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

Defect pyrochlores are promising candidates for thermally-insulating materials for use in technological applications. Preparation of materials of general formula K_1−xCs_xTa_1−yNb_yWO₆ (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 θ_E = 87 K and θ_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_0.75Cs_0.25Ta_0.5Nb_0.5WO₆.