Academic literature on the topic 'PYROCHLORE STRUCTURED MATERIALS'

Herein, we have formulated a simplistic semi-empirical model for Vicker’s hardness of rare earth based pyrochlore compounds. We have considered the structured 97 pyrochlore compounds for Vicker’s hardness calculations. The plasmon energy (ħωp) depends on basic parameters of the material such as Ne-effective number of free electrons per unit volume participating in plasma oscillations, e-electronic charge and m-mass of an electron. The proposed model predicts that the experimental and theoretical values of Vicker’s hardness increases as plasmon energy of pyrochlore increases. We have found that the calculated values are in better agreement with available experimental and theoretical data, which supports the validity of the model. This model supports the modeling of emerging functional pyrochlore compounds and helps to understand their mechanical properties for excellent thermal stability, superconductivities, batteries, ferroelectricity, water spitting, high ionic conductivity, good photoluminescence, inherent oxygen vacancies, exotic magnetism, and now-a-days most importantly in nuclear waste encapsulation and aerospace industry

Background: Pyrochloro structured Sm2Ti2O7 has photocatalytic activity on degradation of organic substances and on hydrogen evolution from water. Powder materials usually encounter the obstacle of separating from treated water. HZSM-5 zeolite is a kind of porous structured material with large surface area. Its role as a support for Sm2Ti2O7 is interesting. Methods: The supported Sm2Ti2O7 was synthesized using sol-gel method. The composite χSm2Ti2O7/HZSM-5 was characterized by XRD, SEM, TEM, FT-IR/FIR, UV-Vis DRS, N2 adsorption- desorption and XPS measurements. Photocatalytic degradation of Reactive Brilliant Red X-3B (RBR X-3B) was measured to evaluate the activity of the composite. Results: Sm2Ti2O7 is in the pyrochlore phase after loading on the surface of HZSM-5 zeolite. The crystal cell of pyrochlore Sm2Ti2O7 continuously expanses with decreasing Sm2Ti2O7 loading content in the composite. Bandgap energy of Sm2Ti2O7 is enlarged after supporting. The specific surface area of Sm2Ti2O7 was enlarged from 9.8 m2/g to 93 m2/g after loading. Both of the adsorption capacity and photocatalytic activity of the χSm2Ti2O7/HZSM-5 are greater than those of pure Sm2Ti2O7. After 120 min of irradiation, 73.1% of the initial RBR X-3B molecules are decomposed on 70%Sm2Ti2O7/HZSM-5, and only 27.7% of the dye is decomposed on the bare Sm2Ti2O7. Conclusion: Sm2Ti2O7 crystal growth is constrained after loading due to dispersion of Sm2Ti2O7 on the surface of HZSM-5. The specific surface area of Sm2Ti2O7 is significantly enlarged after loading. All the supported samples have greatly enhanced photocatalytic activity as compared to the bare Sm2Ti2O7.

Oxide dispersion-strengthened materials are reinforced by a (Y, Ti, O) nano-oxide dispersion and thus can be considered as nanostructured materials. In this alloy, most of the nanoprecipitates are (Y, Ti, O) nano-oxides exhibiting a Y2Ti2O7 pyrochlore-like structure. However, the lattice structure of the smallest oxides is difficult to determine, but it is likely to be close to the atomic structure of the host matrix. Designed to serve in extreme environments—i.e., a nuclear power plant—the challenge for ODS steels is to preserve the nano-oxide dispersion under irradiation in order to maintain the excellent creep properties of the alloy in the reactor. Under irradiation, the nano-oxides exhibit different behaviour as a function of the temperature. At low temperature, the nano-oxides tend to dissolve owing to the frequent ballistic ejection of the solute atoms. At medium temperature, the thermal diffusion balances the ballistic dissolution, and the nano-oxides display an apparent stability. At high temperature, the nano-oxides start to coarsen, resulting in an increase in their size and a decrease in their number density. If the small nano-oxides coarsen through a radiation-enhanced Ostwald ripening mechanism, some large oxides disappear to the benefit of the small ones through a radiation-induced inverse Ostwald ripening. In conclusion, it is suggested that, under irradiation, the nano-oxide dispersion prevails over dislocations, grain boundaries and free surfaces to remove the point defects created by irradiation.

Rare earth doped ceria compounds are fluorite related oxides which show oxide ionic conductivity higher than yttria stabilized zirconia in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for ‘low (below 500°C)’ temperature operation of solid oxide fuel cells (SOFCs). In this study, the nano-sized powders of DyxCe1-xO2-x/2 (x=0.15 and 0.2) were prepared using ammonium carbonate co-precipitation method. To design the nano-structure in aforementioned materials, the round shape particles were prepared in nano-scale. The combined process of Spark Plasma Sintering (SPS) and Conventional Sintering (CS) was examined for fabrication of nano-structured doped CeO2 solid electrolytes. The nano-structural features in the (SPS+CS) specimen and CS specimen were observed using transmission electron microscopy (TEM). This micro-analysis suggested that the micro-domain with distorted pyrochlore structure exists in the grain of these materials. The conducting properties in the specimens were strongly influenced by the micro-domain size. It is found that the present combined process minimized the micro-domain size and maximized the conductivity in the specimens. Also nano-structured Dy doped CeO2 sintered bodies in the present study had wide ionic domain and high transport number of oxygen. This suggests that fabricated sintered bodies are suitable for the solid electrolyte in low temperature operated SOFCs. It is concluded that a control of micro-domain size is a key for development of high quality doped CeO2 electrolytes for fuel cell application. It is expected that advanced solid electrolytes for clean energy production will be produced by a design of nano-structure in rare earth doped CeO2 solid electrolyte.

In a previous publication the order–disorder pyrochlore to fluorite transformation temperatures for a series of A2Hf2O7 pyrochlores were predicted [C.R. Stanek and R.W. Grimes: Prediction of rare-earth A2Hf2O7 pyrochlore phases. J. Am. Ceram. Soc. 2002, 85, p. 2139]. This was facilitated by establishing a relationship between these temperatures and the energy required to introduce a specific defect structure into the perfect pyrochlore lattice. Here an equivalent relationship for A2Zr2O7 pyrochlores was generated, and from this the disorder temperatures for a number of compositions including Eu2Zr2O7 were predicted.

In this thesis I present the synthesis and characterization of materials exhibiting frustrated magnetism. In Chapter 1 I describe magnetic frustration and some of the magnetic states that can arise from it followed by the background on iridates and platinates with honeycomb structures and rare earth pyrochlores.

In Chapter 3 I discuss my work on the synthesis and properties of ternary sodium iridates with formulas Na_xM_2/3Ir_1/3O ₂ and Na_xM_1/3Ir_2/3O₂ (M = Mn, Fe, Co, Ni, Cu, Zn). The ternary iridates are based on the honeycomb compound Na₂IrO₃ but show more disorder in the honeycomb layer than the parent. The six new compounds are all spin glasses but show distinct magnetic properties from one another.

In Chapter 4 I continue my work on honeycombs by exploring new ternary sodium platinates. These three new compounds with formulas Na₃MPt ₂O_6+x (M = Mg, Cu, Zn) are structurally very similar to the iridates discussed in Chapter 2 but have non-magnetic Pt⁴⁺ in place of magnetic Ir⁴⁺. The Mg and Zn variants are non-magnetic while the Cu variant is paramagnetic at 2 K.

Chapter 5 is a synchrotron X-ray diffraction study of the magnetically frustrated rare earth pyrochlores Ho₂Ti₂O₇, Er₂Ti₂O₇ and Yb₂Ti₂O ₇. Previous neutron scattering studies have shown reflections that are forbidden by the assigned space group Fd-3m, therefore high intensity, high resolution X-ray diffraction data was collected to determine if the reflections are present. Slight variations in sample stoichiometry were studied to account for possible sample variation. The forbidden reflections are absent from the X-ray diffraction patterns, providing strong evidence that the extra reflections in neutron scattering experiments are not structural in origin.

This thesis uses solid-state nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations to study local structure and disorder in inorganic materials. Initial work concerns microporous aluminophosphate frameworks, where the importance of semi-empirical dispersion correction (SEDC) schemes in structural optimisation using DFT is evaluated. These schemes provide structures in better agreement with experimental diffraction measurements, but very similar NMR parameters are obtained for any structures where the atomic coordinates are optimised, owing to the similarity of the local geometry. The ³¹P anisotropic shielding parameters (Ω and κ) are then measured using amplified PASS experiments, but there appears to be no strong correlation of these with any single geometrical parameter. In subsequent work, a range of zeolitic imidazolate frameworks (ZIFs) are investigated. Assignment of ¹³C and ¹⁵N NMR spectra, and measurement of the anisotropic NMR parameters, enabled the number and type of linkers present to be determined. For ¹⁵N, differences in Ω may provide information on the framework topology. While ⁶⁷Zn measurements are experimentally challenging and periodic DFT calculations are currently unreliable, calculations on small model clusters provide good agreement with experiment and indicate that ⁶⁷Zn NMR spectra are sensitive to the local structure. Finally, a series of pyrochlore-based ceramics (Y₂Hf₂₋ₓSnₓO₇) is investigated. A phase transformation from pyrochlore to a disordered defect fluorite phase is predicted, but ⁸⁹Y and ¹¹⁹Sn NMR reveal that rather than a solid solution, a significant two-phase region is present, with a maximum of ~12% Hf incorporated into the pyrochlore phase. The use of ¹⁷O NMR to provide insight into the local structure and disorder in these materials is also investigated. Once the different T₁ relaxation and nutation behaviour is considered it is shown that quantitative ¹⁷O enrichment of Y₂Sn₂O₇ is possible, and that ¹⁷O does offer a promising future tool for study.

Doutoramento em Ciência e Engenharia de Materiais
Alkali tantalates and niobates, including K(Ta / Nb)O3, Li(Ta / Nb)O3 and Na(Ta / Nb)O3, are a very promising ferroic family of lead-free compounds with perovskite-like structures. Their versatile properties make them potentially interesting for current and future application in microelectronics, photocatalysis, energy and biomedics. Among them potassium tantalate, KTaO3 (KTO), has been raising interest as an alternative for the well-known strontium titanate, SrTiO3 (STO). KTO is a perovskite oxide with a quantum paraelectric behaviour when electrically stimulated and a highly polarizable lattice, giving opportunity to tailor its properties via external or internal stimuli. However problems related with the fabrication of either bulk or 2D nanostructures makes KTO not yet a viable alternative to STO. Within this context and to contribute scientifically to the leverage tantalate based compounds applications, the main goals of this thesis are: i) to produce and characterise thin films of alkali tantalates by chemical solution deposition on rigid Si based substrates, at reduced temperatures to be compatible with Si technology, ii) to fulfil scientific knowledge gaps in these relevant functional materials related to their energetics and ii) to exploit alternative applications for alkali tantalates, as photocatalysis. In what concerns the synthesis attention was given to the understanding of the phase formation in potassium tantalate synthesized via distinct routes, to control the crystallization of desired perovskite structure and to avoid low temperature pyrochlore or K-deficient phases. The phase formation process in alkali tantalates is far from being deeply analysed, as in the case of Pb-containing perovskites, therefore the work was initially focused on the process-phase relationship to identify the driving forces responsible to regulate the synthesis. Comparison of phase formation paths in conventional solid-state reaction and sol-gel method was conducted. The structural analyses revealed that intermediate pyrochlore K2Ta2O6 structure is not formed at any stage of the reaction using conventional solid-state reaction. On the other hand in the solution based processes, as alkoxide-based route, the crystallization of the perovskite occurs through the intermediate pyrochlore phase; at low temperatures pyrochlore is dominant and it is transformed to perovskite at >800 °C. The kinetic analysis carried out by using Johnson-MehlAvrami-Kolmogorow model and quantitative X-ray diffraction (XRD) demonstrated that in sol-gel derived powders the crystallization occurs in two stages: i) at early stage of the reaction dominated by primary nucleation, the mechanism is phase-boundary controlled, and ii) at the second stage the low value of Avrami exponent, n ~ 0.3, does not follow any reported category, thus not permitting an easy identification of the mechanism. Then, in collaboration with Prof. Alexandra Navrotsky group from the University of California at Davis (USA), thermodynamic studies were conducted, using high temperature oxide melt solution calorimetry. The enthalpies of formation of three structures: pyrochlore, perovskite and tetragonal tungsten bronze K6Ta10.8O30 (TTB) were calculated. The enthalpies of formation from corresponding oxides, ∆Hfox, for KTaO3, KTa2.2O6 and K6Ta10.8O30 are -203.63 ± 2.84 kJ/mol, - 358.02 ± 3.74 kJ/mol, and -1252.34 ± 10.10 kJ/mol, respectively, whereas from elements, ∆Hfel, for KTaO3, KTa2.2O6 and K6Ta10.8O30 are -1408.96 ± 3.73 kJ/mol, -2790.82 ± 6.06 kJ/mol, and -13393.04 ± 31.15 kJ/mol, respectively. The possible decomposition reactions of K-deficient KTa2.2O6 pyrochlore to KTaO3 perovskite and Ta2O5 (reaction 1) or to TTB K6Ta10.8O30 and Ta2O5 (reaction 2) were proposed, and the enthalpies were calculated to be 308.79 ± 4.41 kJ/mol and 895.79 ± 8.64 kJ/mol for reaction 1 and reaction 2, respectively. The reactions are strongly endothermic, indicating that these decompositions are energetically unfavourable, since it is unlikely that any entropy term could override such a large positive enthalpy. The energetic studies prove that pyrochlore is energetically more stable phase than perovskite at low temperature. Thus, the local order of the amorphous precipitates drives the crystallization into the most favourable structure that is the pyrochlore one with similar local organization; the distance between nearest neighbours in the amorphous or short-range ordered phase is very close to that in pyrochlore. Taking into account the stoichiometric deviation in KTO system, the selection of the most appropriate fabrication / deposition technique in thin films technology is a key issue, especially concerning complex ferroelectric oxides. Chemical solution deposition has been widely reported as a processing method to growth KTO thin films, but classical alkoxide route allows to crystallize perovskite phase at temperatures >800 °C, while the temperature endurance of platinized Si wafers is ~700 °C. Therefore, alternative diol-based routes, with distinct potassium carboxylate precursors, was developed aiming to stabilize the precursor solution, to avoid using toxic solvents and to decrease the crystallization temperature of the perovskite phase. Studies on powders revealed that in the case of KTOac (solution based on potassium acetate), a mixture of perovskite and pyrochlore phases is detected at temperature as low as 450 °C, and gradual transformation into monophasic perovskite structure occurs as temperature increases up to 750 °C, however the desired monophasic KTaO3 perovskite phase is not achieved. In the case of KTOacac (solution with potassium acetylacetonate), a broad peak is detected at temperatures <650 °C, characteristic of amorphous structures, while at higher temperatures diffraction lines from pyrochlore and perovskite phases are visible and a monophasic perovskite KTaO3 is formed at >700 °C. Infrared analysis indicated that the differences are due to a strong deformation of the carbonate-based structures upon heating. A series of thin films of alkali tantalates were spin-coated onto Si-based substrates using diol-based routes. Interestingly, monophasic perovskite KTaO3 films deposited using KTOacac solution were obtained at temperature as low as 650 °C; films were annealed in rapid thermal furnace in oxygen atmosphere for 5 min with heating rate 30 °C/sec. Other compositions of the tantalum based system as LiTaO3 (LTO) and NaTaO3 (NTO), were successfully derived as well, onto Si substrates at 650 °C as well. The ferroelectric character of LTO at room temperature was proved. Some of dielectric properties of KTO could not be measured in parallel capacitor configuration due to either substrate-film or filmelectrode interfaces. Thus, further studies have to be conducted to overcome this issue. Application-oriented studies have also been conducted; two case studies: i) photocatalytic activity of alkali tantalates and niobates for decomposition of pollutant, and ii) bioactivity of alkali tantalate ferroelectric films as functional coatings for bone regeneration. Much attention has been recently paid to develop new type of photocatalytic materials, and tantalum and niobium oxide based compositions have demonstrated to be active photocatalysts for water splitting due to high potential of the conduction bands. Thus, various powders of alkali tantalates and niobates families were tested as catalysts for methylene blue degradation. Results showed promising activities for some of the tested compounds, and KNbO3 is the most active among them, reaching over 50 % degradation of the dye after 7 h under UVA exposure. However further modifications of powders can improve the performance. In the context of bone regeneration, it is important to have platforms that with appropriate stimuli can support the attachment and direct the growth, proliferation and differentiation of the cells. In lieu of this here we exploited an alternative strategy for bone implants or repairs, based on charged mediating signals for bone regeneration. This strategy includes coating metallic 316L-type stainless steel (316L-SST) substrates with charged, functionalized via electrical charging or UV-light irradiation, ferroelectric LiTaO3 layers. It was demonstrated that the formation of surface calcium phosphates and protein adsorption is considerably enhanced for 316L-SST functionalized ferroelectric coatings. Our approach can be viewed as a set of guidelines for the development of platforms electrically functionalized that can stimulate tissue regeneration promoting direct integration of the implant in the host tissue by bone ingrowth and, hence contributing ultimately to reduce implant failure.
Tantalatos e niobatos alcalinos, como K(Ta / Nb)O3, Li(Ta / Nb)O3 and Na(Ta / Nb)O3, são uma família atrativa de compostos ferroeléctricos livres de chumbo com estrutura perosvquítica. As suas propriedades versáteis fazem destes potencialmente interessantes para aplicações em microelectrónica, foto catálise, energia e biomédica. Entre os compostos acima citados, os compostos de tantalato de potássio, KTaO3 (KTO), tem atraído bastante atenção como substitutos para o amplamente conhecido titanato de estrôncio, SrTiO3 (STO). KTO é um óxido perovsquítico com comportamento paraelétrico quântico, quando eletricamente estimulado, e elevada polaribilidade tornando viável engenhar as suas propriedades através de estímulos internos e externos. No entanto os problemas na sua produção, quer em macroescala quer em nanoestruturas 2D, tornam estes compostos numa alternativa pouco viável para a substituir o STO. Consequentemente, e de forma a contribuir cientificamente para aumentar o conhecimento sobre as aplicações dos tantalatos, os principais objectivos desta tese são: i) produzir e caracterizar filmes finos de tantalatos alcalinos através de deposição de solução química em substratos rígidos, à base de silício, e a baixas temperaturas de forma a serem compatíveis com a tecnologia de silício; ii) complementar o conhecimento científico sobre estes materiais funcionais relativamente às suas características termodinâmicas; iii) explorar aplicações alternativas para os tantalatos alcalinos, como a foto catálise. No que diz respeito à síntese, foi focalizada no entendimento da formação de fase no tantalato de potássio sintetizado por diferentes métodos, de modo a controlar a cristalização da estrutura perovsquítica desejada e evitar a formação da fase pirocloro a baixas temperaturas e fases deficientes em potássio. Em tantalatos alcalinos o processo de formação da fase desejada está longe de estar plenamente analisado, como é o caso das perovsquites que contêm chumbo, consequentemente o trabalho foi inicialmente focado na compreensão da relação processo-fase para identificar as forças motrizes responsáveis por regular o processo de síntese. Foi realizada um estudo comparativo da formação de fase via método convencional de reação do estado sólido e via método de sol-gel. A análise estrutural revelou que a estrutura piroclórica intermédia K2Ta2O6 não foi formada em nenhuma etapa da reação via método do estado sólido. Por outro lado em processos baseados em solução, como os baseados em alcóxidos, a cristalização perovsquítica ocorre através da indesejada fase pirocloro intermédia; a baixas temperaturas a fase pirocloro é dominante e sofre a transformação para perovsquite a >800 °C. A análise cinética efectuada usando o modelo Johnson-Mehl-Avrami-Kolmogorow e a difração de raio-X quantitativa (DRX), demonstraram que nos pós obtidos pelo método sol-gel, a cristalização ocorre em duas etapas: i) no estágio inicial a reação é denominada por nucleação primária, o mecanismo é controlado por fronteira de fase, e ii) no segundo estágio, o baixo valor do expoente de Avrami, n ~ 0.3, não segue nenhuma categoria reportada impossibilitando assim uma clara identificação do mecanismo. Posteriormente, e em colaboração com o grupo da Professora Alexandra Navrostky da Universidade da Califórnia, Davis, foram realizados estudos de termodinâmica, usando calorimetria de solução de óxidos fundidos a alta temperatura. Foram calculadas as entalpias de formação das três estruturas: pirocloro, perovsquite e tetragonal tungsténio bronze K6Ta10.8O30 (TTB). As entalpias de formação relativas aos óxidos correspondentes, ∆Hfox, para KTaO3, KTa2.2O6 e K6Ta10.8O30, são -203.63 ± 2.84 kJ/mol, 358.02 ± 3.74 kJ/mol e -1252.34 ± 10.10 kJ/mol, respectivamente; enquanto que as relativas aos elementos, ∆Hfel, para KTaO3, KTa2.2O6 e K6Ta10.8O30 são 1408.96 ± 3.73 kJ/mol, -2790.82 ± 6.06 kJ/mol e -13393.04 ± 31.15 kJ/mol, respectivamente. As possíveis reações de decomposição, de KTa2.2O6 para KTaO3 e Ta2O5 (reação 1) ou para K6Ta10.8O30 e Ta2O5 (reação 2), foram propostas e o cálculo das entalpias resultou em 308.79 ± 4.41 kJ/mol e 895.79 ± 8.64 kJ/mol, respectivamente. As reações são fortemente endotérmicas, indicando que estas decomposições são energeticamente desfavoráveis, uma vez que é improvável que qualquer termo de entropia possa sobrepor-se a uma entalpia tão positiva. Os estudos termodinâmicos provaram que o pirocloro é energeticamente mais estável que a perovsquite para temperaturas baixas. Assim, a organização local dos precipitados amorfos canaliza a cristalização para a estrutura mais favorável, que é a pirocloro com uma organização local similar; a distância entre os vizinhos mais próximos na fase amorfa, ou na fase ordenada a baixo alcance, é similar à do pirocloro. Tendo em conta a derivação estequiométrica no sistema KTO, selecionar a técnica de fabricação / deposição de filmes finos mais apropriada é uma questão-chave, especialmente no que concerne aos óxidos ferroeléctricos complexos. A deposição por solução química tem sido o método de processamento mais reportado, para crescimento de filmes finos de KTO, mas o método clássico de alcóxidos permite cristalizar a fase perovsquite a temperaturas >800 °C enquanto que a temperatura máxima de estabilidade para os substratos de silício platinizado é ~700 °C. Portanto, foi usado um processo alternativo baseado em dióis, com precursores carboxilados de potássio, com o objectivo de estabilizar os precursores em solução, evitando assim o uso de solventes tóxicos e diminuindo a temperatura de cristalização da fase perovsquite. A análise dos pós revelou que no caso de KTOac (solução baseada em acetato de potássio), uma mistura de fase perovsquite e pirocloro foi detectada a uma temperatura de apenas 450 °C, e a transformação gradual em estrutura perovsquítica monofásica ocorre quando as temperaturas sobem acima de 750 °C, no entanto a fase KTaO3 monofásica não é obtida. No caso do KTOacac (solução com acetil-acetona de potássio, cadeia alquílica longa carboxilato de metal), um amplo pico é detectado a temperaturas <650 °C, característico de estruturas amorfas, enquanto que a elevadas temperaturas, os planos de difração das fases pirocloro e perovsquite são visíveis e a perovsquite KTaO3 monofásica é conseguida a temperaturas >700 °C. A análise de infravermelhos mostrou que estas diferenças acontecem devido à deformação da estrutura base dos carbonatos sob aquecimento. Uma série de filmes finos de tantalatos alcalinos foram depositados por spincoating em substratos de silício, usando a metodologia baseada em dióis. Filmes monofásicos de perovsquite KTaO3 depositados usando solução de KTOacac foram obtidos a uma temperatura de apenas 550 °C; os filmes foram recristralizados em fornos de aquecimento rápido em atmosfera de oxigénio durante 5 minutos com taxa de aquecimento de 30 °C/seg. Outras composições, LiTaO3 (LTO) e NaTaO3 (NTO), foram depositados com sucesso em substratos de silício a 650 °C. O carácter ferroeléctrico do LTO à temperatura ambiente foi provado. Infelizmente, não foi possível medir as propriedades eléctricas do KTO no condensador paralelo devido às interfaces filme-substrato ou filme-eléctrodo. Assim sendo, estudos futuros são necessários para compreender esta questão. Foram também conduzidos estudos com vista às possíveis aplicações; dois casos de estudo: i) estudo da atividade fotocatalítica de tantalatos e niobatos alcalinos para decomposição de poluentes, e ii) estudo de bioatividade de filmes ferroelétricos de tantalatos alcalinos como revestimento funcional para regeneração óssea. Recentemente, tem sido dedicada muita atenção ao desenvolvimento de novos materiais fotocatalíticos, e as composições à base de óxido de tântalo e nióbio tem demonstrado capacidade de fotocatálise na reação de separação da água devido ao elevado potencial das bandas de condução. Assim, várias composições das famílias dos tantalatos e niobatos alcalinos foram testadas como catalisadores para degradação do azul de metileno. Os resultados mostram valores de atividade promissores para alguns dos compostos, sendo o KNbO3 o mais ativo de entre os testados, alcançando valores acima de 50 % na degradação do pigmento após 7 h sob exposição a UVA. No entanto algumas modificações nas composições dos pós podem melhorar a sua performance. No que concerne à regeneração óssea, é importante obter plataformas que através de estímulos apropriados consigam assegurar a adesão e direcionar o crescimento, a proliferação e a diferenciação celular. Neste contexto, foi aqui explorada uma estratégia alternativa para revestimento de implantes ósseos, baseada na regeneração óssea mediada por sinais elétricos. Esta estratégia implica revestir substratos metálicos de aço inoxidável tipo 316L (316L-SST), com camadas de LiTaO3 ferroeléctrico, funcionalizadas através de polarização elétrica ou de irradiação com luz UV. Foi demonstrado que a formação de fosfato de cálcio na superfície e a adsorção de proteínas é consideravelmente melhorada quando o 316L-SST é revestido com filmes ferroelétricos funcionalizados. Esta estratégia pode ser encarada como um conjunto de orientações para o desenvolvimento de plataformas eletricamente funcionalizadas, capazes de estimular a regeneração de tecidos, promovendo a associação direta do implante com os tecidos hospedeiros, contribuindo assim para a redução de falhas na reabilitação com implantes ósseos.

De nouveaux composés fluorés à chaines magnétiques appartenant aux types weberite et pyrochlore ont été isolés. Les corrélations entre propriétés structurales et magnétiques ont été étudiées. Plusieurs familles a base de palladium(II) ont été caractérisées. Des structures cristallines ont été déterminées (soit sur monocristal, soit sur des échantillons polycristallins par la méthode de Rietveld). Divers types structuraux ont été identifies (types CsPd2F5, LiSbF6 ou dérivant de Na2CuF4): le palladium y est localisé en site octaédrique ou plan carré mais peut posséder simultanément ces deux coordinences. Des structures magnétiques ont été établies par diffraction de neutrons. Dans un certain nombre de ces matériaux des transitions de phases ont été mises en évidence par micro-ATD et analyse radiocristallographique. Leur caractérisation magnétique a été réalisée. Des phases inédites à base de cuivre et d'éléments lanthanidiques apparentées au type KBrF4 ont été synthétisées. La luminescence de l'argent monovalent a été étudiée dans des composés fluorés de la structure elpasolite ou dérivée. Les propriétés ont été analysées en fonction de la coordinence de Ag+.

Materials chemistry is essentially concerned with the design/synthesis of new solids endowed with functional properties that could be of relevance to today’s materials technology. Among the large variety of solid materials that attract attention, metal oxides continue to contribute significantly to current materials chemistry. A wide variety of oxide materials (based on rocksalt, spinel, corundum, perovskite, garnet, pyrochlore and other structures) and their properties have been investigated over the years. Most of these oxides are derived from the transition metals. Transition metal oxides with structures derived from metal-oxygen (MO6) octahedra, in particular, display an array of exotic properties with potential or proven technological application. While it is traditionally believed that the partially filled d shell (dn : 0 < n < 10) of the transition metal atoms plays a crucial role in deciding the electronic properties, the significance of d0 metal atoms for the properties (and structure) of transition metal oxides is not fully recognized. Magnetism (SrRuO3, Fe3O4), metallicity (ReO3, LaNiO3), colossal magnetoresistance (La1-xCaxMnO3) and superconductivity (La2xSrxCuO4, Sr2RuO4) are some of the properties that can be traced to the presence of partially filled d shell, while properties like ferroelectricity (BaTiO3), piezoelectricity (PbZr1-xTixO3) and nonlinear optical response (LiNbO3) could be traced to the presence of transition metals (TiIV, ZrIV, NbV) with d0 electronic configuration. The empty d orbitals on the metal atoms constitute the low lying unoccupied states (LUMO) that mix with the highest occupied states (HOMO) of the ligand atoms (oxygen) through special chemical bonding effects (second order Jahn-Teller effect, SOJT). This mixing results, among others, in out-of-centre distortion(s) of the MO6 octahedra and this distortion is at the heart of several properties mentioned above. Among the transition metal oxide structures based on MO6 octahedra, three structures are noteworthy: the perovskite, the pyrochlore and the rutile. The AMO3 perovskite structure consists of a three-dimensional framework of corner sharing MO6 octahedra in which the A cation occupies the dodecahedral site surrounded by twelve oxide ions. The perovskite structure can accommodate a large variety of substitutions at both the A and the M sites as well as vacancies at the A/O sites, giving a large number of derivatives. Several variants of the perovskite structure are also known, for instance, the layered perovskites and ordered perovskites. Many nonperovskite structures are also known for the composition AMO3 : hexagonal YMnO3 is an alternative structure for AMO3 composition where manganese exists as MnO5 trigonal bipyramids. The A2M2O7 pyrochlore structure is also based on a corner-connected network of MO6 octahedra which interpenetrates an A2O network. The rutile (TiO2) is a well-known structure consisting of chains of edge-sharing MO6 octahedra, which are connected through corners to adjacent chains. A large number of oxide materials based on the above three structure types have been reported : for example, perovskite [Ba3ZnTa2O9 (microwave telecommunication ceramic), Pb3MgNb2O9 (relaxor ferroelectric), Bi4Ti3O12 (high temperature ferroelectric)], pyrochlore [Nd2Mo2O7 (metallic ferromagnet), AOs2O6 for A = K, Rb, Cs (superconductor)] and rutile [TiO2 (photocatalyst), CrO2 (metallic ferromagnet), VO2 (insulator-metal transition)]. Considering the current interest in oxide materials of these three structure types which continue to generate new variants and novel properties, we undertook the present research project to synthesize new derivatives of these structure types, and characterize their structures and relevant electronic properties. In doing so, we recognized that synthesis based on an understanding of the reactivity of the constituents and crystal chemistry of the expected products plays a crucial role in this effort. Accordingly, we tailored several new compositions of AMO3, A2M2O7 and MO2 stoichiometries and adopted appropriate methodologies for their synthesis. We have characterized the structures and properties of the solid products by means of state-of-the-art methods available to us. There are two main approaches to the synthesis of nonmolecular inorganic solids: conventional ceramic route and chimie douce / soft chemistry routes. In the ceramic route, solid reactants are heated at elevated temperatures for long durations with intermittent mixing/grinding until the reaction is complete. Chimie douce routes, on the other hand, utilize gentle reactions such as dehydration, decomposition, intercalation, ion exchange, and so on to synthesize the desired phases. The ceramic route generally provides access to the thermodynamically controlled product(s), while chimie douce routes allow access to metastable phases (kinetically controlled product(s)). Disadvantages notwithstanding, the ceramic route has been the mainstay of materials chemistry and several important materials continue to be discovered / synthesized by this route. The choice of the synthetic route based on an understanding of the crystal chemical preferences and the reactivities of the constituents involved is often crucial to achieve the desired final products. The present thesis is devoted to the synthesis and investigation of MO6 octahedra-based oxides belonging to the perovskite, pyrochlore and rutile structure types wherein we have explored alternate synthetic strategies (perovskite-based Ba3MM'2O9 telecommunication ceramics and a solution route for the synthesis of ruthenium-based pyrochlores) and probed structure-property relations of perovskite oxides (Ba3MM'M''O9 oxides for various M/M'/M'' atoms) as well as formation of new derivatives of layered Aurivillius phases. In addition, we have also synthesized new noncentrosymmetric oxides possessing the YMnO3 structure. Our investigation of rutile based oxides has resulted in the discovery of a new lead-free relaxor ferroelectric material, FeTiTaO6. Given that the lone pair PbII:6s2 plays a crucial role in the ferroelectric properties of Pb-based perovskite oxides, we have also investigated members of the Pb1-xLix/2Lax/2TiO3 system for their structure and dielectric response. The present thesis describes the results of these investigations in eight chapters. Chapter 1 provides a general introduction to oxides of the perovskite, pyrochlore and rutile structures. In Chapter 2, we describe a new one-pot metathesis strategy for the synthesis of dielectric ceramics Ba3MM'2O9 (M = Mg, Ni, Zn; M' = Nb, Ta). Rietveld refinement of X-ray diffraction data shows near-complete ordering of M-site ions in many cases. The dielectric properties of the products synthesized are found to be in reasonable agreement with reported data. The synthesis of ordered materials at lower temperatures (~1100 °C) than that employed in the conventional ceramic route (~1500 °C) is a significant result of this work. Chapter 3 presents a study of Ba3MIIMIVWO9 (MII = Ca, Zn; MIV = Ti, Zr) perovskite oxides for the purpose of synthesizing new dielectric ceramic materials and to gain understanding of the factors that stabilize 3C vs. 6H structures. In general, a 1:2-ordered 6H perovskite structure is stabilized at high temperatures (1300 °C) for all of the Ba3MIITiWO9 oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba2(Zn2/3Ti1/3)(W2/3Ti1/3)O6, is obtained at 1200 °C for Ba3ZnTiWO9. A metastable perovskite, Ba3CaZrWO9, that adopts the 1:1 3C structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d0-induced distortion of metaloxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the Ba3MIIMIVWO9 series. In Chapter 4, we describe the synthesis and investigation of the structure and dielectric properties of Ba3MIIITiMVO9 (MIII = Fe, Ga, Y, Lu; MV = Nb, Ta, Sb) perovskite oxides. The MV = Nb, Ta oxides adopt disordered/partially ordered 3C perovskite structures, where all the MIII/Ti/MV metal-oxygen octahedra are corner-connected. In contrast, the MV = Sb oxides show a distinct preference for the 6H structure, where SbV/TiIV metal-oxygen octahedra share a common face, forming (Sb,Ti)O9 dimers, that are corner-connected to the MIIIO6 octahedra. Investigation of dielectric properties of MIII = Y/Lu, MV = Nb/Ta oxides reveals a normal low loss dielectric behaviour with ε = 30 – 50 in the temperature range 50 – 350 °C. The MIII = Fe, MV = Nb/Ta members show a dielectric behaviour similar to relaxor ferroelectric materials. Chapter 5 deals with a study of isomorphous substitution of several metal atoms in two Aurivillius structures, Bi5TiNbWO15 and Bi4Ti3O12, in an effort to probe structure-property correlations. These investigations have led to the synthesis of new derivatives, Bi4LnTiMWO15 (Ln, = La, Pr; M = Nb, Ta), as well as Bi4PbNb2WO15 and Bi3LaPbNb2WO15, that largely retain the Aurivillius intergrowth structure of the parent oxide Bi5TiNbWO15, but characteristically tend toward a centrosymmetric / tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2TiIV Æ MV + FeIII in Bi4Ti3O12, yields new Aurivillius phases, Bi4Ti3-2xNbxFexO12 (x = 0.25, 0.50) and Bi4Ti3-2xTaxFexO12 (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi4Ti3O12. Chapter 6 describes the design and synthesis of a new series of noncentrosymmetric oxides, R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb, Lu) possessing the YMnO3 structure. Investigation of the Lu-Mn-Cu-V-O system revealed the existence of an isostructural solid solution series, Lu3Mn3-3xCu2xVxO9 for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn3+, Cu2+ and V5+ atoms that preserves the noncentrosymmetric RMnO3 structure. An exploratory investigation of the synthesis, structure and electronic properties of new ruthenium(IV) pyrochlore oxides and their manganese-substituted derivatives is presented in Chapter 7. The richness of the electronic properties of ruthenium-based metal oxides is affirmed by the results which revealed several novel electronic ground states : a metallic and Pauli paramagnetic state for BiPbRu2O6.5 that turns into a semiconducting ferromagnetic spin-glass state at 50 K for BiPbRuMnO6.5 ; a metallic state that likely shows a charge density wave (CDW) instability at 50-225 K for Bi1.50Zn0.50Ru2O6.75, that is suppressed by manganese substitution in Bi1.50Zn0.50Ru1.75Mn0.25O6.50, and a metallic ferromagnetic spin-glass-like state for Pb2Ru1.75Mn0.25O6.15. We describe the investigation of the structure and dielectric properties of rutile-based MTiTaO6 (M = Al, Cr, Fe) in Chapter 8. All the oxides possess disordered rutile structure. FeTiTaO6 shows a strong relaxor ferroelectric effect, while CrTiTaO6 shows a weaker relaxor ferroelectric behaviour. This work is significant for two reasons: the new material is lead-free and it is based on the rutile structure, unlike the conventional relaxors which are mostly derived from the perovskite structure. The work presented in the thesis is carried out by the candidate as a part of the Ph.D. training programme and most of it has been published in the literature. She hopes that the studies reported here will constitute a worthwhile contribution to materials chemistry in general.

Materials chemistry is essentially concerned with the design/synthesis of new solids endowed with functional properties that could be of relevance to today’s materials technology. Among the large variety of solid materials that attract attention, metal oxides continue to contribute significantly to current materials chemistry. A wide variety of oxide materials (based on rocksalt, spinel, corundum, perovskite, garnet, pyrochlore and other structures) and their properties have been investigated over the years. Most of these oxides are derived from the transition metals. Transition metal oxides with structures derived from metal-oxygen (MO6) octahedra, in particular, display an array of exotic properties with potential or proven technological application. While it is traditionally believed that the partially filled d shell (dn : 0 < n < 10) of the transition metal atoms plays a crucial role in deciding the electronic properties, the significance of d0 metal atoms for the properties (and structure) of transition metal oxides is not fully recognized. Magnetism (SrRuO3, Fe3O4), metallicity (ReO3, LaNiO3), colossal magnetoresistance (La1-xCaxMnO3) and superconductivity (La2xSrxCuO4, Sr2RuO4) are some of the properties that can be traced to the presence of partially filled d shell, while properties like ferroelectricity (BaTiO3), piezoelectricity (PbZr1-xTixO3) and nonlinear optical response (LiNbO3) could be traced to the presence of transition metals (TiIV, ZrIV, NbV) with d0 electronic configuration. The empty d orbitals on the metal atoms constitute the low lying unoccupied states (LUMO) that mix with the highest occupied states (HOMO) of the ligand atoms (oxygen) through special chemical bonding effects (second order Jahn-Teller effect, SOJT). This mixing results, among others, in out-of-centre distortion(s) of the MO6 octahedra and this distortion is at the heart of several properties mentioned above. Among the transition metal oxide structures based on MO6 octahedra, three structures are noteworthy: the perovskite, the pyrochlore and the rutile. The AMO3 perovskite structure consists of a three-dimensional framework of corner sharing MO6 octahedra in which the A cation occupies the dodecahedral site surrounded by twelve oxide ions. The perovskite structure can accommodate a large variety of substitutions at both the A and the M sites as well as vacancies at the A/O sites, giving a large number of derivatives. Several variants of the perovskite structure are also known, for instance, the layered perovskites and ordered perovskites. Many nonperovskite structures are also known for the composition AMO3 : hexagonal YMnO3 is an alternative structure for AMO3 composition where manganese exists as MnO5 trigonal bipyramids. The A2M2O7 pyrochlore structure is also based on a corner-connected network of MO6 octahedra which interpenetrates an A2O network. The rutile (TiO2) is a well-known structure consisting of chains of edge-sharing MO6 octahedra, which are connected through corners to adjacent chains. A large number of oxide materials based on the above three structure types have been reported : for example, perovskite [Ba3ZnTa2O9 (microwave telecommunication ceramic), Pb3MgNb2O9 (relaxor ferroelectric), Bi4Ti3O12 (high temperature ferroelectric)], pyrochlore [Nd2Mo2O7 (metallic ferromagnet), AOs2O6 for A = K, Rb, Cs (superconductor)] and rutile [TiO2 (photocatalyst), CrO2 (metallic ferromagnet), VO2 (insulator-metal transition)]. Considering the current interest in oxide materials of these three structure types which continue to generate new variants and novel properties, we undertook the present research project to synthesize new derivatives of these structure types, and characterize their structures and relevant electronic properties. In doing so, we recognized that synthesis based on an understanding of the reactivity of the constituents and crystal chemistry of the expected products plays a crucial role in this effort. Accordingly, we tailored several new compositions of AMO3, A2M2O7 and MO2 stoichiometries and adopted appropriate methodologies for their synthesis. We have characterized the structures and properties of the solid products by means of state-of-the-art methods available to us. There are two main approaches to the synthesis of nonmolecular inorganic solids: conventional ceramic route and chimie douce / soft chemistry routes. In the ceramic route, solid reactants are heated at elevated temperatures for long durations with intermittent mixing/grinding until the reaction is complete. Chimie douce routes, on the other hand, utilize gentle reactions such as dehydration, decomposition, intercalation, ion exchange, and so on to synthesize the desired phases. The ceramic route generally provides access to the thermodynamically controlled product(s), while chimie douce routes allow access to metastable phases (kinetically controlled product(s)). Disadvantages notwithstanding, the ceramic route has been the mainstay of materials chemistry and several important materials continue to be discovered / synthesized by this route. The choice of the synthetic route based on an understanding of the crystal chemical preferences and the reactivities of the constituents involved is often crucial to achieve the desired final products. The present thesis is devoted to the synthesis and investigation of MO6 octahedra-based oxides belonging to the perovskite, pyrochlore and rutile structure types wherein we have explored alternate synthetic strategies (perovskite-based Ba3MM'2O9 telecommunication ceramics and a solution route for the synthesis of ruthenium-based pyrochlores) and probed structure-property relations of perovskite oxides (Ba3MM'M''O9 oxides for various M/M'/M'' atoms) as well as formation of new derivatives of layered Aurivillius phases. In addition, we have also synthesized new noncentrosymmetric oxides possessing the YMnO3 structure. Our investigation of rutile based oxides has resulted in the discovery of a new lead-free relaxor ferroelectric material, FeTiTaO6. Given that the lone pair PbII:6s2 plays a crucial role in the ferroelectric properties of Pb-based perovskite oxides, we have also investigated members of the Pb1-xLix/2Lax/2TiO3 system for their structure and dielectric response. The present thesis describes the results of these investigations in eight chapters. Chapter 1 provides a general introduction to oxides of the perovskite, pyrochlore and rutile structures. In Chapter 2, we describe a new one-pot metathesis strategy for the synthesis of dielectric ceramics Ba3MM'2O9 (M = Mg, Ni, Zn; M' = Nb, Ta). Rietveld refinement of X-ray diffraction data shows near-complete ordering of M-site ions in many cases. The dielectric properties of the products synthesized are found to be in reasonable agreement with reported data. The synthesis of ordered materials at lower temperatures (~1100 °C) than that employed in the conventional ceramic route (~1500 °C) is a significant result of this work. Chapter 3 presents a study of Ba3MIIMIVWO9 (MII = Ca, Zn; MIV = Ti, Zr) perovskite oxides for the purpose of synthesizing new dielectric ceramic materials and to gain understanding of the factors that stabilize 3C vs. 6H structures. In general, a 1:2-ordered 6H perovskite structure is stabilized at high temperatures (1300 °C) for all of the Ba3MIITiWO9 oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba2(Zn2/3Ti1/3)(W2/3Ti1/3)O6, is obtained at 1200 °C for Ba3ZnTiWO9. A metastable perovskite, Ba3CaZrWO9, that adopts the 1:1 3C structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d0-induced distortion of metaloxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the Ba3MIIMIVWO9 series. In Chapter 4, we describe the synthesis and investigation of the structure and dielectric properties of Ba3MIIITiMVO9 (MIII = Fe, Ga, Y, Lu; MV = Nb, Ta, Sb) perovskite oxides. The MV = Nb, Ta oxides adopt disordered/partially ordered 3C perovskite structures, where all the MIII/Ti/MV metal-oxygen octahedra are corner-connected. In contrast, the MV = Sb oxides show a distinct preference for the 6H structure, where SbV/TiIV metal-oxygen octahedra share a common face, forming (Sb,Ti)O9 dimers, that are corner-connected to the MIIIO6 octahedra. Investigation of dielectric properties of MIII = Y/Lu, MV = Nb/Ta oxides reveals a normal low loss dielectric behaviour with ε = 30 – 50 in the temperature range 50 – 350 °C. The MIII = Fe, MV = Nb/Ta members show a dielectric behaviour similar to relaxor ferroelectric materials. Chapter 5 deals with a study of isomorphous substitution of several metal atoms in two Aurivillius structures, Bi5TiNbWO15 and Bi4Ti3O12, in an effort to probe structure-property correlations. These investigations have led to the synthesis of new derivatives, Bi4LnTiMWO15 (Ln, = La, Pr; M = Nb, Ta), as well as Bi4PbNb2WO15 and Bi3LaPbNb2WO15, that largely retain the Aurivillius intergrowth structure of the parent oxide Bi5TiNbWO15, but characteristically tend toward a centrosymmetric / tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2TiIV Æ MV + FeIII in Bi4Ti3O12, yields new Aurivillius phases, Bi4Ti3-2xNbxFexO12 (x = 0.25, 0.50) and Bi4Ti3-2xTaxFexO12 (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi4Ti3O12. Chapter 6 describes the design and synthesis of a new series of noncentrosymmetric oxides, R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb, Lu) possessing the YMnO3 structure. Investigation of the Lu-Mn-Cu-V-O system revealed the existence of an isostructural solid solution series, Lu3Mn3-3xCu2xVxO9 for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn3+, Cu2+ and V5+ atoms that preserves the noncentrosymmetric RMnO3 structure. An exploratory investigation of the synthesis, structure and electronic properties of new ruthenium(IV) pyrochlore oxides and their manganese-substituted derivatives is presented in Chapter 7. The richness of the electronic properties of ruthenium-based metal oxides is affirmed by the results which revealed several novel electronic ground states : a metallic and Pauli paramagnetic state for BiPbRu2O6.5 that turns into a semiconducting ferromagnetic spin-glass state at 50 K for BiPbRuMnO6.5 ; a metallic state that likely shows a charge density wave (CDW) instability at 50-225 K for Bi1.50Zn0.50Ru2O6.75, that is suppressed by manganese substitution in Bi1.50Zn0.50Ru1.75Mn0.25O6.50, and a metallic ferromagnetic spin-glass-like state for Pb2Ru1.75Mn0.25O6.15. We describe the investigation of the structure and dielectric properties of rutile-based MTiTaO6 (M = Al, Cr, Fe) in Chapter 8. All the oxides possess disordered rutile structure. FeTiTaO6 shows a strong relaxor ferroelectric effect, while CrTiTaO6 shows a weaker relaxor ferroelectric behaviour. This work is significant for two reasons: the new material is lead-free and it is based on the rutile structure, unlike the conventional relaxors which are mostly derived from the perovskite structure. The work presented in the thesis is carried out by the candidate as a part of the Ph.D. training programme and most of it has been published in the literature. She hopes that the studies reported here will constitute a worthwhile contribution to materials chemistry in general.

The thesis entitled “Towards the Design of Functional Materials: Evaluation based on Crystal Structure, Photocatalysis and Conductivity Measurements” consist of six chapters. A short introductory note outlines the basis of designing functional materials, different synthetic procedures, characterization techniques and properties such as photocatalysis and ionic conductivity. Chapter 1 describes the effect of Ti doping on photocatalytic activity in orthorhombic perovskite type LnVO3. All the compounds were synthesized by solid state method. Rietveld refinement with high resolution PXRD reveals that the substituent Ti occupies V site rather than Ln Site. Ti substituted compound showed higher photocatalytic activity than the unsubstituted compound and is comparable with that of commercial catalyst. These classes of compounds showed specific degradation towards chlorinated compounds. Chapter 2 discusses the solution combustion synthesis of γ(L)-Bi2MoO6 and its photocatalytic activity under solar radiation. The particle sizes were in the range 300–500 nm with a band gap of 2.51 eV. The degradation of wide variety of cationic and anionic dyes was investigated under solar radiation. Despite the low surface area (<1 m2/g), γ(L)-Bi2MoO6 showed higher photocatalytic activity under solar radiation due to its electronic and morphological properties. Chapter 3 presents a series of visible light photocatalyst M2Ce2O7, synthesized via solution combustion method and characterized by powder X-ray diffraction, solid-state UV-Visible diffuse reflectance spectra, SEM and TEM. The structure of Bi2Ce2O7 has been determined using laboratory as well as synchrotron PXRD. It crystallizes in a disordered F-type structure. The particle sizes are in the range 5–6 nm, band gaps lie within the range 1.7 to 3.2 eV. Bi2Ce2O7 shows high photocatalytic activity, comparable to the commercial Degussa P-25 TiO2 under solar radiation. Chapter 4 examines the effect of Bismuth substitution on crystal chemistry, photocatalysis and conductivity in Sr3V2O8, a variant of palmierite class. These compounds were synthesized by ceramic method and powder X-ray data reveals the limit of the Bi substitution in Sr3-xBi2x/3V2O8 is x=0.4. Single crystal study followed by careful difference Fourier analysis shows that Bi occupies a unique 18h position which is different than Sr1 and Sr2 position. The experimental band gap for Sr3V2O8 was calculated to be 3.45 eV and upon substitution band gap of the material decreases and reaches a value 3.15 eV for the composition x=0.4. Compound exhibits photocatalytic activity specifically towards anionic dyes. However, Bi Substitution leads to lower photocatalytic activity. Chapter 5 describes synthesis, structure, phase transition and ionic conductivity in scheelite type Li0.5Ce0.5MoO4. The compound was synthesized by ceramic method and single crystal study reveals that it crystallizes in the space group I41/a and exhibits conductivity of ~10-3 Ohm-1cm-1 at elevated temperature( 700 °C). It undergoes a first order phase transition around 510 °C. The nature of this transition has been evaluated by laboratory and synchrotron PXRD, DSC, dielectric spectroscopy and variable temperature Raman spectroscopy. The phase transition is shown to be characterized by an iso-structural phase transition which is first example in literature for temperature induced Cowley’s “Type Zero” phase transition. Chapter 6 discusses a new methodology for generating functional materials for fast ion conductors. Several varients of hydrated sodium cadmium bisulfate, Na2Cd2(SO4)3⋅3H2O, Na2Cd(SO4)2⋅2H2O and Na2Cd(SO4)2⋅4H2O have been synthesized and their thermal properties followed by phase transitions have been invesigated. Na2Cd2(SO4)3⋅3H2O (space group P3c). Na2Cd2(SO4)3⋅3H2O loses water completely when heated to 250 °C and transforms to a dehydrated phase (I⎯43d ) whose structure has been established using abinitio powder diffration techniques. Na2Cd(SO4)2⋅2H2O (P21/c) transforms to α−Na2Cd(SO4)2 (space group C2/c) on heating to 150 °C which is a known high ionic conductor. However, when α−Na2Cd(SO4)2 is heated to 570 °C followed by sudden quenching in liquid nitrogen, β−Na2Cd(SO4)2 (P21/c) is formed. β−Na2Cd(SO4)2 takes up water from the atmosphere and gets converted completely to the Kröhnkite type mineral. Further, β−Na2Cd(SO4)2 has a conductivity behavior comparable to α form up to 280 °C, the temperature required for the transformation of β to α form.

The thesis entitled “Towards the Design of Functional Materials: Evaluation based on Crystal Structure, Photocatalysis and Conductivity Measurements” consist of six chapters. A short introductory note outlines the basis of designing functional materials, different synthetic procedures, characterization techniques and properties such as photocatalysis and ionic conductivity. Chapter 1 describes the effect of Ti doping on photocatalytic activity in orthorhombic perovskite type LnVO3. All the compounds were synthesized by solid state method. Rietveld refinement with high resolution PXRD reveals that the substituent Ti occupies V site rather than Ln Site. Ti substituted compound showed higher photocatalytic activity than the unsubstituted compound and is comparable with that of commercial catalyst. These classes of compounds showed specific degradation towards chlorinated compounds. Chapter 2 discusses the solution combustion synthesis of γ(L)-Bi2MoO6 and its photocatalytic activity under solar radiation. The particle sizes were in the range 300–500 nm with a band gap of 2.51 eV. The degradation of wide variety of cationic and anionic dyes was investigated under solar radiation. Despite the low surface area (<1 m2/g), γ(L)-Bi2MoO6 showed higher photocatalytic activity under solar radiation due to its electronic and morphological properties. Chapter 3 presents a series of visible light photocatalyst M2Ce2O7, synthesized via solution combustion method and characterized by powder X-ray diffraction, solid-state UV-Visible diffuse reflectance spectra, SEM and TEM. The structure of Bi2Ce2O7 has been determined using laboratory as well as synchrotron PXRD. It crystallizes in a disordered F-type structure. The particle sizes are in the range 5–6 nm, band gaps lie within the range 1.7 to 3.2 eV. Bi2Ce2O7 shows high photocatalytic activity, comparable to the commercial Degussa P-25 TiO2 under solar radiation. Chapter 4 examines the effect of Bismuth substitution on crystal chemistry, photocatalysis and conductivity in Sr3V2O8, a variant of palmierite class. These compounds were synthesized by ceramic method and powder X-ray data reveals the limit of the Bi substitution in Sr3-xBi2x/3V2O8 is x=0.4. Single crystal study followed by careful difference Fourier analysis shows that Bi occupies a unique 18h position which is different than Sr1 and Sr2 position. The experimental band gap for Sr3V2O8 was calculated to be 3.45 eV and upon substitution band gap of the material decreases and reaches a value 3.15 eV for the composition x=0.4. Compound exhibits photocatalytic activity specifically towards anionic dyes. However, Bi Substitution leads to lower photocatalytic activity. Chapter 5 describes synthesis, structure, phase transition and ionic conductivity in scheelite type Li0.5Ce0.5MoO4. The compound was synthesized by ceramic method and single crystal study reveals that it crystallizes in the space group I41/a and exhibits conductivity of ~10-3 Ohm-1cm-1 at elevated temperature( 700 °C). It undergoes a first order phase transition around 510 °C. The nature of this transition has been evaluated by laboratory and synchrotron PXRD, DSC, dielectric spectroscopy and variable temperature Raman spectroscopy. The phase transition is shown to be characterized by an iso-structural phase transition which is first example in literature for temperature induced Cowley’s “Type Zero” phase transition. Chapter 6 discusses a new methodology for generating functional materials for fast ion conductors. Several varients of hydrated sodium cadmium bisulfate, Na2Cd2(SO4)3⋅3H2O, Na2Cd(SO4)2⋅2H2O and Na2Cd(SO4)2⋅4H2O have been synthesized and their thermal properties followed by phase transitions have been invesigated. Na2Cd2(SO4)3⋅3H2O (space group P3c). Na2Cd2(SO4)3⋅3H2O loses water completely when heated to 250 °C and transforms to a dehydrated phase (I⎯43d ) whose structure has been established using abinitio powder diffration techniques. Na2Cd(SO4)2⋅2H2O (P21/c) transforms to α−Na2Cd(SO4)2 (space group C2/c) on heating to 150 °C which is a known high ionic conductor. However, when α−Na2Cd(SO4)2 is heated to 570 °C followed by sudden quenching in liquid nitrogen, β−Na2Cd(SO4)2 (P21/c) is formed. β−Na2Cd(SO4)2 takes up water from the atmosphere and gets converted completely to the Kröhnkite type mineral. Further, β−Na2Cd(SO4)2 has a conductivity behavior comparable to α form up to 280 °C, the temperature required for the transformation of β to α form.

Abstract Ferromagnetism is a well-known phenomenon wherein solids exhibit spontaneous magnetization even in the absence of an external magnetic field. An important characteristic of ferromagnetic materials is the hysteresis loop found in the relationship between magnetization and magnetic field. The electrical analogue of ferromagnetic materials are ferroelectric materials1 which show a hysteresis loop in the relationship between polarization and electric field, and exhibit spontaneous polarization in the absence of an external electric field. These materials therefore possess permanent dipole moments, the dipoles arising from the absence of a centre of symmetry. A wide variety of compounds are known to exhibit ferroelectricity.7 These include oxides of perovskite structure (e.g. BaTi03), hydrogen bonded solids (e.g. Rochelle salt, KH2P04), tungsten bronze type structures, pyrochlores, simple salts (e.g. (NH4)2S04, NaN02, KN03), alums, organic compounds (e.g. thiourea, glycine sulphate) and binary compounds as simple as HCl, FeS, GeTe, and V3Si.

Spins often prefer to anti-align with their neighbors in antiferromagnetic correlation. Materials with triangle lattices exhibit energetic degeneracy among the possible rearrangements of anti-aligned spins, which is denoted geometric frustration that is associated with strongly depressed transitions to magnetic order. Honeycomb iridates and ruthenates, pyrochlore systems, and double-perovskite iridates all feature triangular lattices as primary building blocks of their structures. Another frustration mechanism evolves from the Kitaev’s exact solution of a spin-liquid model on a honeycomb lattice with strong spin-orbit interactions. The protracted search for a Kitaev spin liquid has recently focused on the honeycomb itidates Na₂IrO₃ and Li₂IrO₃. A newer kind of quantum liquid has been identified in the magnetic insulator Ba₄Ir₃O₁₀, where Ir₃O₁₂ trimers form an unfrustrated square lattice.

Abstract For the application as a thermal barrier coating (TBC) the partially stabilized zirconia approaches some limits of performance, which hardly can be overcome in the near future. To further enhance the efficiency of gas turbines, higher temperature and a longer lifetime of the coating are needed for the next generation of TBC´s. This paper presents the development of new materials and concepts for application as TBC. Materials such as compositions with pyrochlore structure or doped zirconias are compared with new concepts like nanolayer between top- and bondcoat, metal-glass composites and double layer structures. One concept is to use new compositions in a combination with zirconia, as a double, multi or graded layer coating. In this case the benefits of zirconia will be combined with the promising properties of the new top-coating. For the concept of metal glass composites the influences of different plasma spraying processes on the microstructure are described. The quality of these coating systems are evaluated by a burner rig test.

The main objective of this work was to develop fuel reforming technologies to produce a H2-rich synthesis gas to power a solid oxide fuel cell being developed by US DOE for applications like diesel auxiliary power units. In order to accomplish this objective the following efforts were required: 1) examination of the effect of oxygen-conducting supports on reforming catalyst performance, 2) demonstration of the long-term stability under reforming conditions of an oxide powder catalyst deposited onto an oxygen-conducting support, 3) fabrication of a catalyst system by depositing the active catalyst and oxygen-conducting material onto a monolithic support structure for scaled-up reforming tests, 4) demonstration of the scaled-up reforming tests using the monolithic reactor. A successful 1,000-hr diesel reforming test was completed on a powder pyrochlore catalyst developed by NETL deposited onto an oxygen-conducting support. This test demonstrated that the catalyst and support compositions developed have significant potential in a commercial reforming application for the production of synthesis gas. Transforming this powder catalyst into a commercially viable form was the next major step to the development of a usable product. An alumina monolith structure coated with both the oxygen-conducting support and the active pyrochlore phase was fabricated and its performance was validated by short term partial oxidation (POX) tests on pump diesel, and in an integrated reformer-fuel cell test for 100 hrs on a biodiesel fuel under oxidative steam reforming (OSR) conditions.

Abstract Yttrium oxide-stabilized zirconium dioxide is mainly used in thermal barrier coatings. However, the desired higher gas inlet temperatures have initiated the search for novel materials. Low thermal conductivity and high melting points are important criteria for the selection of these materials. This paper investigates a zirconate material with a pyrochlore structure and a high melting point. In addition, it investigates the sintering behavior of the coatings at elevated temperatures. Dilatometer tests are carried out at 1200 deg C for at least 70 hours. For samples which had been annealed for 24 hours at 1250 deg C, the change in the porosity distribution is determined by means of mercury porosimetry. The paper presents the first results of thermal cycling tests on a plasma-sprayed coating. These first results are very promising. Paper includes a German-language abstract.

Plasma-sprayed thermal barrier coating (TBC) systems are widely used in gas turbine blades to increase turbine entry temperature (TET) and better efficiency. Yttria stabilized zirconia (YSZ) has been the conventional thermal barrier coating material because of its low thermal conductivity, relative high thermal expansion coefficient and good corrosion resistance. However the YSZ coatings can hardly fulfill the harsh requirements in future for higher reliability and the lower thermal conductivity at higher temperatures. Among the interesting TBC candidates, materials with pyrochlore structure show promising thermo-physical properties for use at temperatures exceeding 1200 °C. Sm2Zr2O7 bulk material does not only have high temperature stability, sintering resistance but also lower thermal conductivity and higher thermal expansion coefficient. The sintering characteristics of ceramic thermal barrier coatings under high temperature conditions are complex phenomena. In this paper, samarium zirconate (Sm2Zr2O7, SZ) powder and coatings were prepared by solid state reaction and atmosphere plasma spraying process, respectively. The microstructure development of coatings derived from sintering after heat-treated at 1200–1500 °C for 50 h have been investigated. The microstructure was examined by scanning electron microscopy (SEM) and the grain growth was analyzed in this paper as well.

Abstract This study deals with the deposition of coating materials that can be difficult to process by plasma spraying, including lanthanum and gadolinium zirconate, two pyrochlores of interest for thermal barrier applications, and lanthanum strontium cobalt ferrite (LSCF), a perovskite of interest for gas separation membranes. In addition to conventional atmospheric plasma spraying (APS), the feedstock powders were applied by suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD). The spraying processes are described in detail along with the characteristics of the powders and coatings and the effects of various spray parameters on splat behavior and coating composition and structure.

Abstract Lanthanum zirconate (La2Zr2O7) was proposed as a promising material for thermal barrier coatings. At atmospheric plasma spraying (APS) of La2Zr2O7 a considerable amount of La2O3 can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the La2O3-ZrO2 system, in the composition range of pyrochlore structure, the stoichiometric La2Zr2O7 has the highest melting point and other compositions are eutectic. APS experiments were performed with a TriplexPro-200 plasma torch at different power levels to achieve different degrees of evaporation and thus stoichiometry. For comparison, some investigations on Gd2Zr2O7 were included which is less prone to evaporation and formation of non-stoichiometry. Particle temperature distributions were measured by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected at specific torch input powers indicating evaporation and solidification processes. Based on this, process parameters can be defined to provide stoichiometric coatings intended to show good thermal cycling performance.