Статті в журналах: "PYROCHLORE STRUCTURED MATERIALS"

1

Bhati, Rekha, Dheerendra Singh Yadav, Preeti Varshney, Rajesh Chandra Gupta, and Ajay Singh Verma. "Semi-Empirical Predictions for Hardness of Rare Earth Pyrochlores; High-Permittivity Dielectrics and Thermal Barrier Coating Materials." East European Journal of Physics, no. 1 (March 2, 2023): 222–27. http://dx.doi.org/10.26565/2312-4334-2023-1-29.

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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

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2

Zhang, Wenjie, Jiao Yang та Ling Du. "Sol-gel Synthesis of a Novel χSm2Ti2O7/HZSM-5 Composite Photocatalyst for the Promoted Activity on RBR X-3B Degradation". Current Nanoscience 14, № 1 (22 грудня 2017): 17–25. http://dx.doi.org/10.2174/1573413713666170714153328.

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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.

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3

Michailovski, Alexej, Frank Krumeich, and Greta R. Patzke. "Solvothermal synthesis of hierarchically structured pyrochlore ammonium tungstate nanospheres." Materials Research Bulletin 39, no. 7-8 (June 2004): 887–99. http://dx.doi.org/10.1016/j.materresbull.2004.04.004.

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4

Shu, G. J., S. L. Hsu, M.-W. Chu, C. C. Lee, and F. C. Chou. "Site occupancy and magnetic properties of pyrochlore-structured AgOs2O6." Journal of Physics: Condensed Matter 24, no. 38 (September 3, 2012): 385701. http://dx.doi.org/10.1088/0953-8984/24/38/385701.

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5

Kong, Linggen, Inna Karatchevtseva, Mark G. Blackford, Nicholas Scales, and Gerry Triani. "Aqueous Chemical Synthesis of Ln2 Sn2 O7 Pyrochlore-Structured Ceramics." Journal of the American Ceramic Society 96, no. 9 (June 3, 2013): 2994–3000. http://dx.doi.org/10.1111/jace.12409.

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6

Saruhan, B., P. Francois, K. Fritscher, and U. Schulz. "EB-PVD processing of pyrochlore-structured La2Zr2O7-based TBCs." Surface and Coatings Technology 182, no. 2-3 (April 2004): 175–83. http://dx.doi.org/10.1016/j.surfcoat.2003.08.068.

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7

Ribis, Joël, Isabelle Mouton, Cédric Baumier, Aurélie Gentils, Marie Loyer-Prost, Laurence Lunéville, and David Siméone. "Nano-Structured Materials under Irradiation: Oxide Dispersion-Strengthened Steels." Nanomaterials 11, no. 10 (October 1, 2021): 2590. http://dx.doi.org/10.3390/nano11102590.

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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.

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8

Mori, Toshiyuki, John Drennan, Ding Rong Ou, and Fei Ye. "Design of Micro-Structure at Atom Level in Dy Doped CeO₂ Solid Electrolytes for Fuel Cell Applications." Materials Science Forum 539-543 (March 2007): 1437–42. http://dx.doi.org/10.4028/www.scientific.net/msf.539-543.1437.

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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.

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9

Rushton, M. J. D., Robin W. Grimes, C. R. Stanek, and Scott Owens. "Predicted pyrochlore to fluorite disorder temperature for A2Zr2O7 compositions." Journal of Materials Research 19, no. 6 (June 2004): 1603–4. http://dx.doi.org/10.1557/jmr.2004.0231.

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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.

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10

Fujihara, Shinobu, and Kazuaki Tokumo. "Multiband Orange-Red Luminescence of Eu3+Ions Based on the Pyrochlore-Structured Host Crystal." Chemistry of Materials 17, no. 22 (November 2005): 5587–93. http://dx.doi.org/10.1021/cm0513785.

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11

Newman, R., R. D. Aughterson, and G. R. Lumpkin. "Synthesis and Structure of Novel A2BO5 Compounds Containing A = Y, Yb, Gd, Sm, and La and B = Zr, Ti, and Sn." MRS Advances 3, no. 20 (2018): 1117–22. http://dx.doi.org/10.1557/adv.2018.210.

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With the aim of creating novel ceramics for applications in nuclear materials with high radiation tolerance, multiple samples with A-B-O stoichiometries ranging from 215 to 227 were synthesized and characterized by a combination of SEM, XRD, and TEM methods. Single-phase defect-fluorite-type compounds with A = Sm or Yb and B = Ti, Zr, and/or Sn are reported; whereas, pyrochlore structured compounds and lanthanide sesquioxide phases were found as major phases in numerous samples. A series of Y-b-Sn-O samples was successfully prepared as nearly single phase or single phase materials. These are essentially defect fluorites with extra weak peaks, most likely due to X-ray scattering from oxygen or oxygen and metal cations. We describe some interesting TEM data and describe selected area diffraction patterns with complex modulations.

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12

Zhou, Haidong, and Christopher Wiebe. "High-Pressure Routes to New Pyrochlores and Novel Magnetism." Inorganics 7, no. 4 (April 2, 2019): 49. http://dx.doi.org/10.3390/inorganics7040049.

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The pyrochlore structure (A2B2O7) has been an object of consistent study by materials scientists largely due to the stability of the cubic lattice with respect to a wide variety of chemical species on the A or B sites. The criterion for stability under ambient conditions is controlled by the ratio of these cations, which is empirically 1.36 < RA/RB < 1.71. However, under applied pressure synthesis conditions, the pyrochlore lattice is stable up to RA/RB ∼ 2.30, opening up possibilities for new compounds. In this review, we will highlight recent work in exploring new rare-earth pyrochlores such as the germanates RE2Ge2O7 and platinates RE2Pt2O7. We highlight recent discoveries made in these pyrochlores such as highly correlated spin ice behavior, spin liquid ground states, and exotic magnetic ordering.

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13

Talanov, Mikhail V., and Valeriy M. Talanov. "Formation of breathing pyrochlore lattices: structural, thermodynamic and crystal chemical aspects." CrystEngComm 22, no. 7 (2020): 1176–87. http://dx.doi.org/10.1039/c9ce01635j.

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14

Khanvilkar, M. B., A. K. Nikumbh, S. M. Patange, R. A. Pawar, N. J. Karale, D. V. Nighot, P. A. Nagwade, M. D. Sangale, and G. S. Gugale. "Structural, electrical and magnetic properties of substituted pyrochlore oxide nanoparticles synthesized by the co-precipitation method." Physics and Chemistry of Solid State 22, no. 2 (June 16, 2021): 353–71. http://dx.doi.org/10.15330/pcss.22.2.353-371.

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Five substituted pyrochlore nanooxides such as Nd1.9Ho0.1Zr1.8Ce0.2O7, La1.95Ce0.05Zr0.29Ce1.71O7, Y1.79Pr0.21Ru1.99Pr0.01O7, Dy1.9Yb0.1Mn1.93Cu0.07O7 and Dy1.99Sr0.01Sn2O7 were synthesized by coprecipitation method. These precursors were monitored by thermal studies (TGA-DTA). The prepared nanosized substituted pyrochlore oxides were characterized by EDS, XRD, SEM, TEM, d. c. electrical conductivity, Thermoelectric power, Hall effect measurement, dielectric properties and magnetization measurements. XRD confirmed the formation of a single phase crystalline substituted pyrochlores with a cubic nature of nanoparticles. All substituted compounds were adopted a stable pyrochlore structure with rA3+/rB4+ = 1.395 except La1.95Ce0.05Zr0.29Ce1.71O7 compound, which has rA3+/rB4+ = 1.175 indicate disorder pyrochlore structure (i.e. fluorite structure). The temperature dependence of d. c. electrical conductivity for all substituted pyrochlores exhibits two distinct slopes with a break. This discontinuity can be attributed to extrinsic to intrinsic semiconducting properties. The thermoelectric power and Hall effect measurements for all compounds were confirmed the p-type semiconductivity except Y1.79Pr0.21Ru1.99Pr0.01O7 compound and which showed n-type semiconductivity. The dielectric constant (ε’) and dielectric loss (tan δ) i. e dissipation factor decreases with an increase in frequencies and reaching constant at particular frequencies. The applied field dependence of magnetization curve at room temperature (300 K) for Nd1.9Ho0.1Zr1.8Ce0.2O7, Y1.79Pr0.21Ru1.99Pr0.01O7 and Dy1.9Yb0.1Mn1.93Cu0.07O7, showed hysteresis loop with a small kink around the origin and which can be attributed to small but definite ferromagnetic ordering along with significant paramagnetic and superparamagnetic components. The magnetization at 2K showed a clear hysteresis loop for Dy1.9Yb0.1Mn1.93Cu0.07O7 and Dy1.99Sr0.01Sn2O7 pyrochlores are soft (weak) ferromagnets.

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15

Srinivasan, S. A., S. P. Kumaresh babu, L. John Berchmans, and Mehana Usmaniya. "Molten salt synthesis of nano structured pyrochlore lanthanum zirconate: a potential material for high temperature applications." Materials Research Express 6, no. 10 (August 7, 2019): 104001. http://dx.doi.org/10.1088/2053-1591/ab3683.

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16

Cheng, Fuhao, Ziqian Meng, Chufei Cheng, Jiadong Hou, Yufeng Liu, Bei Ren, Haiyan Hu, Feng Gao, Yang Miao, and Xiaomin Wang. "Fluorite-pyrochlore structured high-entropy oxides: Tuning the ratio of B-site cations for resistance to CMAS corrosion." Corrosion Science 218 (July 2023): 111199. http://dx.doi.org/10.1016/j.corsci.2023.111199.

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17

Wang, Yuhao, Chong Jing, Zhao-Ying Ding, Yun-Zhuo Zhang, Tao Wei, Jia-Hu Ouyang, Zhan-Guo Liu, Yu-Jin Wang, and Ya-Ming Wang. "The Structure, Property, and Ion Irradiation Effects of Pyrochlores: A Comprehensive Review." Crystals 13, no. 1 (January 13, 2023): 143. http://dx.doi.org/10.3390/cryst13010143.

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Since the beginning of the use of nuclear energy, humans have been faced with the problem of radionuclide disposal. At present, a large amount of waste is stored in pools or dry tanks at reactor sites. With the development of the nuclear power generation industry worldwide, the high storage cost (including building, maintaining, and operating storage pools) is overwhelming and serious, and urgent radionuclide disposal problems have become increasingly difficult. Safe and economical strategies are urgently needed for long-term storage and disposal of nuclear waste, which has become among the core issues in the utilization of nuclear energy. Pyrochlore ceramics are able to immobilize a variety of radionuclides and have excellent irradiation stability, so they have received extensive attention as hosts of radionuclides waste. This review summarizes the structure, composition, synthesis process, properties, and irradiation stability of pyrochlore ceramics, focusing on the ion irradiation effect of pyrochlore. In general, the cation radii ratio rA/rB is a key parameter related to various properties of pyrochlores. Zirconate pyrochlore is more easily transformed from pyrochlore to defective fluorite, and leads to better irradiation resistance.

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18

Henderson, Stuart J., Olga Shebanova, Andrew L. Hector, Paul F. McMillan, and Mark T. Weller. "Structural Variations in Pyrochlore-Structured Bi2Hf2O7, Bi2Ti2O7and Bi2Hf2-xTixO7Solid Solutions as a Function of Composition and Temperature by Neutron and X-ray Diffraction and Raman Spectroscopy." Chemistry of Materials 19, no. 7 (April 2007): 1712–22. http://dx.doi.org/10.1021/cm062864a.

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19

Mayer, Sergio, Horacio Falcón, María Fernández-Díaz, and José Alonso. "The Crystal Structure of Defect KBB’O6 Pyrochlores (B,B’: Nb,W,Sb,Te) Revisited from Neutron Diffraction Data." Crystals 8, no. 10 (September 20, 2018): 368. http://dx.doi.org/10.3390/cryst8100368.

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Three defect pyrochlores KNbWO6·xH2O, KNbTeO6 and KSbWO6 were synthesized by solid state reaction at 750 °C, from stoichiometric mixtures of K2C2O4, Sb2O3, Nb2O5, WO3 and 20% excess TeO2. A neutron powder diffraction (NPD) data analysis allowed unveiling some structural features. They are all defined in the cubic F d 3 ¯ m space group symmetry, with α = 10.5068(1) Å, 10.2466(1) Å and 10.2377(1) Å, respectively. Difference Fourier synthesis for KNbWO6·xH2O clearly showed the presence of crystallization water, with extra O’ oxygen and H+ atoms that were located from NPD data. These O’ oxygen atoms are placed at 32e Wyckoff sites, conforming a K2O’ sublattice interpenetrated with the covalent framework constituted by (Nb,W)O6 octahedra. The H+ ions coordinate the O’ atoms at partially occupied 96g Wyckoff sites while K+ ions shift also along 32e sites, but closer to the 16c special site (0,0,0). By contrast, extra H2O molecules are absent in the other two pyrochlores: in KNbTeO6 and KSbWO6 K+ ions are shifted along 32e (x,x,x) sites further away from the origin than for the previous material, and the higher covalency of the octahedral network determines more compact structures, with shorter B–O distances and narrower B–O–B angles in the proposed AB2O6 defect pyrochlore structure.

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20

Chyshkala, Volodymyr Oleksiyovych, Serhii Volodymyrovych Lytovchenko, Edwin Spartakovych Gevorkyan, Volodymyr Pavlovych Nerubatskyi, Bogdan Оlexandrovych Mazilin та Oksana Mykolaivna Morozova. "Мastering and modernization of physico-chemical processes of synthesis of oxide compounds with structure of pyrochlorine". Collected scientific works of Ukrainian State University of Railway Transport, № 197 (22 грудня 2021): 82–98. http://dx.doi.org/10.18664/1994-7852.197.2021.248097.

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Modern scientific and technological development of society, further intensification of production together with the provision of proper safety of human life and preservation of the environment necessitate the search for new solutions in the creation of new materials and technologies. The creation of effective materials for the latest and future technologies and technicaldevices is based on new scientific data on the definition and analysis of specific mechanisms of physicochemical processes that implement the desired structural and phase state of solids with the desired set of properties. In recent decades, the most effective way to control the properties of solid materials is the use of nanotechnology and nanomaterials, which have recently been increasingly used in almost all areas of new technologies. The article investigates synthesis processes, structural characteristics and structural-phase processes in multicomponent metal-ceramic oxide materials, physicochemical mechanisms ofsynthesi s of multielement oxide compounds Y2Zr2O7 with pyrochlor structure during consolidation and sintering of yttrium and zirconium oxides, structure formation -phase characteristics of materials with different chemical composition. The structural-phase evolution in the synthesis of new substances and the consolidation of compounds of the Y2O3 – ZrO2 system have been studied. Samples of oxide heat with the proportion of pyrochlorine phase Y2Zr2O7 up to 41 % were obtained. It is established that the kinetics of increasing the proportion of pyrochlorine phase in the samples indicates a desirable increase in the activity of the chemical reaction, which can be achieved by increasing the synthesis temperature to the temperatures of eutectic formation or increasing the reaction surface of powders.

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Playford, Helen, Ravi SINGH, Lieh Jeng Chang, Kripasindhu Sardar, Alex Hannon, Matt Tucker, Martin Lees, Geetha Balakrishnan, and Richard Walton. "Local Structure of Iridate Pyrochlores from Hydrothermal Synthesis." Acta Crystallographica Section A Foundations and Advances 70, a1 (August 5, 2014): C869. http://dx.doi.org/10.1107/s205327331409130x.

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Iridate pyrochlores of general formula M2Ir2O7have potential applications in catalysis [1]. They also often exhibit unusual magnetic and electronic properties caused by spin-orbit coupling and geometric frustration [2]. A detailed understanding of structure is necessary to enable these properties to be understood and exploited. Because of the propensity of the pyrochlore structure to accommodate structural disorder, we have chosen to utilise the technique of total scattering to examine the structure of M2Ir2O7(M = Bi, Nd). The sensitivity of our measurements to all the constituent elements is maximised by the combination of both neutron and X-ray total scattering. We find no evidence for magnetic ordering in our samples of Nd2Ir2O7, in contrast to literature reports [3]. By comparing the local structure of our samples with that of one reported to exhibit magnetic ordering, we explore the possibility of a structural origin for the differences in magnetic behaviour. We have found that synthesis method can directly influence the structure of these iridate pyrochlores. Local structural analysis provides evidences of A-site cation deficiency and partial oxidation of Ir(IV) to Ir(V) in samples produced by hydrothermal techniques. Irreversible changes to the lattice parameter upon heating these samples at 400 – 9000C further support the inference that the cation content is somewhat variable. We report the results of reverse Monte Carlo (RMC) refinements using the program RMCProfile, which is capable of simultaneously fitting to X-ray and neutron data, and therefore provides structural models of the greatest possible accuracy. We also report the results of in situ X-ray total scattering measurements which provide local-scale insight into the interesting thermal behaviour and apparent flexible cation content of these materials.

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Boldrin, D., and A. S. Wills. "Anomalous Hall Effect in Geometrically Frustrated Magnets." Advances in Condensed Matter Physics 2012 (2012): 1–12. http://dx.doi.org/10.1155/2012/615295.

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Geometrically frustrated conducting magnets display extraordinarily large anomalous Hall effects (AHEs) that could be used to realise materials required for the emerging field of spintronics. While the intrinsic Berry phase developed in collinear ferromagnets is well explained through the effects of spin-orbit interactions within the Karplus and Luttinger model, its origins in frustrated magnets are not. The direct space mechanism based on spin chirality that was originally applied to the pyrochlore Nd2Mo2O7appears unsatisfactory. Recently, an orbital description based on the Aharonov-Bohm effect has been proposed and applied to both the ferromagnetic pyrochlores Nd2Mo2O7and Pr2Ir2O7; the first of which features long-ranged magnetic order while the latter is a chiral spin liquid. Two further examples of geometrically frustrated conducting magnets are presented in this paper—the kagome-like Fe3Sn2and the triangular PdCrO2. These possess very different electronic structures to the 3-dimensional heavy-metal pyrochlores and provide new opportunities to explore the different origins of the AHE. This paper summarises the experimental findings in these materials in an attempt to unite the conflicting theoretical arguments.

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23

Stebbins, Jonathan F., Ryan J. McCarty, and Aaron C. Palke. "Solid-state NMR and short-range order in crystalline oxides and silicates: a new tool in paramagnetic resonances." Acta Crystallographica Section C Structural Chemistry 73, no. 3 (February 6, 2017): 128–36. http://dx.doi.org/10.1107/s2053229616015606.

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Most applications of high-resolution NMR to questions of short-range order/disorder in inorganic materials have been made in systems where ions with unpaired electron spins are of negligible concentration, with structural information extracted primarily from chemical shifts, quadrupolar coupling parameters, and nuclear dipolar couplings. In some cases, however, the often-large additional resonance shifts caused by interactions between unpaired electron and nuclear spins can provide unique new structural information in materials with contents of paramagnetic cations ranging from hundreds of ppm to several per cent and even higher. In this brief review we focus on recent work on silicate, phosphate, and oxide materials with relatively low concentrations of paramagnetic ions, where spectral resolution can remain high enough to distinguish interactions between NMR-observed nuclides and one or more magnetic neighbors in different bonding configurations in the first, second, and even farther cation shells. We illustrate the types of information available, some of the limitations of this approach, and the great prospects for future experimental and theoretical work in this field. We give examples for the effects of paramagnetic transition metal, lanthanide, and actinide cation substitutions in simple oxides, pyrochlore, zircon, monazite, olivine, garnet, pyrochlores, and olivine structures.

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24

Lian, Jie, Rodney C. Ewing, L. M. Wang, and K. B. Helean. "Ion-beam irradiation of Gd2Sn2O7 and Gd2Hf2O7 pyrochlore: Bond-type effect." Journal of Materials Research 19, no. 5 (May 2004): 1575–80. http://dx.doi.org/10.1557/jmr.2004.0178.

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Ceramics with III-IV pyrochlore compositions, A3+2B4+2O7 (A = Y and rare earth elements; B = Ti, Zr, Sn, or Hf), show a wide range of responses to ion-beam irradiation. To evaluate the role of the B-site cations on the radiation stability ofthe pyrochlore structure-type, Gd2Sn2O7 and Gd2Hf2O7 have been irradiated by1 MeV Kr+. The results are discussed in terms of the ionic size and type ofbonding of Sn4+ and Hf4+ and compared to previous results for titanate andzirconate pyrochlores. Gd2Sn2O7 is sensitive to ion beam–induced amorphizationwith a critical amorphization dose of approximately 3.4 displacements per atom(dpa) (2.62 × 1015 ions/cm2) at room temperature and a critical amorphization temperature of approximately 350 K. Gd2Hf2O7 does not become amorphous at adose of approximately 4.54 displacement per [lattice] atom (3.13 × 1015 ions/cm2) at room temperature, but instead is transformed to a disordered fluorite structure upon ion-beam irradiation. Although the radius ratio of the A- to B-site cations provides a general indication of the type of radiation response of different pyrochlore compositions, the results for Gd2Sn2O7 emphasize the importance of bond type, particularly the covalency of the 〈Sn–O〉 bond in determining the radiation response.

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25

Muravyov, Vitaliy A., Maria G. Krzhizhanovskaya, Boris A. Makeev, Andrey N. Nizovtsev, Sergey V. Nekipelov, Viktor N. Sivkov, Danil V. Sivkov, and Nadezhda A. Zhuk. "Features of the Preparation of Ni-Doped Bismuth Tantalate Pyrochlore." Crystals 13, no. 3 (March 9, 2023): 474. http://dx.doi.org/10.3390/cryst13030474.

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The possibility of high-temperature solid-phase synthesis of Ni-doped bismuth tantalate pyrochlore, not only from the oxide compounds BiTaO4 and NiO but also using nickel chloride NiCl2 as a precursor, was shown for the first time. The concentration range of the formation of nickel pyrochlore Bi2NixTa2O9−δ (0.85 ≤ x ≤ 1.0) at an equal molar ratio of bismuth(III)/tantalum(V) ions was determined. This indicates that the pyrochlore structure may be stable at 20–33% vacant bismuth sublattice. The influence of non-stoichiometric composition relative to bismuth and tantalum, by the example of Bi2±xNiTa2O9−δ and Bi2NiTa2−xO9−δ (x ≤ 0.5) compositions, on the phase composition of ceramics was studied. X-ray powder diffraction phase analysis of Bi2Ni1+yTa1.75−yO9−δ samples made it possible to determine the solubility limit of nickel ions in the tantalum sublattice. It was shown that the molar ratio of metal atoms (Ta, Ni) in the B2O6 (B-Ni(II), Ta(V)) octahedral sublattice of mixed pyrochlores is also limited.

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26

Kennedy, Brendan, Peter Blanchard, Emily Reynolds, and Zhaoming Zhang. "Transformation from pyrochlore to fluorite by diffraction and X-ray spectroscopy." Acta Crystallographica Section A Foundations and Advances 70, a1 (August 5, 2014): C234. http://dx.doi.org/10.1107/s2053273314097654.

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We have studied the long-range average and local structures in a number of zirconium containing materials of the type A2B2O7 ( A = Ln or Y; B = Zr, Hf or Sn) using synchrotron X-ray and neutron powder diffraction and X-ray absorption spectroscopy. Studies of the system Gd2-xTbxZr2O7 include neutron diffraction data, obtained at λ ≍ 0.497 Å to minimise absorption, not only provide evidence for independent ordering of the anion and cation sublattices, but also suggest that the disorder transition across the pyrochlore-defect fluorite boundary of Ln2Zr2O7 is rather gradual. In general we observe that while the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect-fluorite at specific compositions corresponding to a critical ionic radius ratio of the A and B cations (rA/rB) x ~ 1.0-1.2, X-ray absorption near-edge structure (XANES) results reveal a gradual structural evolution across the compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defect-fluorite region. Where possible the experimental results were supplemented by ab initio atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures were predicted, and the trends agree well with the experimental XANES results. X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the defect fluorites.

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27

Rapenne, L., C. Jiménez, T. Caroff, C. Million, S. Morlens, P. Bayle-Guillemaud, and F. Weiss. "High-resolution transmission electron microscopy observations of La2Zr2O7 thin layers on LaAlO3 obtained by chemical methods." Journal of Materials Research 24, no. 4 (April 2009): 1480–91. http://dx.doi.org/10.1557/jmr.2009.0162.

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Анотація:

La2Zr2O7 (LZO) films have been grown by metalorganic decomposition (MOD) to be used as buffer layers for coated conductors. LZO can crystallize into two similar structures: fluorite or pyrochlore. Coated conductor application focuses on pyrochlore structure because it is a good barrier against oxygen diffusion. Classical x-ray diffraction is not able to separate the contribution of these two structures. Transmission electron microscopy and high-resolution transmission electron microscopy were used to determine the local distribution of these two phases in epitaxial LZO layers grown on LaAlO3. A characteristic feature of LZO thin films deposited by MOD is the formation of nanovoids in an almost single-crystal structure of LZO pyrochlore phase. For comparison, LZO layers deposited by metalorganic chemical vapor deposition were also studied. In this last case, the film is compact without voids and the structure corresponds to pyrochlore phase. Thus, the formation of nanovoids is a characteristic feature of MOD grown films.

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28

Wang, Hong, Desheng Zhang, Xiaoli Wang, and Xi Yao. "Effect of La2O3 substitutions on structure and dielectric properties of Bi2O3–ZnO–Nb2O5-based pyrochlore ceramics." Journal of Materials Research 14, no. 2 (February 1999): 546–48. http://dx.doi.org/10.1557/jmr.1999.0078.

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Анотація:

The effect of La2O3 substitutions on structure and dielectric properties of Bi2O3 –ZnO–Nb2O5-based ceramics was investigated. Bi1.5-xLaxZn0.5(Zn0.5Nb1.5)O7 samples were prepared by conventional ceramic processing technology. The crystal structure of the Bi1.5Zn0.5(Zn0.5Nb1.5)O7 sample was characterized as a pure cubic pyrochlore. With a lower amount of La2O3 substitution, the crystal structures were still cubic pyrochlore. Superlattice x-ray diffraction line was identified for some compositions. With the increasing amount of La2O3 substitution, the crystal structure gradually transformed from pure cubic pyrochlore to LaNbO4 phase. The dielectric properties regularly changed with the structure change. The structure-properties relations of the ceramics are discussed.

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29

Azeem, M. Mustafa, and Qingyu Wang. "Atomic Insights into the Structural Properties and Displacement Cascades in Ytterbium Titanate Pyrochlore (Yb2Ti2O7) and High-Entropy Pyrochlores." Journal of Composites Science 7, no. 10 (October 5, 2023): 413. http://dx.doi.org/10.3390/jcs7100413.

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Pyrochlore oxides (A2B2O7) are potential nuclear waste substrate materials due to their superior radiation resistance properties. We performed molecular dynamics simulations to study the structural properties and displacement cascades in ytterbium titanate pyrochlore (Yb2Ti2O7) and high-entropy alloys (HEPy), e.g., YbYTiZrO7, YbGdTiZrO7, and Yb0.5Y0.5Eu0.5Gd0.5TiZrO7. We computed lattice constants (LC) (ao) and threshold displacement energy (Ed). Furthermore, the calculation for ao and ionic radius (rionic) were performed by substituting a combination of cations at the A and B sites of the original pyrochlore structure. Our simulation results have demonstrated that the lattice constant is proportional to the ionic radius, i.e., ao α rionic. Moreover, the effect of displacement cascades of recoils of energies 1 keV, 2 keV, 5 keV, and 10 keV in different crystallographic directions ([100], [110], [111]) was studied. The number of defects is found to be proportional to the energy of incident primary knock-on atoms (PKA). Additionally, the Ed of pyrochlore exhibits anisotropy. We also observed that HEPy has a larger Ed as compared with Yb2Ti2O7. This establishes that Yb2Ti2O7 has characteristics of lower radiation damage resistance than HEPy. Our displacement cascade simulation result proposes that HEPy alloys have more tendency for trapping defects. This work will provide atomic insights into developing substrate materials for nuclear waste applications.

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30

Douma, Mohamed, Hossain El, Raquel Trujillano, and Vicente Rives. "Structural determination of new solid solutions [Y2-xMx][Sn2-xMx]o7-3x/2 (M = Mg or Zn) by Rietveld method." Processing and Application of Ceramics 4, no. 4 (2010): 237–43. http://dx.doi.org/10.2298/pac1004237d.

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Анотація:

New [Y2-xMx][Sn2-xMx]O7-3x/2 (0 ?x? 0.30 for M = Mg and 0 ?x? 0.36 for M = Zn) solid solutions with the pyrochlore structure were synthesized via high-temperature solid-state reaction method. Powder X-ray diffraction (PXRD) patterns and Fourier transform infrared (FT-IR) spectra showed that these materials are new non-stoichiometric solid solutions with the pyrochlore type structure. The structural parameters for the solids obtained were successfully determined by Rietveld refinement based on the analysis of the PXRD diagrams. Lattice parameter (a) of these solid solutions decreases when x increases in both series. All samples obtained have the pyrochlore structure Fd-3m, no. 227 (origin at center -3m) with M2+ (M = Mg2+ or Zn2+) cations in Y3+ and Sn4+ sites, thus creating vacancies in the anionic sublattice.

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31

Zhuk, N. A., M. G. Krzhizhanovskaya, A. V. Koroleva, V. G. Semenov, A. A. Selyutin, A. M. Lebedev, S. V. Nekipelov, et al. "Fe,Mg-Codoped Bismuth Tantalate Pyrochlores: Crystal Structure, Thermal Stability, Optical and Electrical Properties, XPS, NEXAFS, ESR, and 57Fe Mössbauer Spectroscopy Study." Inorganics 11, no. 1 (December 24, 2022): 8. http://dx.doi.org/10.3390/inorganics11010008.

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Анотація:

The effect of Fe and Mg-codoping on the crystal structure, optical and dielectric properties of bismuth tantalate-based pyrochlores has been studied. Samples of Bi2MgxFe1−xTa2O9.5−Δ (x ≤ 0.7) are characterized by a porous dendrite-like microstructure. Fe,Mg-codoped bismuth tantalate pyrochlores are thermally stable up to a temperature of 1140 °C (x = 1). The Bi2Mg0.5Fe0.5Ta2O9.5−Δ thermal expansion coefficient increases uniformly and weakly from 3.6 to 9.3 × 10−6 °C−1 (30–1050 °C). The unit cell parameter of solid solutions increases uniformly from 10.5009(1) Å (x = 0.3) up to 10.5225(7) Å (x = 0.7). The structural parameters of disordered pyrochlore are determined by the Rietveld method (sp. gr. Fd3¯m:2 (227), Z = 8). According to near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy data, ions in solid solutions are in the charge states Bi (+3), Mg (+2), Fe (+3), Ta (+5-δ). The Mössbauer spectrum is represented by a symmetric doublet with parameters IS = 0.365 ± 0.0020 mm/s, QS = 0.604 ± 0.034 mm/s, related to Fe3+ ions in regular axial octahedral positions. The samples exhibit the properties of dielectrics. The permittivity and the tangent of dielectric losses at 20 °C increases with the growth of iron content in the samples in the range of 28.5–30.5 and 0.001 (1 MHz). The width of the band gap of the obtained materials for direct allowed electronic transitions is in the range of 2.16(5)–2.41(5) eV. The studied samples satisfy the condition of efficient conversion of solar energy into an electrical one and are promising as catalysts and light-absorbing elements for solar panels.

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32

Bespalko, Yuliya, Nikita Eremeev, Ekaterina Sadovskaya, Tamara Krieger, Olga Bulavchenko, Evgenii Suprun, Mikhail Mikhailenko, Mikhail Korobeynikov, and Vladislav Sadykov. "Synthesis and Oxygen Mobility of Bismuth Cerates and Titanates with Pyrochlore Structure." Membranes 13, no. 6 (June 13, 2023): 598. http://dx.doi.org/10.3390/membranes13060598.

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Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi1.6Y0.4Ti2O7 were synthesized by the citrate route; Bi2Ce2O7 and Bi1.6Y0.4Ce2O7—by the Pechini method. The structural characteristics of materials after conventional sintering at 500–1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi1.6Y0.4Ti2O7, occurs after high-temperature calcination. Complex oxides Bi2Ce2O7 and Bi1.6Y0.4Ce2O7 have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO2-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes.

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33

Tabira, Yasunori, Ray Withers, John Thompson та Siegbert Schmid. "Structured Diffuse Scattering as an Indicator of Inherent Cristobalite-like Displacive Flexibility in the Rare Earth Zirconate Pyrochlore LaδZr1−δO2−δ/2, 0.49<δ<0.51". Journal of Solid State Chemistry 142, № 2 (лютий 1999): 393–99. http://dx.doi.org/10.1006/jssc.1998.8054.

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34

Lang, M., F. X. Zhang, R. C. Ewing, Jie Lian, Christina Trautmann, and Zhongwu Wang. "Structural modifications of Gd2Zr2-xTixO7 pyrochlore induced by swift heavy ions: Disordering and amorphization." Journal of Materials Research 24, no. 4 (April 2009): 1322–34. http://dx.doi.org/10.1557/jmr.2009.0151.

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Анотація:

The isometric, pyrochlore structure type, A2B2O7, exhibits a wide variety of properties that find application in a large number of different technologies, from electrolytes in solid oxide fuel cells to actinide-bearing compositions that can be used as nuclear waste forms or inert matrix nuclear fuels. Swift xenon ions (1.43 GeV) have been used to systematically modify different compositions in the Gd2Zr2-xTixO7 binary at the nanoscale by radiation-induced phase transitions that include the crystalline-to-amorphous and order-disorder structural transformations. Synchrotron x-ray diffraction, Raman spectroscopy, and transmission electron microscopy provide a complete and consistent description of structural changes induced by the swift heavy ions and demonstrate that the response of pyrochlore depends strongly on chemical composition. The high and dense electronic energy deposition primarily results in amorphization of Ti-rich pyrochlore; whereas the formation of the fully disordered, defect-fluorite structure is the dominant process for Zr-rich pyrochlore.

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35

Chen, Yan, Nina Orlovskaya, Nicholas Miller, Harry Abernathy, Daniel Haynes, David Tucker, and Randall Gemmen. "La_1.97Sr_0.03Zr₂O₇ Pyrochlore Powder for Advanced Energy Application." Advances in Science and Technology 62 (October 2010): 56–60. http://dx.doi.org/10.4028/www.scientific.net/ast.62.56.

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Materials with A2B2O7 (pyrochlore) structure have received a significant attention for their applications as new protonic conductors and materials used in electronic devices. One of the unique synthesis routes for La2Zr2O7 (pyrochlore) powders is the glycine-nitrate combustion method, which shows superior properties of the synthesized powder by using glycine as a complexing agent. The Sr doped La2Zr2O7 powders in pure pyrochlore structure were produced using this approach. Selected characteristics of the synthesized powders, such as crystal structure, lattice parameters, crystallite size, the vibrational properties, the morphology of the particles, along with the specific surface area and particle size have been investigated. The dependence of some properties on annealing temperatures of the powders has been studied.

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36

Goh, Gregory K. L., Sossina M. Haile, Carlos G. Levi, and Fred F. Lange. "Hydrothermal synthesis of perovskite and pyrochlore powders of potassium tantalate." Journal of Materials Research 17, no. 12 (December 2002): 3168–76. http://dx.doi.org/10.1557/jmr.2002.0458.

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Potassium tantalate powders were hydrothermally synthesized at 100 to 200 °C in 4 to 15 M aqueous KOH solutions. A defect pyrochlore, Kta2O5(OH). nH2O (n ≈ 1.4), was obtained at 4 M KOH, but at 7–12 M KOH, this pyrochlore was gradually replaced by a defect perovskite as the stable phase. At 15 M KOH, there was no intermediate pyrochlore, only a defect perovskite, 0.85Ta0.92O2.43(OH)0.57 0.15H2O. Synthesis at higher KOH concentrations led to greater incorporation of protons in the perovskite structures. The potassium vacancies required for charge compensation of incorporated protons could accommodate water molecules in the perovskite structure.

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37

Pokhrel, Madhab, Nicholas Dimakis, Chamath Dannangoda, Santosh K. Gupta, Karen S. Martirosyan, and Yuanbing Mao. "Structural Evolution and Magnetic Properties of Gd2Hf2O7 Nanocrystals: Computational and Experimental Investigations." Molecules 25, no. 20 (October 21, 2020): 4847. http://dx.doi.org/10.3390/molecules25204847.

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Structural evolution in functional materials is a physicochemical phenomenon, which is important from a fundamental study point of view and for its applications in magnetism, catalysis, and nuclear waste immobilization. In this study, we used x-ray diffraction and Raman spectroscopy to examine the Gd2Hf2O7 (GHO) pyrochlore, and we showed that it underwent a thermally induced crystalline phase evolution. Superconducting quantum interference device measurements were carried out on both the weakly ordered pyrochlore and the fully ordered phases. These measurements suggest a weak magnetism for both pyrochlore phases. Spin density calculations showed that the Gd3+ ion has a major contribution to the fully ordered pyrochlore magnetic behavior and its cation antisite. The origin of the Gd magnetism is due to the concomitant shift of its spin-up 4f orbital states above the Fermi energy and its spin-down states below the Fermi energy. This picture is in contrast to the familiar Stoner model used in magnetism. The ordered pyrochlore GHO is antiferromagnetic, whereas its antisite is ferromagnetic. The localization of the Gd-4f orbitals is also indicative of weak magnetism. Chemical bonding was analyzed via overlap population calculations: These analyses indicate that Hf-Gd and Gd-O covalent interactions are destabilizing, and thus, the stabilities of these bonds are due to ionic interactions. Our combined experimental and computational analyses on the technologically important pyrochlore materials provide a basic understanding of their structure, bonding properties, and magnetic behaviors.

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38

Matsunami, M., T. Hashizume, and A. Saiki. "Ion-Exchange Reaction Of A-Site In A2Ta2O6 Pyrochlore Crystal Structure." Archives of Metallurgy and Materials 60, no. 2 (June 1, 2015): 941–44. http://dx.doi.org/10.1515/amm-2015-0234.

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Abstract Na+ or K+ ion rechargeable battery is started to garner attention recently in Place of Li+ ion cell. It is important that A+ site ion can move in and out the positive-electrode materials. When K2Ta2O6 powder had a pyrochlore structure was only dipped into NaOH aqueous solution at room temperature, Na2Ta2O6 was obtained. K2Ta2O6 was fabricated from a tantalum sheet by a hydrothermal synthesize with KOH aqueous solution. When Na2Ta2O6 was dipped into KOH aqueous solution, K2Ta2O6 was obtained again. If KTaO3 had a perovskite structure was dipped, Ion-exchange was not observed by XRD. Because a lattice constant of pyrochlore structure of K-Ta-O system is bigger than perovskite, K+ or Na+ ion could shinny through and exchange between Ta5+ and O2− ion site in a pyrochlore structure. K+ or Na+ ion exchange of A2Ta2O6 pyrochlore had reversibility. Therefore, A2Ta2O6 had a pyrochlore structure can be expected such as Na+ ion rechargeable battery element.

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39

Fan, Long, Yi Xie, and Xiao Yan Shu. "Fabrication of Pyrochlore Gd₂Zr₂O₇by High Temperature Solid State Reaction." Advanced Materials Research 1061-1062 (December 2014): 87–90. http://dx.doi.org/10.4028/www.scientific.net/amr.1061-1062.87.

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In order to investigate the fabrication of pyrochlore Gd2Zr2O7, Gd2O3 and ZrO2 were used as raw materials in the process. Pyrochlore Gd2Zr2O7 were fabricated by high temperature solid state reaction at 1100 – 1600 °C under atmospheric pressure for 72 h. XRD and SEM studies were exploited to characterize the crystal structure and microstructure of the synthetic samples. The results revealed that Gd2Zr2O7 with a single pyrochlore structure was fabricated successfully at 1500 °C. The microstructure of the sample was uniform and dense, and the grain size was in the range of 1 - 3 μm.

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40

Rothensteiner, Matthäus, Alexander Bonk, Ulrich F. Vogt, Hermann Emerich, and Jeroen A. van Bokhoven. "Structural changes in equimolar ceria–hafnia materials under solar thermochemical looping conditions: cation ordering, formation and stability of the pyrochlore structure." RSC Advances 7, no. 85 (2017): 53797–809. http://dx.doi.org/10.1039/c7ra09261j.

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Equimolar ceria–hafnia oxides form a pyrochlore Ce₂Hf₂O₇ under the reducing conditions of a solar thermochemical looping reactor for the two-step dissociation of water or carbon dioxide.

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41

Wang, Hong, and Xi Yao. "Structure and dielectric properties of pyrochlore–fluorite biphase ceramics in the Bi2O3–ZnO–Nb2O5 system." Journal of Materials Research 16, no. 1 (January 2001): 83–87. http://dx.doi.org/10.1557/jmr.2001.0016.

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Анотація:

The structure and dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics with pyrochlore–fluorite biphase structure were investigated. Mixed-sintered ceramics were prepared by two precalcined constituents in the system of x[Bi1.5Zn0.5(Zn0.5Nb1.5)O7]−(1 − x)Bi3/4Nb1/4O7/4 (0.05 ≤ x ≤ 0.35), where Bi1.5Zn0.5(Zn0.5Nb1.5)O7 is a cubic pyrochlore (α) and Bi3/4Nb1/4O7/4 is a defect cubic fluorite (F). The phase composition of the mixed-sintered ceramics were characterized as a biphasic structure with a distorted pyrochlore (β) and a fluorite (F) coexisting. The phase ratio of β pyrochlore and F fluorite is related to the chemical composition x. The dielectric properties of the ceramics gradually change with their structure.

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42

Sheetal, A. Elghandour, R. Klingeler, and C. S. Yadav. "Field induced spin freezing and low temperature heat capacity of disordered pyrochlore oxide Ho₂Zr₂O₇." Journal of Physics: Condensed Matter 34, no. 24 (April 7, 2022): 245801. http://dx.doi.org/10.1088/1361-648x/ac5fd8.

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Abstract Spin ice materials are the model systems that have a zero-point entropy as T → 0 K, owing to the frozen disordered states. Here, we chemically alter the well-known spin ice Ho2Ti2O7 by replacing Ti sites with isovalent but larger Zr ion. Unlike the Ho2Ti2O7 which is a pyrochlore material, Ho2Zr2O7 crystallizes in disordered pyrochlore structure. We have performed detailed structural, ac magnetic susceptibility and heat capacity studies on Ho2Zr2O7 to investigate the interplay of structural disorder and frustrated interactions. The zero-field ground state exhibits large magnetic susceptibility and remains dynamic down to 300 mK without showing Pauling’s residual entropy. The dynamic state is suppressed continuously with the magnetic field and freezing transition evolves (∼10 K) at a field of ∼10 kOe. These results suggest that the alteration of chemical order and local strain in Ho2Ti2O7 prevents the development of spin ice state and provides a new material to study the geometrical frustration based on the structure.

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43

Moroz, Y., M. Lozynskyy, A. Lopanov, K. Chebyshev, and V. Burkhovetsky. "THE RESEARCH OF THE THERMOLYSIS PRODUCTS OF CESIUM TUNGSTOPHOSPHATES." Bulletin of Belgorod State Technological University named after. V. G. Shukhov 5, no. 12 (January 8, 2021): 126–35. http://dx.doi.org/10.34031/2071-7318-2020-5-12-126-135.

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The article deals with the synthesis, study of thermal decomposition and identification of the thermolysis products of cesium tungstophosphates that are promising compounds in the field of materials science, catalysis and other fields of science and technology. Compounds with the Keggin anion structure are synthesized from aqueous solutions: Cs3[PW12O40] ∙ 9H2O; Cs5Na2[PW11O39(H2O)] ∙ 5H2O and Cs5[PW11O39Ni0,5Cu0,5(H2O)] ∙ 4H2O. The processes of their thermal decomposition are investigated and some regularities of their thermolysis are established. Thermolysis products are identified: Cs3PW12O40, phases with the structure of pyrochlore and hexagonal tungsten bronze of the composition Cs10/13Na4/13P2/13W22/13O6 and Cs10/13P2/13Ni1/13Cu1/13W22/13O6. The unit cell parameters of the phase with the pyrochlore structure are determined. Research results confirm that phosphorus, nickel and copper ions are included in the structure of pyrochlore and hexagonal tungsten bronze. Phases similar to this chemical composition are not previously known in the literature. The studied tungstophosphates and their thermolysis products are promising compounds for obtaining heterogeneous catalysts for the oxidation of organic compounds and selective sorbents. The research results can be useful for predicting the thermal properties and phase composition of thermolysis products of similar polyoxometallates in order to obtain new compounds with the structure of pyrochlore and hexagonal tungsten bronze, as well as composite materials based on them.

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44

Teng, Zhen, Yongqiang Tan, and Haibin Zhang. "High-Entropy Pyrochlore A2B2O7 with Both Heavy and Light Rare-Earth Elements at the A Site." Materials 15, no. 1 (December 24, 2021): 129. http://dx.doi.org/10.3390/ma15010129.

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Анотація:

A novel class of high-entropy pyrochlore ceramics (HEPCs) with multiple heavy and light rare-earth elements at the A site were successfully synthesized via solid-state reaction. Both the XRD patterns and Raman spectroscopy demonstrated the single pyrochlore structure feature of seven kinds of HEPCs. Electron microscopic images revealed the typical morphology and the homogeneous distribution of all rare-earth elements. It can be concluded that the significance of configuration entropy in the HEPC system has promoted the tervalent lanthanide nuclides to form a single pyrochlore structure. This work is expected to provide guidance for the further design of high-entropy pyrochlore/fluorite ceramics.

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45

Gupta, Santosh K., Brindaban Modak, J. Prakash, N. S. Rawat, P. Modak, and K. Sudarshan. "Modulating the optical and electrical properties of oxygen vacancy-enriched La₂Ce₂O₇:Sm³⁺ pyrochlore: role of dopant local structure and concentration." New Journal of Chemistry 46, no. 9 (2022): 4353–62. http://dx.doi.org/10.1039/d1nj04854f.

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46

Zorzi, Janete E., Cintia L. G. de Amorim, Raquel Milani, Carlos A. Figueroa, J. A. H. da Jornada, and Claudio A. Perottoni. "Ball milling-induced pyrochlore-to-tungsten bronze phase transition in RbNbWO6." Journal of Materials Research 24, no. 6 (June 2009): 2035–41. http://dx.doi.org/10.1557/jmr.2009.0247.

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A set of Bragg peaks consistent with a hexagonal Bravais lattice was observed in the x-ray powder diffraction pattern of cubic pyrochlore rubidium tungstoniobate (RbNbWO6) subjected to high-energy ball milling. The calculated lattice parameters for this hexagonal phase are similar to those of compounds with tungsten bronze structure. In fact, the powder pattern of the hexagonal phase could be refined with a structural model based on the tungsten bronze structure. The hexagonal phase produced by high-energy ball milling of RbNbWO6 transforms back to the pyrochlore structure upon heating to 773 K in air. A similar phase was obtained by ball milling the mixture RbNbWO6 + WO3, but, in this case, the stoichiometric hexagonal tungsten bronze compound thus obtained remained stable up to 1273 K.

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47

Zhao, Hanqi, Jianbin Fan, and Quansheng Wang. "Phase Structure and Phase Stability Studies of La-Y Co-doped HfO₂ Materials and Coatings." E3S Web of Conferences 406 (2023): 01028. http://dx.doi.org/10.1051/e3sconf/202340601028.

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In this study, The 30 mol% La-Y co-doped HfO2 was prepared by solid-phase synthesis and the effect of the doping content of La element on the phase structure and high temperature phase stability was investigated by XRD. The results show that LaYSH is pyrochlore p-phase + fluorite c-phase when the doping content of La element is 6 and 9 mol%, and a single fluorite c-phase when the doping content is 0-3 mol%. After sintering at 1600 °C for 50 h, 0, 1 and 2 LaYSH remained as a single fluorite c-phase, 3 LaYSH produced 2.54 wt% pyrochlore p-phase, and the pyrochlore p-phase content in 6 and 9 LaYSH decreased to varying degrees. The prepared 0, 1 and 2 LaYSH coatings all had a single c-phase and remained unchanged in the single c-phase when sintered at 1600°C for 50 hours, demonstrating their good high temperature phase stability.

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48

Isupov, V. A. "Physical problems of capacitor materials with the pyrochlore structure." Technical Physics 42, no. 10 (October 1997): 1155–57. http://dx.doi.org/10.1134/1.1258792.

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49

Gorshkov, Nikolay, Egor Baldin, Dmitry Stolbov, Viktor Rassulov, Olga Karyagina, and Anna Shlyakhtina. "Oxygen–Ion Conductivity, Dielectric Properties and Spectroscopic Characterization of “Stuffed” Tm2(Ti2−xTmx)O7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) Pyrochlores." Ceramics 6, no. 2 (April 10, 2023): 948–67. http://dx.doi.org/10.3390/ceramics6020056.

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Tm2(Ti2−xTmx)O7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) solid electrolytes have been investigated as potential electrolyte materials for solid oxygen fuel cells (SOFCs), operating in the medium temperature range (600–700 °C). The design of new oxygen-conducting materials is of importance for their possible utilization in the solid oxide fuel cells. The oxygen–ion conductivity of the Tm2(Ti2−xTmx)O7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) “stuffed” pyrochlores ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) in dry and wet air. The synthesis of precursors via co-precipitation and the precipitate decomposition temperature have been shown to be of key importance for obtaining dense and highly conductive ceramics. At ~770 °C, the highest total conductivity, ~3.16 × 10−3 S/cm, is offered by Tm2Ti2O7. The conductivity of the fluorite-like solid solution Tm2(Ti2−xTmx)O7−x/2 (x = 0.74) is an order of magnitude lower. However, for the first time a proton contribution of ~5 × 10−5 S/cm at 600 °C has been found in Tm2(Ti2−xTmx)O7−x/2 (x = 0.74) fluorite. Until now, compositions with proton conductivity were not known for the intermediate and heavy rare earth titanates Ln2(Ti2−xLnx)O7−x/2 (Ln = Ho − Lu) systems. The use of X-ray diffraction (structural analysis with Rietveld refinement), optical spectroscopy and dielectric permittivity data allowed us to follow structural disordering in the solid solution series with increasing thulium oxide content. High and low cooling rates have been shown to have different effects on the properties of the ceramics. Slow cooling initiates’ growth of fluorite nanodomains in a pyrochlore matrix. The fabrication of such nanostructured dense composites is a promising direction in the synthesis of highly conductive solid electrolytes for SOFCs. We assume that high-temperature firing of nanophase precursors helps to obtain lightly doped “stuffed” pyrochlores, which also provide the high oxygen–ion conductivity.

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50

Bhuiyan, M. S., M. Paranthaman, S. Sathyamurthy, A. Goyal, and K. Salama. "Growth of rare-earth niobate-based pyrochlores on textured Ni–W substrates with ionic radii dependency." Journal of Materials Research 20, no. 4 (April 1, 2005): 904–9. http://dx.doi.org/10.1557/jmr.2005.0110.

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Epitaxial films of rare-earth (RE) niobates, RE3NbO7 with pyrochlore structures, were grown on biaxially textured nickel–3 at.% tungsten (Ni–W) substrates using a chemical solution deposition process. A precursor solution of 0.3–0.50 M concentration of total cations was spin coated on to short samples of Ni–W substrates, and the films were crystallized at 1050–1100 °C in a gas mixture of Ar–4% H2 for 15 min. Detailed studies revealed that RE-niobates with ionic radius ratio RRE/RNb (R = ionic radius) from 1.23 to 1.40 (i.e., Sm, Eu, Gd, Ho, Y, and Yb) grow epitaxially with the pyrochlore structure. X-ray studies showed that the films of pyrochlore RE niobate films were highly textured with cube-on-cube epitaxy. Scanning electron and atomic force microscopy investigations of RE3NbO7 films revealed a fairly dense and smooth microstructure without cracks and porosity. The rare-earth niobate layers may be potentially used as buffer layers for YBa2Cu3O7−δ coated conductors.

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