Ultralow lattice thermal conductivity of chalcogenide perovskite CaZrSe3 contributes to high thermoelectric figure of merit

An emerging chalcogenide perovskite, CaZrSe3, holds promise for energy conversion applications given its notable optical and electrical properties. However, knowledge of its thermal properties is extremely important, e.g. for potential thermoelectric applications, and has not been previously reported in detail. In this work, we examine and explain the lattice thermal transport mechanisms in CaZrSe3 using density functional theory and Boltzmann transport calculations. We find the mean relaxation time to be extremely short corroborating an enhanced phonon-phonon scattering that annihilates phonon modes, and lowers thermal conductivity. In addition, strong anharmonicity in the perovskite crystal represented by the Gruneisen parameter predictions, and low phonon number density for the acoustic modes, results in the lattice thermal conductivity to be limited to 1.17 Wm(-1) K-1. The average phonon mean free path in the bulk CaZrSe3 sample (N ->infinity) is 138.1 nm and nanostructuring CaZrSe3 sample to similar to 10 nm diminishes the thermal conductivity to 0.23Wm(-1) K-1. We also find that p-type doping yields higher predictions of thermoelectric figure of merit than n-type doping, and values of ZT similar to 0.95-1 are found for hole concentrations in the range 10(16)-10(17) cm(-3) and temperature between 600 and 700 K.