Strong anharmonicity and high thermoelectric performance of cubic thallium-based fluoride perovskites TlXF3 (X = Hg, Sn, Pb)

Abstract

State-of-the-art first-principles calculations are performed to investigate the thermoelectric transport properties in thallium-based fluoride perovskites TlXF₃ (X = Hg, Sn, Pb) by considering anharmonic renormalization of the phonon energy and capturing reasonable electron relaxation times. The lattice thermal conductivity, κ_L, of the three compounds is very low, among which TlPbF₃ is only 0.42 W m⁻¹ K⁻¹ at 300 K, which is less than half of that of quartz glass. The low acoustic mode group velocity and strong three-phonon scattering caused by the strong anharmonicity of the Tl atom are the origin of the ultralow κ_L. Meanwhile, the strong ionic bonds between X (X = Hg, Sn, Pb) and F atoms provide good electrical transport properties. The results show that the ZT value of TlHgF₃ at 900 K is 1.58, which is higher than the 1.5 value of FeNbSb at 1200 K. TlSnF₃ and TlPbF₃ also exceed 1, which is close to the classical thermoelectric material PbTe:Na. Furthermore, we present the methods and expected effects of improving the ZT value through nanostructures.