Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality

In this article we provide some perspectives on a range of pyrochlore and defect fluorite type compounds with nominal A2B2O7, A2BO5, ABC2O7, and other stoichiometries. Typically, the phase transformations and stability fields in these systems are mapped as a function of the ionic radii of the A and...

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Autores principales:	Gregory R. Lumpkin, Robert D. Aughterson
Formato:	article
Lenguaje:	EN
Publicado:	Frontiers Media S.A. 2021
Materias:	defect fluorite modulated structures pyrochlore weberite zirconolite Chemistry QD1-999
Acceso en línea:	https://doaj.org/article/074bd5724c444f2eaff64730ff443aa9
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spelling	oai:doaj.org-article:074bd5724c444f2eaff64730ff443aa92021-11-12T05:04:46ZPerspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality2296-264610.3389/fchem.2021.778140https://doaj.org/article/074bd5724c444f2eaff64730ff443aa92021-11-01T00:00:00Zhttps://www.frontiersin.org/articles/10.3389/fchem.2021.778140/fullhttps://doaj.org/toc/2296-2646In this article we provide some perspectives on a range of pyrochlore and defect fluorite type compounds with nominal A2B2O7, A2BO5, ABC2O7, and other stoichiometries. Typically, the phase transformations and stability fields in these systems are mapped as a function of the ionic radii of the A and B-site cations, e.g., the A/B cation radius ratio (rA/rB). This provides a useful guide to compatible structures and compositions for the development of advanced materials. Pyrochlore commonly transforms to a defect fluorite structure at high temperature in many systems; however, it is not uncommon to observe defect fluorite as the initial metastable phase at low temperature. The patterns of order-disorder observed in these materials are primarily due to the energetics of layer stacking, the defect formation and migration energies of cations and anions, or modulations of the parent cubic structure in 3 + n dimensional space. The first lead to predominantly non-cubic derivatives of the parent defect fluorite structure (e.g., zirconolite polytypes), the second control the order-disorder processes, and the latter lead to a variety of subtle additional scattering features within the cubic parent structure. Although the energetics of cation disorder and anion-vacancy disorder have become more accessible via atomistic approaches (e.g., MD and DFT), we continue to find interesting physical-chemical problems in these materials. For example, although there are significant differences in composition (Tb/Zr ratio and O content) between Tb2Zr2O7 and Tb2ZrO5, both of which are defect fluorites, we note that the modulations found in these two compounds by electron scattering are virtually identical with regard to the direction and magnitude of displacement from the normal Bragg diffracted beams. This suggests that neither the A/B cation ratio nor the oxygen stoichiometry have a significant effect on the modulations. The general observations on the systems of compounds noted in this paper rest primarily in the context of industrial materials for nuclear waste disposal, potential applications in inert matrix fuel designs, and other important technological applications such as ionic conductivity, electrical conductivity, and magnetism. Scientific advances in these areas have been underpinned by recent advances in ion irradiation, synchrotron X-ray, neutron scattering, and modelling and simulation capabilities. Furthermore, there has been some renewed interest in natural samples, e.g., Th-U zirconolite and pyrochlore as analogues for potential host phases in nuclear waste forms. In particular, the natural pyrochlores have provided additional details with regard to radiation damage ingrowth, percolation transitions, and the relationships between accumulated dose and physical properties including hardness, elastic modulus. Specific details of the thermal annealing of these samples have also been elucidated in considerable detail.Gregory R. LumpkinRobert D. AughtersonFrontiers Media S.A.articledefect fluoritemodulated structurespyrochloreweberitezirconoliteChemistryQD1-999ENFrontiers in Chemistry, Vol 9 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	defect fluorite modulated structures pyrochlore weberite zirconolite Chemistry QD1-999
spellingShingle	defect fluorite modulated structures pyrochlore weberite zirconolite Chemistry QD1-999 Gregory R. Lumpkin Robert D. Aughterson Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality
description	In this article we provide some perspectives on a range of pyrochlore and defect fluorite type compounds with nominal A2B2O7, A2BO5, ABC2O7, and other stoichiometries. Typically, the phase transformations and stability fields in these systems are mapped as a function of the ionic radii of the A and B-site cations, e.g., the A/B cation radius ratio (rA/rB). This provides a useful guide to compatible structures and compositions for the development of advanced materials. Pyrochlore commonly transforms to a defect fluorite structure at high temperature in many systems; however, it is not uncommon to observe defect fluorite as the initial metastable phase at low temperature. The patterns of order-disorder observed in these materials are primarily due to the energetics of layer stacking, the defect formation and migration energies of cations and anions, or modulations of the parent cubic structure in 3 + n dimensional space. The first lead to predominantly non-cubic derivatives of the parent defect fluorite structure (e.g., zirconolite polytypes), the second control the order-disorder processes, and the latter lead to a variety of subtle additional scattering features within the cubic parent structure. Although the energetics of cation disorder and anion-vacancy disorder have become more accessible via atomistic approaches (e.g., MD and DFT), we continue to find interesting physical-chemical problems in these materials. For example, although there are significant differences in composition (Tb/Zr ratio and O content) between Tb2Zr2O7 and Tb2ZrO5, both of which are defect fluorites, we note that the modulations found in these two compounds by electron scattering are virtually identical with regard to the direction and magnitude of displacement from the normal Bragg diffracted beams. This suggests that neither the A/B cation ratio nor the oxygen stoichiometry have a significant effect on the modulations. The general observations on the systems of compounds noted in this paper rest primarily in the context of industrial materials for nuclear waste disposal, potential applications in inert matrix fuel designs, and other important technological applications such as ionic conductivity, electrical conductivity, and magnetism. Scientific advances in these areas have been underpinned by recent advances in ion irradiation, synchrotron X-ray, neutron scattering, and modelling and simulation capabilities. Furthermore, there has been some renewed interest in natural samples, e.g., Th-U zirconolite and pyrochlore as analogues for potential host phases in nuclear waste forms. In particular, the natural pyrochlores have provided additional details with regard to radiation damage ingrowth, percolation transitions, and the relationships between accumulated dose and physical properties including hardness, elastic modulus. Specific details of the thermal annealing of these samples have also been elucidated in considerable detail.
format	article
author	Gregory R. Lumpkin Robert D. Aughterson
author_facet	Gregory R. Lumpkin Robert D. Aughterson
author_sort	Gregory R. Lumpkin
title	Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality
title_short	Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality
title_full	Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality
title_fullStr	Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality
title_full_unstemmed	Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality
title_sort	perspectives on pyrochlores, defect fluorites, and related compounds: building blocks for chemical diversity and functionality
publisher	Frontiers Media S.A.
publishDate	2021
url	https://doaj.org/article/074bd5724c444f2eaff64730ff443aa9
work_keys_str_mv	AT gregoryrlumpkin perspectivesonpyrochloresdefectfluoritesandrelatedcompoundsbuildingblocksforchemicaldiversityandfunctionality AT robertdaughterson perspectivesonpyrochloresdefectfluoritesandrelatedcompoundsbuildingblocksforchemicaldiversityandfunctionality
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Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality

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