Ultra-low lattice thermal conductivity in Cs2BiAgX6 (X=Cl, Br): Potential thermoelectric materials

TL;DR

This study investigates the electronic and thermoelectric properties of Cs2BiAgX6 halides, revealing their ultra-low lattice thermal conductivity and promising thermoelectric performance, making them potential candidates for thermoelectric applications.

Contribution

The paper provides first-principles calculations demonstrating ultra-low lattice thermal conductivity and high thermoelectric efficiency in Cs2BiAgX6 halides, a novel finding for these materials.

Findings

Lattice thermal conductivities are 0.078 and 0.065 W/m·K at 300K.

Maximum power factors are achieved at 700K.

Thermoelectric figures of merit are approximately 0.77 for both compounds.

Abstract

We have explored electronic and thermoelectric properties of bismuth-based double-perovskite halides Cs2BiAgX6 by using first principles calculations. The calculated indirect bandgaps 2.85 eV and 1.99 eV for Cs2BiAgCl6 and Cs2BiAgBr6, respectively well agree with the measured value (2.77 eV of Cs2BiAgCl6 and 2.19 eV of Cs2BiAgBr6). We have calculated the relaxation time and lattice thermal conductivity by using relaxation time approximation (RTA) within supercell approach. The lattice thermal conductivities for both compounds are remarkably low and the obtained values at 300K for Cs2BiAgCl6 and Cs2BiAgBr6 are 0.078 and 0.065 Wm-1K-1, respectively. Such quite low lattice thermal conductivity arises due to low phonon group velocity in the large weighted phase space and large phonon scattering. The large Seebeck coefficient obtained for both halides at 400K. We have obtained the maximum power factors at 700K and the corresponding thermoelectric figure of merit for Cs2BiAgCl6 and Cs2BiAgBr6 are 0.775 and 0.774, respectively. The calculated results reveal that both halides are potential thermoelectric materials.