Issue 34, 2024

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

Abstract

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.

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

Supplementary files

Article information

Article type
Paper
Submitted
30 Oct 2023
Accepted
22 Jul 2024
First published
29 Jul 2024
This article is Open Access
Creative Commons BY-NC license

J. Mater. Chem. A, 2024,12, 22668-22678

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

N. Ormerod, A. V. Powell, R. Grau-Crespo, R. K. B. Gover and C. J. Cox, J. Mater. Chem. A, 2024, 12, 22668 DOI: 10.1039/D3TA06618E

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