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

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, κL, 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 κL. 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.