Disorder by design: long- and short-range pyrochlore ordering
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Date
2020-11-11
Journal Title
Journal ISSN
Volume Title
Publisher
Australian Institute of Nuclear Science and Engineering (AINSE)
Abstract
Carbon-neutral energy generation is being developed in order to combat climate change. Two technologies of current interest, which are related to renewable and nuclear energy respectively, are next-generation oxygenion conductors for fuel cells and materials suitable for long-term storage and disposal of radioactive nuclear wastes [1-2]. Pyrochlores of the structure A2B2O7 have found immense applications in each of the above areas. Ionic conductors for fuel cells require flexibility and movement in their anionic sublattice, whereas the storage of radioactive nuclear wastes needs a robust lattice from which ions cannot escape. This is a seemingly contradiction in requirements. It is believed that the oxygen vacancies present in the pyrochlore structure allow for the development of short-range disorder, whilst keeping the long-range order intact [3]. The pyrochlore structure can be viewed as a superstructure of the defect-fluorite structure. The defect-fluorite structure (A2BO5) consists of a random distribution of cations and oxygen vacancies. However, by choosing A and B with a particular ionic radii ratio, the ordered pyrochlore superstructure may form under ambient conditions. This ordering of oxygen vacancies may be analysed using neutron powder diffraction and used to reason the enhanced properties and applications of pyrochlores [4]. The current work aims to characterise oxygen-vacancy disorder in defect pyrochlores so to enable the rational design of defect pyrochlores that are optimised for specific applications. We have done this by looking at ‘stuffed’ pyrochlores of the form A2(B2−xAx)O7−x/2 where the smaller B-type cation, in this case Ti4+, is progressively replaced by a larger A-type cation (Tm3+). We wish to determine whether controlling the disorder in the cation sublattice will allow us to tailor-make stuffed pyrochlores targeting specific applications across ionic conductivity, magnetism, photocatalysis and the storage of long-term radioactive waste. Series of stuffed pyrochlores have been synthesised using conventional solid-state methods and their longrange average structures characterised by Rietveld refinement against combined neutron and synchrotron X-ray diffraction data. The local short-range order has been characterised by Raman spectroscopy and XANES. Other measurements have also been performed regarding their applications, demonstrating a vast improvement in their ionic conductivity at high temperatures. These results will be presented, along with a judgement as to whether inducing certain types of disorder within the pyrochlore structure can lead to them being purposely engineered for specific applications. © 2020 The Authors
Description
Keywords
Carbon, Energy, Radioactive wastes, Fuel cells, Oxygen, X-ray diffraction, Spectroscopy, ANSTO, Materials, Data, Pyrochlore
Citation
Mullens, B., Kennedy, B., Zhang, Z., & Saura-Múzquiz, M. (2020). Disorder by design: long- and short-range pyrochlore ordering. Paper presented to the ANBUG-AINSE Neutron Scattering Symposium, AANSS 2020, Virtual Meeting, 11th - 13th November 2020, (pp. 96). Retrieved from: https://events01.synchrotron.org.au/event/125/attachments/725/1149/AANSS_Abstract_Booklet_Complete_-_1_Page_Reduced.pdf