Extremely Strong Four-Phonon Scattering and Ultra-Low Lattice Thermal Conductivity Due to Quartic Anharmonicity in Fluoride Perovskites Xhgf 3 (X = K, Rb)

Using high-throughput first-principles calculations, combined with self-consistent phonon theory and Boltzmann transport equations, the lattice thermal transport properties of fluoride perovskites XHgF 3 (X = K, Rb) with strong anharmonicity are investigated. After considering the quartic anharmonic renormalization, the imaginary frequency in the phonon spectrum disappears and the low-lying modes soften, resulting in a low group velocity of the acoustic phonons. The strong quartic anharmonicity also causes extremely strong four-phonon scattering, which is even greater than three-phonon scattering in the low frequency phonon mode. The results show that the lattice thermal conductivity κ L of XHgF 3 (X = K, Rb) is ultra-low and the temperature dependence is unusually weak. The κ L of KHgF 3 at 300 K is only 0.18 Wm −1 K −1 , which is one-fifth of the classical low thermal conductivity material, quartz glass. Our study points the way to addressing the thermal transport properties of materials with strong anharmonicity and demonstrates that XHgF 3 (X = K, Rb) can not only serve as a good thermal insulation material but also has potential applications in the thermoelectric field. • Fluoride perovskites XHgF 3 (X = K, Rb) have strong quartic anharmonicity. • The frequency of the low-lying modes is significantly boosted after considering the quartic anharmonic renormalization. • The low-frequency four-phonon scattering of XHgF 3 (X = K, Rb) is even stronger than the three-phonon scattering. • XHgF 3 (X = K, Rb) have ultra-low lattice thermal conductivity and unusually weak temperature dependence. • The lattice thermal conductivities of KHgF 3 and RbHgF 3 are much lower than that of the classical thermoelectric material PbTe.