Ultralow thermal conductivity in defect pyrochlores: balancing mass fluctuation scattering and rattling modes

Ormerod, N., Powell, A. V., Grau-Crespo, R. ORCID: https://orcid.org/0000-0001-8845-1719, Gover, R. K.B. and Cox, C. J. (2024) Ultralow thermal conductivity in defect pyrochlores: balancing mass fluctuation scattering and rattling modes. Journal of Materials Chemistry A, 12 (34). pp. 22668-22678. ISSN 2050-7496 doi: 10.1039/D3TA06618E

Abstract/Summary

Defect pyrochlores are promising candidates for thermally-insulating materials for use in technological applications. Preparation of materials of general formula K1−xCsxTa1−yNbyWO6 (0 ≤ x ≤ 1; y = 0, 0.5) has enabled the impact on thermal conductivity of chemical substitution at both framework and non-framework sites to be investigated. Water is detected in the as-prepared potassium-containing materials (x < 1.0) below 200 °C, the amount of which correlates with the potassium content. Structural changes on dehydration have been followed by synchrotron powder X-ray diffraction, which reveals migration of the K+ cations towards the centre of metal–oxide cages as water is removed. Measurements of thermal diffusivity reveal that partial substitution of both non-framework A-cations and the B-type framework cations reduces the thermal conductivity of KTaWO6 by up to 33%. The magnitude of the thermal conductivity is determined by the competition between increased mass-fluctuation scattering and the decrease in the energy of the rattling mode, as potassium is progressively replaced by caesium. This is consistent with Einstein temperatures θE = 87 K and θE = 109 K, determined experimentally for the rattling vibrations of Cs+ and K+ respectively, and with our ab initio molecular dynamics simulations. The minimum thermal conductivity, κ = 0.46 W m−1 K−1, for the anhydrous materials, is observed at 300 °C in the partially-substituted phase, K0.75Cs0.25Ta0.5Nb0.5WO6.

Item Type	Article
URI	https://reading-clone.eprints-hosting.org/id/eprint/118684
Identification Number/DOI	10.1039/D3TA06618E
Refereed	Yes
Divisions	Life Sciences > School of Chemistry, Food and Pharmacy > Department of Chemistry Interdisciplinary centres and themes > Chemical Analysis Facility (CAF) > Optical Spectroscopy (CAF) Interdisciplinary centres and themes > Chemical Analysis Facility (CAF) > Thermal Analysis (CAF) Interdisciplinary centres and themes > Chemical Analysis Facility (CAF) > Xray (CAF)
Publisher	Royal Society of Chemistry (RSC)
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