle/riufc/25008.
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BARBOSA, R. de L. Propriedades dielétricas intrínsecas de perovskitas duplas RE2CoMnO6 (RE = íon terra-rara). 2017. 69 f. Dissertação (Mestrado em Física) – Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2017.Submitted by Pós-Graduação em Física (posgrad@fisica.ufc.br) on 2017-08-23T16:49:17Z No. of bitstreams: 1 2017_dis_rlbarbosa.pdf: 2986314 bytes, checksum: fa8e3135695dd91e18a33451cfe520b7 (MD5)
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Dielectric ceramics with ordered double perovskite structure and RE2CoMnO6 stoichiometry, in which RE is a rare-earth ion, are usually multiferroic with critical magnetic and electrical transition temperatures depending on the RE ion. This family of materials crystallizes in the P21/n symmetry when there is ordering of the Mn and Co ions. This monoclinic structure has a distortion directly dependent on the RE ionic size, influencing directly the properties of these materials. In this work, we investigate the intrinsic dielectric properties of these samples for RE = Tb, Dy, Ho, Yb and Tm. These properties were investigated by infrared reflection spectroscopy, allowing to estimate both the intrinsic static dielectric constant and the dielectric losses due to the polar phonons, which allows estimating the maximum microwave quality factor obtained when a resonator is formed with the respective materials. Our results show that, as obtained for La2CoMnO6 (LaCMO), the main compound of this family, the intrinsic static dielectric constant is reduced. In the case of LaCMO, dielectric measurements performed by other authors showed a colossal effect, whose origin was extrinsic effects. Therefore, our result allows to predict that any effect that leads to very high dielectric constants is extrinsic in this family of materials. In addition, the analysis of the quality factor showed that such ceramics have a quality factor compatible with those employed in microwave dielectric resonators since the low dielectric constant is not a problem.
As cerâmicas dielétricas com estrutura perovskita dupla ordenada com estequiometria RE2CoMnO6, na qual RE é um íon terra-rara, são usualmente multiferróicas com temperaturas críticas de transição magnética e elétrica dependendo do íon RE. Esta família de materiais cristaliza-se na simetria P21/n quando há o ordenamento dos íons Mn e Co. Esta estrutura monoclínica, tem um distorção diretamente ligada ao tamanho do íon RE, o que influencia diretamente suas propriedades. Neste trabalho investigamos as propriedades dielétricas intrínsecas destas amostras para RE = Tb, Dy, Ho, Yb e Tm. Tais propriedades foram investigadas por espectroscopia de reflexão especular no infravermelho, permitindo estimar tanto a constante dielétrica estática intrínseca devido aos fônons polares, como as perdas dielétricas devido a estes fônons, o que permite estimar o fator de qualidade em micro-ondas máximo obtido quando se conforma um ressonador dielétrico com o respectivo material. Nossos resultados mostram que, assim como obtido para o La2CoMnO6 (LaCMO), principal composto desta família, a constante dielétrica estática intrínseca é reduzida. No caso do LaCMO , medidas dielétricas realizadas por outros pesquisadores mostraram efeito colossal, cuja origem eram efeitos extrínsecos. Portanto, nosso resultado permite prever que qualquer efeito que leve a obter constantes dielétricas muito altas tem natureza extrínseca nesta família de materiais. Além disso, a análise do fator de qualidade mostrou que tais cerâmicas tem fator de qualidade compatível com aqueles empregados em ressonadores dielétricos para micro-ondas desde que a constante dielétrica baixa não seja um problema.
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Mishra, Rohan. "First Principles Study of Double Perovskites and Group III-V Compounds." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1345489862.
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Ball, Molly R. "First Principles Study of Electronic and Magnetic Structures in Double Perovskites." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1483702986122186.
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LIU, FANG. "Structural and optical properties of highly emitting lead-free double perovskites." Doctoral thesis, Università degli Studi di Cagliari, 2022. http://hdl.handle.net/11584/333072.
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This thesis introduces lead-free double perovskite compounds with broad-band white light emission and the potential application of various doped and alloyed double perovskites in the illumination field. In addition, the mechanism of highly efficient warm white light emission is also studied.
Double perovskites are promising lead-free perovskite materials which alternatively replace the divalent Pb cations in lead-halide perovskite with a combination of a monovalent and trivalent cation. They exhibit excellent stability under moisture, heat and light. In this work, halide double perovskite materials such as Cs2AgInCl6, Cs2NaInCl6, Cs2AgBiCl6, and Cs2NaBiCl6 are firstly successfully synthesized and their potential application in the optoelectronic field is studied. Cs2AgBiCl6 and Cs2NaBiCl6 possess indirect band gaps while Cs2AgInCl6 and Cs2NaInCl6 have direct band gaps but the dipole transitions between the band edge states are parity-forbidden, which leads to relatively large optical band gaps. However, alloyed double perovskite materials Cs2Na1-xAgxIn1-yBiyCl6 show excellent properties for its highly efficient warm white emission which could be used as a single-component white light emitter. Here, transient absorption and photoluminescence are measured for investigating the outstanding emission properties of Cs2Na1-xAgxIn1-yBiyCl6.
Furthermore, Sb alloyed Cs2NaInCl6, Cs2InCl5.H2O and Cs3InCl6 are also prepared which exhibit efficient blue, yellow and blue-green emission respectively under UV light. These materials are promising phosphor in lighting applications. The understanding of luminescence mechanism in double perovskite will help the search and the design of more efficient phosphors.
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Janczak, Julia. "Theory of Spectral Function and Optical Conductivity for Half-Metallic Double Perovskites." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1354713689.
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Dixit, Manisha. "Structure-Property Correlations in Double Perovskite Systems." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1366345489.
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Soliz, Jennifer Rose. "The Interplay of the Chemical, Orbital and Spin Disorder in Ca2-xLaxMnRuO6 Perovskites." The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1253094110.
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Shabadi, Vikas [Verfasser], Lambert [Akademischer Betreuer] Alff, and Wolfgang [Akademischer Betreuer] Donner. "Epitaxial engineering of ferrimagnetic double perovskites / Vikas Shabadi ; Lambert Alff, Wolfgang Donner." Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2017. http://d-nb.info/1135386048/34.
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Ricciardo, Rebecca Ann. "Chemical, Magnetic, and Orbital Order of Polycrystalline and Thin film Double Perovskites." The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1250277883.
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Warren, Matthew Timothy. "Time-Domain Terahertz Studies of Strongly Correlated GeV4S8 and Osmate Double-Perovskites." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1512066420271281.
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Tran, Phuong Minh. "Investigating the Relationship between Structure and Magnetic Ground States of 5d Double Perovskites." The Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1577937896742109.
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Roknuzzaman, Md. "Ab initio atomistic insights into lead-free perovskites for photovoltaics and optoelectronics." Thesis, Queensland University of Technology, 2020. https://eprints.qut.edu.au/198196/1/Md_Roknuzzaman_Thesis.pdf.
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This project focuses on the development of advanced non-toxic materials for the applications in energy generation and consumption devices like solar cells, light-emitting diodes, lasers and photodetectors. A first-principles density functional theory calculations are conducted to investigate the properties of a number of inorganic, hybrid and double perovskites compounds to predict their potential for applications in photovoltaics and optoelectronics. The achieved outcomes provide a better understanding of the structural, electronic, optical and mechanical properties of a group of potential compounds and provide new scientific knowledge to develop non-toxic high-quality organic-inorganic materials for photovoltaic and optoelectronic applications.
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Russell, David D. "Design, synthesis, crystal structure and magnetic properties of novel osmium-based B-site ordered double perovskites." Thesis, California State University, Long Beach, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10011271.
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Transition metal oxides (TMOs) with face centered cubic arrangement of magnetic ions are composed of triangular sub-lattices. When antiferromagnetic (AFM) interactions of the same strengths between all three pathways in triangular settings are in place, spin constraints cannot be fulfilled simultaneously and the system undergoes geometric magnetic frustration (GMF). The purpose of the work presented in this thesis is to better understand the criteria for a system to undergo GMF. To achieve this, the novel B-site ordered double perovskites Ca2ScOsO6 and Ca2.2Mg0.8OsO6 were synthesized in polycrystalline form utilizing the conventional solid-state method. The crystal structure of these compounds were characterized through X-ray diffraction, and magnetic properties were explored through magnetic susceptibility measurements. Employing the spin-dimer analysis method, relative magnetic exchange interactions were calculated and modeled. These novel osmium-based B-site ordered double perovskites were then compared to isostructural compounds to study the effects of the osmium oxidation state on crystal structure and the exhibited properties.
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Gao, Haitao [Verfasser]. "Substitution effects and electronic structure studies in double perovskites : how crystal structure influences physical properties / Haitao Gao." Mainz : Universitätsbibliothek Mainz, 2014. http://d-nb.info/1047379112/34.
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Barnes, Paris W. "Exploring structural changes and distortions in quaternary perovskites and defect pyrochlores using powder diffraction techniques." Columbus, Ohio : Ohio State University, 2003. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1064346592.
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Thesis (Ph. D.)--Ohio State University, 2003.Title from first page of PDF file. Document formatted into pages; contains xxvi, 255 p.; also includes graphics. Includes abstract and vita. Advisor: Patrick M. Woodward, Dept. of Chemistry. Includes bibliographical references (p. 221-228).
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Burrows, Oliver James. "Structural and magnetic properties of the geometrically frustrated 3d and 5d s = ½ Double Perovskites Sr₂CuWO₆, Ba₂YWO₆ and LaSrMgWO₆." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/22069.
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Double perovskites with a single s = ½ magnetic ion and rocksalt order can show geometric frustration, due to arrangement of electron spins such that they cannot satisfy all nearest-neighbour antiferromagnetic interactions simultaneously. This can give rise to exotic magnetic states at low temperature. Compounds with unpaired 4d and 5d electrons have in this respect been studied far less than 3d compounds. Here, results of studies on the compounds Sr2CuWO6, Ba2YWO6 and LaSrMgWO6 are presented. The synthesis of the previously reported compound Ba2Y1WO6 (W5 +, 5d1) was attempted by many methods, but yttrium deficiencies were present in all samples. This led to the series Ba2YxWO6 with ⅔ ≤ x ≤ 0.8, dependent on the synthesis conditions. The x = ⅔ compound is known to exist as a metastable cubic phase and an 18H rhombohedral thermodynamic phase. The one-third vacant B sites on the x = ⅔ cubic material are doped with lithium, resulting in the Ba2Y2/3LixWO6 compound. This thesis focuses on the new x = 0.75 phase, and characterises its structural and magnetic properties. The Jahn-Teller distorted Sr2CuWO6, with the Cu2+ (3d9) magnetic ion, has an elongated c axis leading to separation of ab planes. This compound had been proposed as a pseudo-2D spin liquid model candidate, following a lack of evidence of transition to long-range order in SQUID magnetometry and heat capacity. However, recent μSR measurements did show a transition to long-range ordered state at 24K. This thesis details further bulk and local probe measurements which indicate that the low-temperature state is type-2 antiferromagnetic, and which point to a thermally activated spin-liquid-like state which occurs between 24K and ∼100K. La0.5Sr1.5MgWO6, has also been synthesised. The 1:1 compound LaSrMgWO6 had previously been reported as W5+ and described as “pseudo-cubic”: X-ray and neutron diffraction studies here characterise the low-temperature structure within the P2₁/n monoclinic space group, and suggest that no ordering of the atoms on the A site is observed.
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Martins, Júnior Antonio Luiz. "FÔNONS ÓPTICOS DE PEROVSKITAS DUPLAS Ca₃Mn₂B O₉ (B = W e Nb)." Universidade Federal do Maranhão, 2013. http://tedebc.ufma.br:8080/jspui/handle/tede/746.
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Previous issue date: 2013-10-30Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
In recent years, researchers and scientists has been devoted more to the study of inorganic materials, in particular the oxides known as perovskites, because its technological use. The analysis of their properties have been made by various techniques, among these we can mention the Raman spectroscopy and infrared, which show very sensitive to changes in the structure of these oxides. Thus, this work presents the study of phonons at room temperature the double perovskites Ca₃Mn₂WO₉ (CMWO) and Ca₃Mn₂NbO₉ (CMNO) by means of Raman scattering techniques and transmittance Infrared. Both compounds crystallize in a monoclinic system space group P2₁/n, with Z = 2. The Raman and infrared spectra are interpreted by the group factor analysis in terms of the space group P2₁/n. Also the vibrational modes of stretching and folding B were reported "- O. The observed spectra are very similar to the cubic perovskite structure prototype dual Fm3m indicating that the phases of the study result of minor distortions cubic cell. It is also observed an unexpected band in the Raman spectra of both materials. We suggest that this additional bandwidth would result from occupational disorder of the sites of the cation B, which is evidenced in both phases.
Nos últimos anos, pesquisadores e cientistas tem se dedicado mais ao estudo de materiais inorgânicos, em particular aos óxidos denominados perovskitas, devido o seu aproveitamento tecnológico. A análise de suas propriedades tem sido feita por diversas técnicas, entre estas podemos citar as espectroscopias Raman e de Infravermelho, que se mostram muito sensíveis às mudanças na estrutura desses óxidos. Sendo assim, este trabalho apresenta o estudo de fônons à temperatura ambiente das perovskitas duplas Ca₃Mn₂WO₉ (CMWO) e Ca₃Mn₂NbO₉ (CMNO), por meio das técnicas de espalhamento Raman e transmitância de Infravermelho. Ambos os compostos cristalizam em um sistema monoclínico com grupo espacial P2₁/n, com Z = 2. Os espectros Raman e infravermelho são interpretados por meio da análise do grupo fator em termos do grupo espacial P2₁/n. Também foram reportados os modos vibracionais de estiramento e dobramento do B - O. Os espectros observados são muito semelhantes ao do protótipo cúbico de estrutura perovskitas dupla Fm m indicando que as fases em estudo resultam de pequenas distorções da célula cúbica. Observa-se também uma banda inesperada nos espectros Raman de ambos materiais. Nós sugerimos que essa banda adicional seria resultado da desordem ocupacional dos sítios do cátion B, que é evidenciada nas duas fases.
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Azad, Abul Kamal. "Synthesis, structure, and magnetic properties of double perovskites of the type A₂MnBO₆ and A₂FeBO₆ : (A=Ca, Sr, Ba, La; B=W, Mo, Cr) /." Göteborg : Göteborg university, 2004. http://catalogue.bnf.fr/ark:/12148/cb401256467.
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Silva, Antonio Vinnie dos Santos. "Structural, vibrational and morphological properties of La2CoMnO6 multiferroics films." Universidade Federal do CearÃ, 2016. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=17901.
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Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgicoFundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico
FundaÃÃo de Amparo à Pesquisa e ao Desenvolvimento CientÃfico e TecnolÃgico do MaranhÃo
CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior
Perovskitas duplas com fÃrmula na forma A2BBâO6 (A = La, Sr, Ca, B/Bâ = Co/Mn ou Fe/Mo) sÃo materiais multiferrÃicos de grande interesse na ciÃncia devido Ãs notÃveis propriedades, que permitem aplicaÃÃes em capacitores e em memÃrias, por exemplo. AlÃm disso, mudanÃas estruturais sÃo observadas entre a forma policristalina e o filme fino. Essas propriedades podem mudar de acordo com a rota utilizada para sintetizaÃÃo do material. No nosso trabalho estudamos filmes de La2CoMnO6 sintetizados pelo mÃtodo de Pechini Modificado (MPM), com trÃs deposiÃÃes e tratados em diferentes temperaturas: 700, 800 e 900 ÂC. Os filmes foram caracterizados por difraÃÃo de raios X, Microscopia EletrÃnica de Varredura (MEV), Microscopia de ForÃa AtÃmica e espectroscopia Raman à temperatura ambiente e tambÃm variando a temperatura. As tÃcnicas de MEV e AFM foram bastante eficientes na determinaÃÃo do diÃmetro mÃdio dos filmes, fornecendo resultados correspondentes. A tÃcnica de EDS trouxe dados qualitativos da composiÃÃo do material. A difraÃÃo de Raios X confirmou a fase do filme, alÃm de ter identificado a composiÃÃo do substrato: silÃcio e platina. A espectroscopia Raman em diversos pontos possibilitou identificar a regiÃo de melhor formaÃÃo do LCMO. O estudo da mudanÃa do fÃnon mais intenso com a temperatura mostrou diferentes temperaturas para a transiÃÃo de fase paramagnÃtica-ferromagnÃtica, possibilitando confirmar que a temperatura de calcinaÃÃo dos filmes determinou elevada desordem estrutural para os filmes de LCMO tratados a 700 e 900 ÂC. Discutimos nossos resultados com outros estudos na literatura, que mostram as medidas magnÃticas como um fator determinante para comprovar a temperatura do acoplamento spin-fÃnon.
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Tseggai, Mehreteab. "Synthesis, Nuclear Structure, and Magnetic Properties of some Perovskite Oxides." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-4826.
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Bufaiçal, Leandro Félix de Sousa. "Investigação das propriedades estruturais, eletrônicas e magnéticas dos óxidos com estrutura perovskita dupla Ca2-xLaxFeIrO6." [s.n.], 2006. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278507.
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Orientador: Pascoal Jose Giglio PagliusoDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Um grande número de óxidos de metais de transição se forma em estrutura perovskita simples ou em uma de suas variantes, e muitos deles apresentam propriedades físicas interessantes, como supercondutividade, magneto-resistência colossal e ferroeletricidade. Uma das variantes da estrutura perovskita simples, a perovskita dupla ordenada (PDO), pode se cristalizar em simetria cúbica ou em variantes distorcidas, e possui fórmula geral A2B¿B" O6, em que o íon A ocupa os vértices do cubo enquanto os cátions B¿ e B¿¿ se alternam nos centros dos octaedros de oxigênio. Dentre os compostos com estrutura perovskita dupla já reportados na literatura, Sr2FeRe O6e Sr2FeMoO6são particularmente interessantes devido à suas interessantes propriedades físicas e à sua potencialidade como dispositivos magneto-eletrônicos. As propriedades estruturais, de transporte e magnéticas desses materiais estão altamente conectadas, e acredita-se que essas propriedades das PDO sejam fortemente dependentes do grau de hibridização dos orbitais d dos cátions B" (ex.: Re e Mo). Portanto, para se comprovar a potencialidade dos compostos PDO como dispositivos magnetoeletrônicos, além entender os mecanismos microscópicos responsáveis por suas propriedades magnéticas e eletrônicas, é essencial que se investigue em detalhes outros exemplos de PDOs que possam confirmar as idéias correntes propostas na literatura. Nesse trabalho descrevemos o processo de síntese e as propriedades físicas da série inédita Ca2-xL axFeIrO6, onde o Ir, assim como o Re e o Mo, é metal de transição, no caso com caráter 5d, e pode possuir diferentes estados de valência. Espectros de difração de raios-x e Refinamento Rietveld mostraram que os compostos da série se cristalizam em uma estrutura monoclínica, com grupo espacial P21/n, com uma grande presença de desordem catiônica nos sítios Fe/Ir. Medidas de magnetização indicaram que, aparentemente, os compostos tendem a evoluir de antiferromagnéticos nas extremidades da série, x = 0 e x = 2, para ferromagnéticos em regiões intermediárias da série. Medidas de espectroscopia Mössbauer mostraram que a valência do ferro é +3 ao longo da toda série, de modo que, possivelmente, a mudança na natureza das interações magnéticas estaria sendo causada pela alteração da valência do Ir devido a dopagem com La. Medidas de calor específico revelaram uma anomalia característica de uma transição magnética somente para a mostra de x = 0. Foram feitas medidas de resistividade em função da temperatura, em que se observou que os materiais apresentam comportamento isolante e praticamente nenhum efeito magneto-resistivo. Para as amostras no centro da série (em torno de x = 1,0) a presença de loops de histerese nas curvas MxH e um comportamento irreversível nas curvas de MxT sugerem uma competição entre fases ferrimagnéticas e antiferromagnética para esta região de concentração. Qualitativamente, nossos resultados podem ser interpretados considerando-se a mudança de valência do Ir, as regras de Goodenough-Kanamori-Anderson e a presença de desordem catiônica
Abstract: Perovskite structure and its variants host a great number of transition metals oxides that present a variety of interesting physical properties such as superconductivity, ferroelectricity and colossal magneto-resistance. A variant of the simple perovskite structure, the ordered double perovskite (ODP), also can grow in cubic (or lower) symmetry, with a general formula A2B¿B" O6, where the cation A occupies the vertices of the cube while B¿ and B" sits alternately at the center of the oxygen-octahedron. Among the compounds with ordered double-perovskite structure, Sr2FeReO6 and Sr2FeMoO6 are particularly interesting due to their interesting physical properties and their potentiality as magneto-electronic devices, having highly connected structural, transport and magnetic properties. It has been proposed that the magnetic and conducting ground states of ODP systems are strongly dependent on the delocalization level of the cation B" 5d electrons (ex: Re, Mo). However in order to prove the potentiality of composites ODP as magnetoeletronics devices, and to understand the microscopical mechanisms responsible for its magnetic and electronic properties, it is necessary to further investigate other examples of ODP that can confirm the current ideas proposed in literature. In this work we have synthesized and studied the Ca2-xLaxFeIrO6 series, where Ir, as well as Re and Mo, is transition metal with a 5d character which can possess different valence states. X-rays diffraction spectra and Rietveld Refinement analyses have shown that the Ca2-xLaxFeIrO6 compounds series crystallized in a monoclinic structure, space group P21/n, with an unavoidable cationic Fe/Ir site disorder. Measurements of temperature/field dependent magnetization have indicated that the magnetic interactions in these compounds evolves from antiferromagnetic in the extremities of the series, x = 0 and x = 2, for ferromagnetics in intermediate regions of the series. Mössbauer spectroscopy measurements revealed that the valence of the Fe is +3 in the whole series, in a way that the change in the nature of the magnetic interactions can be possibly caused by the variation of Ir valence due the La doping. Specific heat measurements have revealed an anomaly associated to a magnetic phase transition only for the x = 0 compound while measurements of electrical resistivity as a function of the temperature have shown insulating behavior and absence of magneto resistance for all studied samples. For the samples with x ~ 1.0, the presence of ferromagnetic loops and ZFC and FC hysteresys in the MxT curves indicates the competition between ferrimagnetic and antiferromagnetic phases in a disordered system. Qualitatively, all our results can be understood in terms of a valence changes in the Ir ions, the Goodenough-Kanamori-Anderson rules and the role of cationic disorder
Mestrado
Física da Matéria Condensada
Mestre em Física
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Silva, Rosivaldo Xavier da. "Síntese e caracterização de perovskitas complexas multiferróicas com estrutura dupla ordenada." Universidade Federal do Maranhão, 2015. http://tedebc.ufma.br:8080/jspui/handle/tede/748.
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Previous issue date: 2015-12-11Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
The RE2CoMnO6 (RE = La e Y) and Gd(Co0.5Mn0.5)O3 samples were obtained via modified Pechini method (MPM). Raman spectroscopy (RS), Fourier transform infrared (FTIR) spectroscopy, X ray powder diffraction, X ray photoelectron spectroscopy (XPS), SQUID and scanning electron microscopy (SEM) were used to characterize the samples, investigate structural and microstructural evolution, as well as evaluate their vibrational, magnetic and intrinsic dielectric properties. The impact of synthesis conditions on the structural ordering was investigated. We obtained a good control of structural order depending on the annealing temperature for LCMO. We observed an increase in the saturation magnetization, lifetime of the phonons and dielectric constant while suppression of anti-site defects and reduction of dielectric loss. Our investigations on polar phonons by FTIR revealed the extrinsic character of CDC effect on LCMO and clarified the contributions of phonons for dielectric constant in these systems. The temperature dependence Raman spectra of GCMO was investigated between 40 and 300 K and revealed an intriguing spin-phonon coupling, characterized by an increase in the energy of the most intense stretching mode near the magnetic transítion. The correlation between the Raman data and the magnetization suggests that the structure influences the magnitude of the spin-phonon coupling. Correlated analysis of LCMO, GCMO and YCMO systems show that all samples have vibrational properties quite similar. The intrinsic dielectric constants were obtained from the polar phonons dispersive parameters ε intr ̴ 15.8, 17.9 and 16.0, making the contributions to this value explicit, as well as the quality factors, reciprocal of dielectric losses , 𝑄𝑢×𝑓 ≈ 124,74 e 83 THz, extrapolated to microwave region at 10 GHz, to LCMO, GCMO and YCMO, respectively. XPS measures showed that oxidation state for Co and Mn ions are similar each another, being mostly Co2 + and Mn4 + for all investigated systems, however to GCMO, the spin-phonon coupling behavior and losses relatively large indicate that this compound has a high level of structural disordering.
Amostras de RE2CoMnO6 (RE = La e Y) e Gd(Co0.5Mn0.5)O3 foram obtidas pelo método Pechini modificado (MPM). Espectroscopia Raman (RS), espectroscopia no infravermelho por transformada de Fourier (FTIR), difração de raios X (DRX), espectroscopia de fotoelétrons excitados por raios X (XPS), magnetometria e microscopia eletrônica de varredura (MEV) foram utilizados para caracterizar as amostras, investigar evolução estrutural e microestrutural bem como avaliar suas propriedades vibracionais, magnéticas e dielétricas intrínsecas. O efeito da temperatura de tratamento térmico sobre o ordenamento estrutural foi investigado. Um bom controle do ordenamento estrutural em função da temperatura de tratamento térmico para o La2CoMnO6 (LCMO) foi obtido. Observa-se um incremento da magnetização de saturação, aumento do tempo de vida dos fônons, supressão de defeitos de anti-sítio, redução de perdas dielétricas e incremento da constante dielétrica. A investigação dos fônons polares via FTIR revelaram o caráter extrínseco da constante dielétrica colossal (CDC) no LCMO e explicitou as contribuições dos fônons para a constante dielétrica nesses sistemas. A dependência dos espectros Raman com a temperatura do Gd(Co1/2Mn1/2)O3 (GCMO) foi investigada entre 40 e 300 K revelando um acoplamento spin-fônon, caracterizado pelo incremento na energia do modo de estiramento mais intenso próximo à transição magnética. A correlação entre os resultados obtidos pela espectroscopia Raman e a magnetização sugere que a ordem estrutural influencia a magnitude do acoplamento spin-fônon. Análise correlacionada dos sistemas LCMO, GCMO e YCMO, mostram que todas as amostras apresentam características vibracionais bastante semelhantes. A partir dos parâmetros de dispersão dos fônons polares foram obtidas as constantes dielétricas intrínsecas ε intr ̴ 15,8, 17,9 e 16,0, explicitando as contribuições dos fônons para esses valores, e o fator de qualidade, recíproco da perdas dielétricas, 𝑄𝑢×𝑓 ≈ 124,74 e 83 THz, extrapolado para a região de micro-ondas em 10 GHz, para o LCMO, GCMO e YCMO, respectivamente. Medidas de XPS mostraram que os estados de oxidação dos íons de Co e Mn são semelhantes entre si, e principalmente do tipo Co2+/ Mn4+ para todos os sistemas estudados, sendo que para o GCMO, o acoplamento spin-fônon e as perdas dielétricas relativamente maiores indicaram que esse sistema possui elevada desordem estrutural.
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Morrow, Ryan. "Competing Superexchange Interactions in Double Perovskite Osmates." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429787669.
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Coutrim, Leandro Tolentino. "Propriedades estruturais, eletrônicas e magnéticas dos óxidos La2−xCaxCoIrO6." Universidade Federal de Goiás, 2015. http://repositorio.bc.ufg.br/tede/handle/tede/4574.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
In this work we describe the synthesis of the novel series La2−xCaxCoIrO6 and its structural, electronic and magnetic characterization. The main objective of this project is to investigate the magnetic evolution of the system when modified due to the chemical substitution of Ca at La site, which can induce changes in the magnetic interactions. The policristaline samples were synthesized by Solid State Reaction process, and studied combining global characterization techniques as powder x-ray diffraction, temperature dependence of magnetic susceptibility and magnetization as a function of applied magnetic field. All compounds crystallize in monoclinic structure, with space group P21/n. Magnetic susceptibility results indicate that the superexchange antiferromagnetic interaction between Co2+ and Ir4+ in La2CoIrO6 lead to ferrimagnetic behavior due to the different magnetic moment of the transition metal ions. For the Ca2+ doped at La3+ site compounds, a magnetic evolution is observed with doping, induced by Co and Ir valence changes. As expected, this system presents a strong correlation between its structural, electronic and magnetic properties, in a way that combining the different results of each compound we can explain its behavior and understand the big picture of this system.
Neste trabalho descrevemos a síntese e a caracterização estrutural, eletrônica e magnética dos compostos policristalinos da série inédita La2−xCaxCoIrO6. O principal objetivo deste projeto foi investigar a evolução da magnetização do sistema quando os materiais são modificados a partir da substituição química do Ca no sítio do La, o que pode induzir mudanças na natureza das interações magnéticas. As amostras policristalinas foram sintetizadas pela técnica de Reação de Estado Sólido convencional, e estudadas combinando-se técnicas de caracterização globais como difração de raios X de pó, magnetização em função da temperatura e em função do campo magnético aplicado, e também calor específico em função da temperatura. Todos os compostos da série cristalizam-se em estrutura monoclínica, com grupo espacial P21/n. As medidas de susceptibilidade magnética indicaram que a interação de supertroca antiferromagnética entre os metais de transição Co2+ e Ir4+ leva o composto La2CoIrO6 a apresentar comportamento ferrimagnético devido aos diferentes momentos magnéticos apresentados pelos dois íons, com TC em torno de 100K. Para os compostos dopados com Ca2+ no sítio do La3+, observou-se mudanças nas propriedades magnéticas, induzidas pelas alterações nas valências dos íons de Co e Ir. Como esperado, este sistema apresenta uma forte correlação entre as suas propriedades estruturais, eletrônicas e magnéticas, de modo que a partir da interpretação conjunta dos dados acerca da estrutura e da magnetização, podemos entender o comportamento de cada composto da série.
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Bos, J. W. G. "Synthesis, structures and properties of new double perovskite oxides." Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596791.
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Double perovskite oxides have attracted much interest since the discovery of spin-polarized conduction and large low field magnetoresistances in Sr2FeMoO6. This behaviour exists to high temperatures, making Sr2FeMoO6 and related materials candidates for application in data storage devices. This thesis describes the synthesis, structures and properties of some new double perovskite oxides. These can be divided into the (LaM)BNbO6 (M = Ca, Sr, Ba; B = Mn, Co) and LaSrFeNbO6 materials that have one magnetic transition metal (TM) cation, and the (La1+xM1-x)CoRuO6 (M = Ca, Sr; -0.25 £ x £ 0.25 x = 1) materials that have two magnetic TM cations. The (LaM)CoNbO6 double perovskites crystalize with the monoclinic P21/2 (M = Ca, Sr) and tetragonal 14/m (M = Ba) superstructures, which have rocksalt ordered arrangements of Co2+ (3d7) and Nb5+ (4d0), with almost no Co/Nb inversion (<4%). All three phases are anti-ferromagnetic with Neel temperatures of 17 K (M = Ca), 16 K (M = Sr) and 10K (M = Ba). The magnetic ordering is described by the magnetic propagation vector k = ( ½ 0 ½ ) for all materials. For M = Ca and Sr, two “non-collinear” and two “collinear” magnetic structures are found. These are experimentally indeterminate because of the small monoclinic angle (b = 89.90). The M = Ba material has a collinear antiparallel magnetic structure. The saturated cobalt moment decreases from 2.97(2) mB to 2.52 (3) mB to 1.85 (5) mB for M = Ca, Sr and Ba, respectively. The frustration due to competing 900 exchange interactions. The (LaK)MnNbO6 materials also have the monoclinic P21/n (M = Ca, Sr) and tetragonal 14/m (M = Ba) superstructures, with <3% inversion of Mn2+ (3d5) and Nb5+ (4d0). All materials are antiferromagnetic with Neel temperatures of 9 K (M = Ca), 8 K (M = Ba) and 6.5 K (M = Ba). The magnetic Bragg reflections are indexed by k = (0 ½ ½ ), and the possible magnetic models are identical to those for (LaM)CoNbO6. The manganese saturation moment decreases from 4.77(3) mB (M = Ca) to 4.43 (4) mB (M = Sr) to 41.9 (4) mB (M = Ba). (LaSr)FeNbO6 has the monoclinic P21/n superstructure with 14% inversion of Fe2+ (3d6) and Nb5+ (4d0). This material is antiferromagnetic (TN = 22 K), and has a ( ½ 0 ½) magnetic propagation vector (mFe = 3.45(3) mB). The lattice parameters, antisite disorder, Neel temperatures and resistivities of the (La1-xM1-x)CoRuO6 double perovskites are found to vary in a systematic manner. The Co/Ru antisite disorder is minimal for x = 0, showing that this disorder is driven by the mixed Co or Ru electronic states at x ≠ 0 rather than by La/M cation mixing. The x = 0 materials have TN = 96K (M = Ca) and TN = 85 K (M = Sr), and are variable range hopping semiconductors. Hole doping )x < 0) results in a rapid reduction in the resistivity while the Neel temperature increases only moderately, whereas electron doping (x > 0) results in a significant reduction in TN but has little effect on the resistivity. X-ray absorption near edge spectroscopy shows that for x < 0, the Co2+ states are selectively hope doped while for x > 0 only the Ru5+ states are electron doped, which is in keeping with the strongly asymmetric doping response.
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Moreira, Alessandro Ferreira Lisboa. "Espalhamento Raman em perovskitas duplas." [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277693.
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Orientador: Eduardo Granado Monteiro da SilvaDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Nesse trabalho foram realizados estudos em duas famílias de compostos de perovskitas duplas Ba2FeReO6 e S r2UO6 (B'= Co, Ni) por espectroscopia Raman e difração de raios-X com luz síncrotron. No primeiro composto há uma transição de uma estrutura cúbica Fm3m paramagnética para uma tetragonal I4/mmm ferrimagnética abaixo de TC~305 K. O mecanismo de acoplamento magnético neste material ainda permanece uma incógnita. Dois grupos de medidas de espalhamento Raman foram realizados em regiões distintas da amostra à base de FeRe. Foi verificado que o fônon associado às vibrações de estiramento dos oxigênios nos octaedros sofre um desvio na freqüência em relação ao comportamento convencional abaixo de TC nos dois grupos de medidas. Isso pode ser associado a dois possíveis mecanismos: acoplamento spin-fônon gigante e acoplamento órbita-spin-fônon. Na segunda família de compostos contendo urânio, os compostos estudados apresentam uma estrutura cristalina monoclínica com grupo espacial P 21/n e um ordenamento antiferromagnético abaixo de TN~21 K para B" = Ni e TN~10 K para B" = Co. A partir dos espectros Raman desses compostos observa-se que a área integrada do modo de estiramento do oxigênio sofre um decréscimo anômalo sob aquecimento até T*~300 K. Além disso, este modo sofre um endurecimento anômalo de ~1 cm -1 sob aquecimento até T*. Para verificar a hipótese de que T* poderia estar associada a uma transição estrutural nesta família, medidas de difração de raios-X de pó com luz síncrotron foram realizadas no LNLS. A análise estrutural de ambos os compostos não mostraram nenhum grau observável de desordem de antisítios entre B' e U e tampouco uma transição de fase ou comportamento anômalo dos parâmetros de rede foi encontrada em torno de T*. Portanto, as anomalias encontradas em espectroscopia Raman nestes materiais são de origem eletrônica
Abstract: In this work, studies on two families of double perovskite compounds Ba2FeReO6and Sr2B'U O6(B' = Ni, Co) were carried out by Raman spectroscopy and synchrotron X-ray diffraction. In the first compound, a transition from a cubic Fm3m paramagnetic to a tetragonal ferromagnetic structure takes place below TC ~305 K. The magnetic coupling mechanism in this material remains unknown. Two groups of Raman scattering measurements were performed in distinct regions of the FeRe-based sample. It was verified that the phonon associated to stretching vibrations of oxygen octahedra shows a frequency shift with respect to the conventional behavior below TC, in both groups of measurements. This may be associated with two possible mechanisms: giant spin-phonon coupling and orbit-spin-phonon coupling. In the second family of compounds with uranium, the studied compounds present a monoclinic crystal structure with space group P21/n and an antiferromagnetic ordering below TN~21 K for B'' = Ni and TN ~10 K for B'' = Co. From the Raman spectra of these compounds, an anomalous decrease of the integrated area of the oxygen stretching mode was observed upon heating up to T*~300 K. Also, this mode shows an anomalous hardening of ~1 cm -1 under heating up to T*. In order to verify the hypothesis that T* could be associated to a structural transition in this family, powder x-ray diffraction measurements with synchrotron light were perfomed at LNLS. The structural analysis of both compounds did not show any observable degree of antisite disorder between B' and U, and no phase transition or anomalous behavior of lattice parameters was found close to T*. Therefore, the anomalies found by Raman spectroscopy in these materials are electronic in origin
Mestrado
Física da Matéria Condensada
Mestre em Física
34
Asano, H., N. Koduka, K. Imaeda, M. Sugiyama, and M. Matsui. "Magnetic and junction properties of half-metallic double-perovskite thin films." IEEE, 2005. http://hdl.handle.net/2237/6775.
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Johnson, Alexanne Holcombe. "Growth and Characterization of Double Perovskite Buffer Materials for Thin Film Applications." The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1397479975.
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Garcia, Flores Ali Francisco. "Acoplamento spin-fonon em sistemas magneticamente frustrados." [s.n.], 2007. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277694.
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Orientador: Eduardo Granado Monteiro da SilvaTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Nesta tese são investigados dois sistemas magneticamente frustrados, a série de compostos isolantes RMn2O5 (R = Eu, Bi, Dy) e o composto intermetálico GdAI3, além do material isolante não frustrado de perovskita dupla Sr2CoUO6. O estudo destes sistemas foi realizado utilizando principalmente a técnica de Espectroscopia Raman e complementado por medidas de susceptibilidade magnética. Para o sistema RMn2O5(R = Eu, Bi, e Dy) apresentamos um estudo dos fônons ópticos de baixa e alta freqüência. Todos os cristais estudados mostraram deslocamentos anômalos nas freqüências dos fônons abaixo de uma nova temperatura característica, T* ~ 60-65 K. O sinal e magnitude dos deslocamentos dos fônons parecem estar correlacionados com o raio iônico de R. Por exemplo, analisando os fônons de alta freqüência, observamos amolecimento dos fônons para R = Bi e um endurecimento para R = Dy, e um comportamento intermediário para R = Eu no intervalo de temperatura entre TC / TN e T*, onde TC e TN são as temperaturas de transição ferrolétrica e antiferromagnética, respectivamente. Já para os fônons de baixa freqüência é observado um amolecimento dos fônons para R = Bi e um endurecimento para R = Eu e Dy na mesma região paramagnética, (TC/TN < T < T* ). Anomalias dos fônons foram também identificadas abaixo de TN ~ 40-43 K, refletindo o início de uma ordem magnética e/ou ferroelétrica de longo alcance da subrede de Mn. Medidas complementares de susceptibilidade magnética dc (x(T)) para o cristal RMn2O5 no intervalo de temperatura entre 2 e 800 K revelaram uma temperatura de Curie-Weiss q CW = -253(3) K, apresentando um grande parâmetro de frustração ( q CW = TN). A curva do inverso de x (T), subtraindo o termo diamagnético, sofre um desvio do comportamento de Curie-Weiss devido a correlações magnéticas abaixo de temperaturas da ordem de ~ qCW . Também, um aspecto interessante é a derivada do inverso da susceptibilidade, a qual mostra pontos de in exfleao a ~ 160 K e ~ T*, sendo esta última, a temperatura abaixo da qual as anomalias dos fônons foram observadas. Estes dados magnéticos dão apoio a nossas interpretações de medidas Raman, onde deslocamentos anômalos de fônons abaixo de T* são associados ao acoplamento spin-fônon, em um cenário de fortes correlações magnéticas. Portanto, nossos resultados sustentam uma frustração magnética significante, introduzindo uma nova temperatura característica T* e sugerindo um comportamento interessante para as correlações magnéticas na fase paramagnética neste sistema RMn2O5 (R = Eu, Bi, e Dy). No material GdAl3 as interações entre o grau de liberdade de spin e os deslocamentos atômicos foram estudadas por meio do espalhamento Raman polarizado em função da temperatura. Neste composto a camada de Gd 4 f 7é esférica, indicando que os efeitos de campo cristalino são de ordem superiores. O estudo do acoplamento spin-rede pode fornecer evidências do mecanismo de troca e o grau de correlações magnéticas neste sistema. Nossas medidas de espalhamento Raman em superfícies frescas mostraram fônons com comportamento de freqüência convencional, enquanto que superfícies crescidas naturalmente e polidas apresentam anomalias na freqüência dos fônons abaixo de uma temperatura característica T** ~ 50 K. Tais anomalias são possivelmente devido a uma modulação da energia magnética pelas vibrações da rede em uma fase paramagnética fortemente correlacionada. Um estado de spin totalmente correlacionado imediatamente acima de TN é deduzido de nossos resultados neste sistema frustrado. Também, sugerimos que o acoplamento spin-fônon em metais pode depender das condições de superfície da amostra devido a que anomalias dos fônons foram observadas só nas amostras de superfícies envelhecidas. Finalmente, para a perovskita dupla Sr2CoUO6, os modos Raman de primeira e alta ordem foram estudos em função da temperatura. Nossos dados de espalhamento Raman revelam a existência de duas temperaturas características, T1 ~ 150 K e T2 ~ 300 K, onde são observados comportamentos não convencionais das intensidades dos espectros Raman e um amolecimento anômalo na freqüência de um fônon de alta freqüência abaixo de T2. Estas anomalias sugerem a possibilidade de transições de fase estruturais e/ou eletrônicas a essas temperaturas características. Baseado nas modificações relevantes dos espectros Raman e o comportamento quase constante das posições dos modos de alta ordem com a temperatura, sugerimos uma contribuição do mecanismo Franck-Condon neste sistema
Abstract: In this thesis we present an investigation of two magnetically frustrated systems, namely the RMn2O5 series (R = Eu, Bi, Dy) and the intermetallic compound GdAl3, in addition to the double perovskite compound Sr2CoUO 6. These systems were studied mainly using the Raman spectroscopy technique and complemented with magnetic susceptibility measurements. For the system RMn2O5 (R = Bi, Eu, and Dy), we present a study of the low- and high-frequency optical phonons. All studied materials show anomalous phonon shifts below a new characteristic temperature, T*~ 60-65 K. The sign and magnitude of such shifts appear to be correlated with the ionic radius of R. For instance, the high-energy phonons evolve from softenings for R = Bi to hardenings for R = Dy, and show an intermediary behavior for R = Eu in the temperature range between TC/TN and T*. On the other hand, the low-frequency anomalous phonon behaviors show softenings for R = Bi and hardening for R = Eu and Dy in the same paramagnetic range ( TC/TN < T < T*). Additional phonon anomalies were identified below ~ TN ~ 40-43 K, re ecting the onset of long-range ferroelectric and/or magnetic order of the Mn sublattice. Complementary dc-magnetic susceptibility [x(T)] measurements for BiMn2O5 up to 800 K yield a Curie-Weiss temperature q CW = -253(3) K, revealing a fairly large frustration ratio ( q CW / TN). Deviations of the inverse magnetic suceptibility, substracting the diamagnetic term, from a Curie-Weiss paramagnetic behavior due to magnetic correlations were observed below temperatures of the order of q CW . An interesting feature is the derivative of the inverse susceptibility, which shows in ection points at ~ 160 K and ~ T*. Supported by x(T) data, the anomalous Raman phonons shifts below T* are interpreted in terms of the spin-phonon coupling, in a scenario of strong magnetic correlations. Overall, these results support significant magnetic frustration, introducing a new characteristic temperature ( T*), and suggest a surprinsingly rich behavior for the magnetic correlations in the paramagnetic phase in this system. In GdAl3, the interaction among the spin degree of freedom and the atomic displacements were investigated by means of polarized Raman scattering. In this compound the Gd 4 f 7 shell is spherical, indicating that the crystal field e ect can be neglected. The spin-lattice coupling provide a fingerprint of the exchange mechanism and degree of magnetic correlations in this system. Raman scattering in fresh broken surfaces shows phonons with conventional frequency behavior, while naturally grown and polished surfaces present frequency anomalies below a characteristic temperature T** ~ 50 K. Such anomalies are possibly due to a modulation of the magnetic energy by the lattice vibrations in a strongly spin-correlated paramagnetic phase. A fully spin-correlated state inmediately above TN is inferred from our results in this frustrated system. Also, we suggest that the spin-phonon coupling in metals may depend in the surface conditions due to phonon anomalies were observed only in old-surface samples. Finally, for the Sr2CoUO6 double perovskite, the first- and higher-order Raman modes were studied as a function of temperature. Our Raman scattering results revealed the existence of two characteristic temperatures, T1 ~ 150 K e T2 ~ 300 K, where we observed unconvencional intensity behaviors of the Raman spectra and an anomalous softening of a particular high-energy mode below T2. These anomalies suggest structural and/or electronic phase transitions at such characteristic temperatures. Based on relevant modifications of the Raman spectra and the near-constant position behavior with temperature, we suggest a contribution of the Franck-Condon mechanism in this system
Doutorado
Física da Matéria Condensada
Doutor em Ciências
37
Qadri, Syed N. "Development of a double-layered perovskite as alternative anode material for high temperature steam electrolysis." Thesis, University of St Andrews, 2014. http://hdl.handle.net/10023/5826.
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The research presented is based on alternative anode materials for high temperature steam electrolysis. The key to commercially viable renewable energy economy is based on energy storage of intermittent sources. Hydrogen is the preferred form of energy storage for solid oxide electrolysis cells. However, conventional anode material lanthanum strontium manganite (LSM), suffers from poor ionic conductivity, thus prohibiting much of the bulk electrode from providing an enhanced electrochemical performance. This study explores the use of a double-layered perovskite system with mixed electronic and ionic conductivity for use as anode material. Specifically, the SmBa₁₋ₓSrₓCo₂O[sub](5+δ) system (SBSCO) is analyzed for characteristics that may enhance the performance and feasibility of SBSCO as an alternative anode material to LSM. Previous in-house work showed SmBa₀.₅Sr₀.₅Co₂O[sub](5+δ) had the lowest area specific resistance of any double- layered material reported. Here the system is further explored by studying the full range of compositions. From X-ray diffraction analysis, increased Sr substitution leads to a tetragonal phase change in SBSCO. High temperature x-ray diffraction of compositions showed thermal stability of structure. Magnetization measurements are reported for selected compositions. The stability of SBSCO was examined in CO₂ containing atmospheres. Despite containing alkaline earth metals, the system offers limited CO₂ tolerance. A set of thermodynamic parameters is presented based on CO₂ partial pressure and temperature. Model indicates SBSCO is a stable electrode material for both electrolysis and fuel cell modes. Compositions were tested for steam electrolysis performance with the use of YSZ electrolyte, and Ni-YSZ and La₀.₄Sr₀.₄Ni₀.₀₆Ti₀.₉₄O₂.₉₄ cathodes. SmBa₀.₃Sr₀.₇Co₂O[sub](5+δ) had the highest performance for compositions (0≤x≤1) based on I-V curves and impedance measurements. Stability tests were conducted in potentiostatic mode and no delamination was observed for SBSCO in microstructural analysis after testing. From these studies, SBSCO is demonstrated to be a suitable for application in electrolysis and an alternative for LSM as anode material.
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Azimonte, Bottan Carla. "Sondando o acoplamento magnetoelástico e magnetoelétrico em novos materiais magnetoresistentes e multiferróicos através de interações dos raios-x com a matéria." [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/276976.
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Orientador: Eduardo Granado Monteiro da SilvaTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Nesta tese de doutorado empregamos a união de diferentes técnicas com intuito de estudar as correlações entre as propriedades magnéticas, elétricas e cristalinas em alguns óxidos de metais de transição de interesse atual. Dentre eles se encontram os compostos da família das perovskitas duplas A2FeReO6 (A = Ba, Ca) e o multiferróico DyMn2O5. Esses materiais apresentam extraordinário potencial para aplicações e otimizações de novos dispositivos baseados nos materiais parentes ferroelétricos e ferromagnéticos. Realizamos medidas de dicroísmo circular magnético de raios-x, difração de raios-x e magnetização-dc. Esses dados, conjuntamente com resultados prévios de difração de nêutrons e cálculos de teoria de banda, mostraram um incipiente ordenamento orbital no sistema meiometal Ba2FeReO6, onde os elétrons de condução se encontram na fronteira da transição metalisolante. Para o composto Ca2FeReO6, a união dessas técnicas nos proporcionou um maior entendimento sobre a transição metal-isolante (TMI ~ 150 K), levando à proposta de um mecanismo até então nunca descrito para uma transição deste tipo, onde há competição entre um estado colinear otimizando a interação de Dupla Troca e um estado não colinear e localizado estabilizado pela energia magnetocristalina do Re. No multiferróico DyMn2O5, as medidas de difração anômala de raios-x com aplicação de campo elétrico, fundamentadas na teoria dos pares de Friedel, possibilitaram pela primeira vez a verificação experimental do deslocamento atômico médio dos íons de Mn nessa estrutura que leva à ferroeletricidade, até então não identificado por medidas convencionais de difração de raios-x ou nêutrons. De maneira geral, o trabalho descrito nesta tese esteve focado no estudo dos acoplamentos magnetocristalino e magnetoelétrico em materiais meio-metálicos que apresentam magnetoresistência de tunelamento e em multiferróicos. O desenvolvimento desta área é condição básica e necessária para viabilizar uma nova geração de dispositivos relacionados à spintrônica
Abstract: In this doctoral thesis, different techniques were employed to study the correlations between electrical, magnetic and crystalline properties in some transition metal oxides of current interest, such as the double perovskite family A2FeReO6 (A = Ba, Ca) and the multiferroic DyMn2O5. These materials offer extraordinary potential for the application and optimization of new devices based on ferroelectric and ferromagnetic related materials. X-ray magnetic circular dichroism (XMCD), x-ray diffraction and dc-magnetization measurements were performed. These data, associated with previous neutron results and band structure calculations, show an incipient orbital order in half-metallic Ba2FeReO6 where the conduction electrons are in the proximity of a metal-insulator transition. For the Ca2FeReO6 compound, the combination of these techniques provided insight into the metal-insulator transition (TMI ~ 150K), leading to the proposal of a new mechanism for this type of transition, according to which a competition between a double-exchange-like collinear state and a localized non-collinear state stabilized by the Re magnetocrystaline energy takes place. In the multiferroic DyMn2O5, anomalous x-ray diffraction experiments with the application of electric fields, and an interpretation based on Friedel pairs theory, allowed us to see, for the first time, the experimental confirmation of Mn ferroelectric atomic displacements in this structure leading to the ferroelectricity, so far unidentified by conventional x-ray diffraction and neutron diffraction experiments. In summary, the research described in this thesis was focused in the study of the magnetocrystalline and magnetoelectric coupling in half-metallic materials with tunneling magnetoresistance, as well as in multiferroics. Further development of this area is a basic and necessary condition to bring life to a new generation of devices related to spintronics
Doutorado
Física da Matéria Condensada
Doutor em Ciências
39
Tassetti, Anna. "NMR investigation of spin and charge dynamics in the electron doped Dirac-Mott insulator Ba2Na1-xCaxOsO6 double Perovskite." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2021.
Find full textAbstract:
Materials which possess both strong spin orbit coupling and strong electron correlation reveal exceptional magnetic and conductive properties, such as the case of the transition metal oxide object of this study. In the present dissertation we use Nuclear Magnetic Resonance technique in order to study the effect of charge doping on the Dirac Mott insulator double perovskites Ba2Na1-xCaxOsO6. We analyse in depth the material characteristic relaxation time T1 and T2 as a function of doping and temperature to investigate the spin and charge dynamics of the system.
The study has revealed an unexpected and anomalous dependence at high temperature, well above the magnetic transitions, fingerprint of a thermally activated processes. The comparison with previous muSR data and a detailed analysis through Bloembergen-Purcell-Pound theory indicate that they are
due to thermally activated fluctuation of the crystalline electric field gradient.
The temperature and doping dependence of the observed dynamics, the material properties, along with the comparison with earlier manganites studies, suggests that the phenomenon can be identified with the presence of small polarons, which has never been observed before for double perovskites.
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Fisher, James C. II. "A Novel Fuel Cell Anode Catalyst, Perovskite LSCF: Compared in a Fuel Cell Anode and Tubular Reactor." University of Akron / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=akron1152215855.
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Forino, Paola Caterina. "Nuclear magnetic resonance study of the electron doped Dirac-Mott Insulator double perovskite Ba2Na(1-x)CaxOsO6." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020.
Find full textAbstract:
Osmium-based materials exhibit unconventional magnetism due to the interplay between spin orbit coupling and strong electronic correlations.
In this thesis we investigate for the first time the effect of charge doping on the Dirac Mott insulator double perovskites Ba2Na(1-x)CaxOsO6 by using Nuclear Magnetic Resonance technique. We study in details the evolution of the magnetic phase diagram as a function of doping and applied magnetic field.
The system rapidly evolves from a canted to a collinear antiferromagnetic ground state with a monotonic increase of the magnetic transition temperature. Furthermore, the system remains insulating in the whole range of doping, despite the injection of extra electrons. This indicates that the Dirac-Mott insulating state of this material is unusually robust.
In particular, the results show an unexpected thermally activated charge dynamics which suggests the presence of polarons dominating the high temperature excitations of this Dirac-Mott insulator.
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Kluczny, Maksymilian. "Synthesis and Electrochemical Evaluation of Perovskite related oxide for Active Cathode for Solid Oxide Fuel Cells (SOFCs)." Thesis, KTH, Kemiteknik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-223612.
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Solid oxide fuel cells are used as stationary power plants for electricity production. Despite having a very high efficiency of 90% they haven’t gained a world-wide commercial usage, due to their very high operating temperatures, and high production cost. However, there is a lot of ongoing research with the aim of developing intermediate-temperature solid oxide fuel cells (IT-SOFCs) that could operate at temperatures below 800°C. Cathodes are the most studied components of IT-SOFCs, since decreasing operating temperature results in slow oxygen reduction reaction(ORR) kinetics and large polarization losses. Perovskite related metal oxides have become very popular materials that could make suitable cathodes for IT-SOFCs. In this work an evaluation of several materials belonging to three different material groups have been studied: single layer perovskites, with a general formula of ABO3, double layer perovskites, with a general formula of AA’B2O6 and Ruddlesden-Popper phase, with a general formula of An+1BnO3n+1. Power generating capabilities of those materials have been studied on an electrolyte supported cell, cathode/LSGM9182/Ni-Fe. IR drop and overpotential of the cathode was measured and activation energy of the ORR for each material has been calculated. The double layer perovskite cobaltites offer a significant drop in overpotential, increase in conductivity compared to their single layer counterpart, while being able to generate significant amount of power. Ruddlesden-Popper phase materials offer the lowest activation energy values amongst the researched materials, but offer limited power generation values in the setup they were tested. Both of double layer perovskites and Ruddlesden-Popper based materials have opportunities for their performance to be improved.Fastoxidbränsleceller används som stationära kraftverk för elproduktion. Trots att de har en mycket hög effektivitet på 90% har de inte fått en världsomspännande kommersiell användning på grund av deras mycket höga driftstemperaturer och hög produktionskostnad. Det är emellertid mycket pågående forskning med sikte på att utveckla intermediär temperatur fastoxidbränsleceller (IT-SOFC) som kan fungera vid temperaturer under 800 ° C. Katod är de mest studerade komponenterna i IT-SOFC, eftersom minskad driftstemperatur resulterar i kinetik med långsam syrereduktion (ORR) och stora polarisationsförluster. Perovskite-relaterade metalloxider har blivit mycket populära material som kan göra lämpliga katoder för IT-SOFC. I detta arbete har en utvärdering av flera material som hör till tre olika materialgrupper studerats: singelskikt perovskiter, med en generell formel för ABO3, dubbelskikt perovskiter, med en generell formel av AA'B2O6 och Ruddlesden-Popper-fasen med en allmän formel för An + 1BnO3n + 1. Effektgenereringskapaciteten hos dessa material har studerats på en elektrolytbärbar cell, katod / LSGM9182 / Ni-Fe. IR-droppe och överpotential hos katoden mättes och aktiveringsenergin för ORR för varje material har beräknats. Dubbelskiktet perovskit koboltiter ger en signifikant minskning av överpotentialen, ökad ledningsförmåga jämfört med deras enkelskikt motpart, samtidigt som man kan generera betydande mängden kraft. Ruddlesden-Popper-fasmaterial erbjuder de lägsta aktiveringsenergivärdena bland de undersökta materialen, men erbjuder begränsade kraftproduktionsvärden i den inställning de testades. Både av dubbelskiktet perovskiter och Ruddlesden-Popper-baserade material har möjligheter att förbättra deras prestanda.
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Ardito, Fábio Machado 1984. "Medidas simultâneas de espectroscopia Raman e propriedades de trasnporte eletrônico." [s.n.], 2010. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277700.
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Orientadores: Eduardo Granado Monteiro da Silva, Juan Carlos Paredes CampoyDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Espectroscopia Raman é uma ferramenta muito versátil e poderosa na investigação de mudanças de fase estruturais ou eletrônicas, principalmente quando aliada à possibilidade de se atingir altos campos magnéticos e baixas temperaturas. Neste projeto, introduzimos uma montagem capaz de caracterizar as propriedades de transporte eletrônico simultaneamente à obtenção de espectros Raman nestas condições de temperatura e campo, visando possibilitar a verificação in-situ do comportamento macroscópico do material a ser estudado. Descrevemos aqui o procedimento de montagem experimental, configuração, automação otimização deste sistema. Níveis de ruído muito baixos foram obtidos,consistentes como esperado pelas especificações dos instrumentos utilizados. A montagem foi aplicada a dois casos de interesse à Física do Estado Sólido. Primeiramente, níveis de Landau em grafite pirolítico altamente orientado (HOPG), oriundos da quantização dos níveis de energia dos portadores de carga sob ação de um campo magnético, foram diretamente observados por espectroscopia Raman. Simultaneamente, foram realizadas medidas de magnetorresistência feito Hall no HOPG, que comprovaram a reprodutibilidade de efeitos quânticos já reportados na literatura. Também foram investigadas as origens do acoplamento spin-fônon gigante na perovskita dupla Ba2FeReO6, obtendo-se a frequência de um modo de estiramento do oxigênio, em baixas temperaturas sob um campo magnético de 1,5 tesla, simultaneamente a medidas de magnetorresistência. Estas últimas serviram como uma sonda indireta da orientação relativa dos domínios magnéticos neste material, comprovaram que as medidas de Raman com campo foram tomadas com os domínios orientados. A ausência de qualquer mudança na frequência do modo de vibração com o campo aplicado indica que o grau de liberdade orbital dos elétrons 5d do Re não é important no mecanismo de acoplamento spin- fônon neste material
Abstract: The Raman Spectroscopy is a powerful and versatile tool in the investigation of structural and electronic phases transitions, specially when allied with the possibility of reaching high magnetic fields and low temperatures. In this project, we introduce an assembly capable of characterizing electronic transport properties simultaneously to the Raman spectra acquisition, in such conditions, allowing in situ verification of the macroscopic behavior of the material under study. We describe in this dissertation the procedures of assembly, configuration, automation and optimization of such system. Very low noise levels were obtained, in agreement with the specifications of the instruments used in this system. The assembly was employed in two cases of interest for solid stat physics. First, Landau Levels in highly ordered pyrolytic graphite (HOPG), due to energy quantization of charge carriers by a magnetic field, were directly observed by Raman spectroscopy. Simultaneously, magnetor sistanc and Hall effect measurements were carried out for HOPG, attesting the reproducibility of quantum effects already reported in the literature . Also, the origin of giant spin-phonon coupling into the double perovskit Ba2FeReO6 was investigated by obtaining the frequency of the oxygen stretching mode at low temperatures and under a magnetic field of 1.5 tesla, simultaneously to magnetoresistance measurements. The latter were taken as an indirect probe of the relative orientation of the magnetic domains in the material, proving that the Raman experiments with field were obtained with oriented domains. The absence of any change in the frequency of this mode with field indicat that the orbital degree of freedom of the Re 5d electrons is not important for the mechanism of spin-phonon coupling in this material
Mestrado
Física da Matéria Condensada
Mestre em Física
44
Hauser, Adam J. "Unlocking the potential of half-metallic Sr2FeMoO6 thin films through controlled stoichiometry and double perovskite ordering." The Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1291048729.
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Lucy, Jeremy M. "Exploration and Engineering of Physical Properties in High-Quality Sr2CrReO6 Epitaxial Films." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1436888316.
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Pers, Paul. "Développement et optimisation de matériaux d’électrodes et d’électrolytes, pour cellules PCFC." Thesis, Montpellier, 2015. http://www.theses.fr/2015MONTS193/document.
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Ce travail de thèse s'inscrit dans le cadre du développement des piles à combustible à céramique conductrice protonique (PCFC) anode support, opérant dans le domaine de température 400 – 600 °C. Une attention particulière a été portée sur la diminution des températures d'élaboration des composants constituants les piles. Les stratégies mises en œuvre pour l'élaboration des anodes et des couches minces électrolytiques à basse température ont été la nano-structuration et l'ajout d'additifs aidant au frittage. La réalisation de couche mince électrolytique a fait l'objet d‘un développement par pulvérisation de suspensions. Le choix des matériaux et leur optimisation ont permis de minimiser les résistances spécifiques surfaciques (ASR). Les tests en pile de cellules élémentaires PCFC ont montré des résultats prometteurs concurrentiels aux autres types de pile à combustible de l'ordre de 400mW/cm²The aim of present work was to develop PCFC materials and fuel cells working in 400-600°C. The work deals with the optimization of materials and elaboration processes with the aim of decreasing the sintering temperature. In order to achieve high performances, nanostructured and architecture electrodes and optimized electrolytes have been investigated. Efficient anode support PCFCs were fabricated using wet powder spraying whiten simply method easily suitable on order to scaling-up. The maximum power densities obtained in this work are among, one of the best reported for PCFC
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Esser, Bryan David. "High Resolution Characterization of Magnetic Materials for Spintronic Applications." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1524154928191728.
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Zuo, Peng. "Synthèses, études structurales et physiques de doubles pérovskites ordonnées NaLnCoWO6 : recherche de nouveaux composés multiferroïques basés sur la ferroélectricité hybride impropre." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAI113/document.
Full textAbstract:
Ce travail porte sur la synthèse et la caractérisation de nouveaux matériaux multiferroïques basés sur le concept très récent de la Ferroélectricité Hybride Impropre.Deux classes de matériaux ont été envisagées : les oxydes de type Ruddlesden-Popper NaRMO4 (R=Y, La; M= Mn, Cr) et les doubles pérovskites ordonnées NaLnCoWO6 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb). Les essais de synthèse sur la première classe n’ont pas permis d’obtenir les composés visés. Pour la seconde classe, l’ensemble des composés ont pu être obtenus par synthèse par voie solide à haute température. Les composés NaLnCoWO6 (Ln=La, Pr, Nd) ont été synthétisés à pression ambiante. L’usage des techniques de Hautes Pressions – Hautes Températures (HP-HT) a permis de stabiliser les composés contenant des terres rares plus petites et d’obtenir ainsi neuf nouveaux composés aux propriétés inédites.L’utilisation combinée de la diffraction sur poudre des rayons X au synchrotron et des neutrons a permis une étude structurale fine de la famille des doubles pérovskites ordonnées NaLnCoWO6. Les groupes d’espace ont été déterminés grâce aux affinements Rietveld des diffractogrammes de Rayons X sur poudre haute résolution. Les composés NaLnCoWO6 (Ln=La, Pr, Nd) cristallisent dans le groupe d’espace centrosymétrique C2/m tandis que les 9 nouveaux composés (Ln= Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) cristallisent dans le groupe d’espace polaire P21. Des mesures de génération de seconde harmonique confirment la structure non-centrosymétrique des nouveaux composés. L’analyse adaptée des modes de symétrie des composés cristallisant dans la structure polaire montre que l’amplitude du mode polaire induit augmente avec la diminution de la taille du cation Ln3+. La polarisation estimée à partir des affinements de la structure pourrait atteindre jusqu’à ~20µC/cm2.Une transition de phase présentant une très large hystérésis en température (~150K) a été observée par diffraction des neutrons pour le composé NaLaCoWO6. De plus, les images obtenues en Microscopie Electronique en Transmission révèlent la présence de bandes dans la phase haute température. Cette superstructure présente une périodicité de 12ap selon la direction [100]p ou [010]p . On a pu montrer à l’aide d’observations en STEM (microscopie à balayage en transmission) combinées avec des mesures en EELS (spectroscopie de pertes d’énergie des électrons) que ce contraste de bandes n’est pas lié à une variation de composition mais bien à une variation structurale. Différents modèles de rotations des octaèdres d’oxygènes ont été élaborés pour valider les données expérimentales obtenues par diffraction des rayons X et de neutrons. Le schéma de rotation qui décrit au mieux les données est a-a-c0. Concernant la phase basse température le groupe d’espace attribué est le groupe polaire P21.Les caractérisations magnétiques ont été réalisées pour toutes ces phases. Tous les composés NaLnCoWO6 s’ordonnent dans une configuration antiferromagnétique. Les températures de Néel varient entre 4 et 13K en fonction de la nature de la terre-rare. Les moments effectifs déterminés par la loi de Curie-Weiss sont en accord avec les moments théoriques attendus. Toutes les températures de Weiss sont négatives traduisant le fait que les interactions antiferromagnétiques sont prépondérantes dans ces systèmes. Les structures magnétiques ont été déterminées pour les composés Ln= Y, La, Tb, and Ho. Pour ces mêmes composés, des mesures diélectriques en fonction de la température et du champ magnétique ont permis de mettre en évidence un couplage magnéto-diélectrique conséquent pour Ln=Y and Ho. Les mesures de courant pyroélectrique autour de la transition magnétique montrent qu’il existe une polarisation induite par l’ordre magnétique dans le composé NaYCoWO6.. C’est la première mise en évidence expérimentale d’un couplage magnéto-électrique dans la famille des doubles pérovskites ordonnées AA’BB’O6In this study, new magneto-electric materials were synthesized on the basis of the very recently recognized ferroelectric inducing mechanism, hybrid improper ferroelectricity, and structural and physical properties characterizations were carried out on these new phases.Two classes of materials were focused on: the Ruddlesden-Popper oxides NaRMO4 (R=Y, La; M= Mn, Cr) and the doubly ordered perovskites NaLnCoWO6 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb). Attempts to synthesize the former class failed to give the target phases. All compounds in the latter class were prepared successfully by solid-state reactions at high temperature, among which the compounds NaLnCoWO6 (Ln=La, Pr, Nd) were prepared at ambient pressure while the other nine compounds were synthesized at high pressure.The structural study of the doubly ordered perovskite family NaLnCoWO6 was performed by synchrotron X-ray powder diffraction (SXRPD) and neutron powder diffraction (NPD). Based on the Rietveld refinement of the SXRPD patterns, the space groups were assigned. NaLnCoWO6 (Ln=La, Pr, Nd) compounds crystallize in the centrosymmetric C2/m symmetry, whereas the other nine new compounds crystallize in the polar space group P21. Second harmonic generation measurements on powder confirmed the non-centrosymmetric structure of the new compounds. Symmetry mode analysis demonstrates that the amplitude of the induced polar mode increases with a decreasing Ln cation size. The amplitude of the polarization was estimated from the refined structures, and can be as large as ~20µC/cm2.A structural phase transition was observed by NPD in NaLaCoWO6 with a large temperature hysteresis of ~150K. In addition, stripes were observed on the high-resolution transmission electron microscopy (TEM) images in the high temperature phase. The periodicity of this superstructure is 12ap along either the [100]p or [010]p direction. Further investigations by scanning TEM and electron energy loss spectroscopy revealed that the contrast of the stripes is due to a structural modulation rather than a compositional variation. Octahedral tilt twinning models were built with different tilting schemes to fit the observed SXRPD and NPD patterns. The tilting scheme a-a-c0 describes successfully the data. The low temperature phase was unambiguously determined to possess the polar space group P21.Magnetic and electric properties were experimentally characterized. All NaLnCoWO6 compounds order antiferromagnetically below TN which is between 4 and 13K. Curie-Weill fits were performed for all compounds, yielding reasonable effective magnetic moments compared to the theoretical ones. Weiss temperatures were all determined to be negative further indicating that antiferromagnetic interactions are dominant in these systems. Magnetic structures were determined for four NaLnCoWO6 (Ln= Y, La, Tb, and Ho) compounds, of which two have non-magnetic Ln cations (Y and La) and two have magnetic ones (Tb and Ho). Magneto-dielectric coupling was experimentally observed in compounds NaLnCoWO6 (Ln=Y, Tb, Ho) by dielectric measurements as a function of temperature and magnetic field. Polarization was derived for the Y and Ho compounds from pyroelectric current measurement, however, only the NaYCoWO6 compound demonstrates a polar behavior which cannot be switched. This is the first evidence that electric polarization can be induced by the magnetic ordering in the AA’BB’O6 class materials
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Євсєєва, М. В., Т. І. Панченко, M. V. Evseeva, and T. I. Panchenko. "Гетероядерні (купрум, лантаноїд)вмісні ацетилацетонати як вихідні речовини для синтезу наноматеріалів." Thesis, ВНТУ, 2019. http://ir.lib.vntu.edu.ua//handle/123456789/24079.
Full textAbstract:
Синтезовано гетероядерні (купрум, лантаноїд)вмісні ацетилацетонати загальної формули CuLn(AA)5·nH2O (Ln(III) = La, Nd, Sm, Gd; НАА = Н3С–С(О)–СН2–С(О)–СН3); n = 1-4). Методом рентгенофазового аналізу встановлено, що продуктами їх піролізу є подвійні оксиди Ln2CuО4 (Ln = Lа, Nd, Sm, Gd) структури перовскітуThe synthesis of heteronuclear compounds (copper, lanthanide)-containing acetylacetonates of the general formula CuLn(AA)5·nH2O (Ln(III) = La, Nd, Sm, Gd; НАА = Н3С–С(О)–СН2–С(О)–СН3); n = 1–4). The method of X-ray diffraction analysis showed that the products of their pyrolysis are the double oxides Ln2CuО4 (Ln = Lа, Nd, Sm, Gd) of the perovskite structure.
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Richard, Mélissandre. "Application de la technique d'échange isotopique à l'étude de systèmes catalytiques innovants : activation et mobilité d'O2 sur YSZ au sein d’un double-lit et réactivité de l’azote dans les matériaux nitrures pour la catalyse hétérogène." Thesis, Poitiers, 2015. http://www.theses.fr/2015POIT2293/document.
Full textAbstract:
Ce travail porte sur l’étude de systèmes catalytiques innovants par la technique d’échange isotopique (EI) permettant d’apprécier des propriétés fondamentales (activation des molécules en surface, mobilité et réactivité des atomes de réseau) pour comprendre les mécanismes de réaction mis en jeu en catalyse hétérogène et développer des systèmes plus performants. Aussi, l’identification d’espèces adsorbées intermédiaires est possible en couplant la spectrométrie de masse (analyse de la phase gaz) à l’observation de la surface catalytique par spectroscopie DRIFT.L’EI 16O/18O montre des effets de dispersion ou de synergie de LaMnO3 (LM) supportée sur YSZ ou TiO2 expliquant les performances de cette structure pérovskite pour l’oxydation catalytique de C7H8 via un mécanisme suprafacial. L’activité en EI C16O2/C18O2 démontre la mobilité exceptionnelle des atomes O de réseau de YSZ dès 150 °C via la formation d’espèces (hydrogéno)carbonates en surface. En catalyse d’oxydation, à T < 800 °C, cette mobilité est pourtant limitée par l’activation d’O2 à la surface de YSZ. La solution proposée ici est la génération préalable d’une espèce oxygène réactive sur un lit de matériau réductible type LM. Le double-lit LM-YSZ montre d’excellentes performances pour abaisser la température d’oxydation de CH4 à 425 °C via un mécanisme Mars van Krevelen (MvK) où les atomes O de YSZ participent à la réaction par l’intermédiaire d’espèces formiates.L’EI 14N/15N est également utilisé dans ce travail pour analyser la réactivité des atomes N de réseau dans les nitrures métalliques. En particulier, Co3Mo3N et Ni2Mo3N montrent des propriétés remarquables, dépendant de la méthode de préparation ou du prétraitement appliqué. Leur comportement suggère la participation des atomes N dans la réaction de synthèse de NH3 sur le principe d’un mécanisme MvKThis work concerns the study of new catalytic systems by isotopic exchange (IE) technique allowing to appreciate basic properties (molecules surface activation, mobility and reactivity of lattice atoms) to better understand catalytic mechanisms and to develop efficient catalysts. The identification of intermediate adsorbed species is possible by coupling mass spectrometry (gas-phase analysis) with the catalytic surface analysis by DRIFT spectroscopy.IE 16O/18O shows dispersal and synergetic effect of supported LaMnO3 perovskite (LM) on YSZ or TiO2 which explain catalytic performances of this perovskite structure for toluene oxidation via a suprafacial mechanism. IE C16O2/C18O2 activity demonstrates the remarkable lattice O atoms mobility of YSZ from 150 °C via adsorbed (hydrogeno)carbonates. To the contrary, in oxidation catalysis, under 800 °C, this mobility is very limited by O2 activation on YSZ surface. The solution proposed in this work is the previous generation of reactive oxygen species on a first catalytic bed of reducible material as LM. LM+YSZ dual-bed shows very efficient activity to reduce methane oxidation temperature at 425 °C via a Mars-van Krevelen (MvK) mechanism in which lattice O atoms of YSZ take part in the reaction by intermediate formate species.IE 14N/15N is thereafter used to analyse lattice N atoms reactivity of metal nitrides materials. In particular, Co3Mo3N and Ni2Mo3N show interesting properties depending on preparation or pre-treatement routes. This behaviour supposes that ammonia synthesis reaction could be procced via MvK type mechanism with the participation of lattice N of this nitrides