Rattling Vibrations and Phonon Density of States in Low Thermal Conductivity Defect Pyrochlores

We have used chemical substitution of non-framework (A) cations to reduce the thermal conductivity of defect pyrochlores, to produce materials for use as thermal insulation in thermal batteries. The weakly bound A cations, located within a cavity formed by metal-centered octahedra, act as Einstein oscillators, undergoing localized vibrations, termed rattling modes. These scatter heat-carrying phonons and reduce thermal conductivity. That of mixed-phases K1-xCsxTaWO6 is determined by the balance between two effects. The lighter, more weakly-bound K+ is expected to show increased rattling vibrations within the cavity, decreasing the thermal conductivity of potassium-rich phases but the increased mass-fluctuation scattering of phonons, resulting from substitution of K+ by Cs+, also reduces the thermal conductivity. The balance of these two effects results in a minimum thermal conductivity at x = 0.25. Calculations of the radial distribution function of the two ions indicate greater delocalization of K+, consistent with increased rattling. Fitting the temperature dependence of atomic displacement parameters from X-ray diffraction data leads to estimates of 144 K and 84 K for the Einstein temperatures of K+ and Cs+ respectively. We wish to measure the rattling modes of K+ and Cs+ by inelastic neutron scattering and investigate the impact of changes in the K/Cs ratio on the rattling modes and phonon density of states of the two end-member and three mixed phases in this series.