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

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2024-08, Vol.12 (34), p.22668-22678
• Main Authors: Ormerod, Natasha, Powell, Anthony V, Grau-Crespo, Ricardo, Gover, Richard K. B, Cox, Christina J
• Format: Article
• Language: English
• Subjects: Cations; Cesium; Defects; Dehydration; Heat conductivity; Heat transfer; Insulation; Mass balance; Molecular dynamics; Potassium; Pyrochlores; Scattering; Substitutes; Thermal conductivity; Thermal diffusivity; Vibrations; X ray powder diffraction; X-ray diffraction
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/d3ta06618e

Defect pyrochlores are promising candidates for thermally-insulating materials for use in technological applications. Preparation of materials of general formula K 1− x Cs x Ta 1− y Nb y WO 6 (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 KTaWO 6 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 E = 87 K and E = 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, K 0.75 Cs 0.25 Ta 0.5 Nb 0.5 WO 6 . Progressive replacement of non-framework K + cations with Cs + cations in defect pyrochlores leads to reductions in thermal conductivity, the extent of which reflects a balance between the effects of mass fluctuation and rattling vibrations.