Quartic anharmonicity and anomalous thermal conductivity in cubic antiperovskites A3BO (A=K, Rb; B=Br, Au)

Abstract

We use ab initio self-consistent phonon theory combined with compressive sensing techniques to investigate the role of quartic anharmonicity in the lattice dynamics and thermal transport properties of the cubic antiperovskites ${A}_{3}B\text{O}$ ($A=\text{K}$, Rb; $B=\text{Br}$, Au). Our findings indicate that the strong quartic anharmonicity of alkali-metal atoms plays a crucial role in the phonon quasiparticles free from imaginary frequencies in ${\mathrm{K}}_{3}\mathrm{BrO}$, ${\mathrm{Rb}}_{3}\mathrm{BrO}$, and ${\mathrm{Rb}}_{3}\mathrm{AuO}$, and gives rise to an evident hardening of vibrational frequencies of the low-lying modes in ${\mathrm{K}}_{3}\mathrm{AuO}$. Based on these results, the lattice thermal transport properties are predicted through the Boltzmann transport equation within the relaxation time approximation. The results exhibit that the four cubic antiperovskites have remarkably low thermal conductivities ${\ensuremath{\kappa}}_{L}$, e.g., 0.73--1.70 W/mK at 300 K, and an anomalously weak temperature dependence of the ${\ensuremath{\kappa}}_{L}$, e.g., ${\ensuremath{\kappa}}_{L}\ensuremath{\sim}{T}^{\ensuremath{-}0.3}$, of which the latter has not been reported in normal perovskites and other conventional semiconductors to date. We demonstrate that the strong three-phonon scattering along with strong quartic anharmonic renormalization of the soft modes and the presence of high-frequency dispersive optical modes are responsible for the thermal conductivity spectrum ${\ensuremath{\kappa}}_{L}(\ensuremath{\omega})$ and thus the low ${\ensuremath{\kappa}}_{L}$ and its anomalously weak temperature dependence.