Anharmonic phonon renormalization and thermoelectric properties of CsPbX₃ (X = Cl, Br, and I): first-principles...

Halide perovskites with ultralow thermal conductivity have emerged as promising candidates for thermoelectric materials. We study the lattice dynamics and thermoelectric properties of cubic all-inorganic lead halide perovskites CsPbX₃ (X = Cl, Br, and I) through first-principles calculations. Combined with self-consistent phonon theory, we have successfully renormalized the phonon frequency using a quartic anharmonic term, allowing us to accurately reproduce the phonon dispersion of the high-temperature cubic phase of CsPbX₃ without any imaginary frequencies. Cubic CsPbX₃ exhibit ultralow lattice thermal conductivities (0.61-1.71 Wm^-1 K^-1) at room temperature. Because of the strong quartic anharmonic renormalization and hardening of the soft modes, the lattice thermal conductivities of cubic CsPbX₃ all exhibit weak temperature dependence. Notably, CsPbCl₃ exhibits remarkably high thermal conductivity and a long phonon lifetime. This can be attributed to the smallest atomic mean square displacement and the weakest tilting and distortions of PbCl₆ octahedra, resulting from the strongest Pb-Cl covalent bonding. Furthermore, the maximum ZT value of 0.63 at 900 K is obtained for the n-type CsPbBr₃.