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 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 degrees 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 theta(E) = 87 K and theta(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, kappa = 0.46 W m(-1 )K(-1), for the anhydrous materials, is observed at 300 degrees C in the partially-substituted phase, K0.75Cs0.25Ta0.5Nb0.5WO6.