Strong Anharmonicity at the Origin of Anomalous Thermal Conductivity in Double Perovskite Cs2NaYbCl6

Anomalous thermal transport of Cs2NaYbCl6 double-halide perovskite above room temperature is reported and rationalized. Calculations of phonon dispersion relations and scattering rates up to the fourth order in lattice anharmonicity have been conducted to determine their effective dependence on temperature. These findings show that specific phonon group velocities and lifetimes increase if the temperature is raised above 500 K. This, in combination with anharmonicity, provides the microscopic mechanism responsible for the increase in lattice thermal conductivity at high temperatures, contrary to the predictions of phonon transport theories based on solely cubic anharmonicity. The model accurately and quantitatively reproduces the experimental thermal conductivity data as a function of temperature.

Automated summary

This study reports anomalous thermal transport of Cs2NaYbCl6 double-halide perovskite above room temperature and provides a rational explanation. Calculations of phonon dispersion relations and scattering rates show that specific phonon group velocities and lifetimes increase at high temperatures. The combination of anharmonicity and other factors is responsible for the increase in lattice thermal conductivity, contrary to previous predictions. The proposed model accurately reproduces the experimental data.