Anharmonic Lattice Dynamics, Phase Transitions and Thermal Transport in Light-Element Crystalline Perovskites

A fundamental understanding of lattice dynamics and heat conduction in highly anharmonic compounds is essential for realizing high-performance thermoelectrics, photovoltaics and thermal barrier coatings. In this work, we employ the stochastic self-consistent harmonic approximation (SSCHA), combined with the unified theory of thermal transport, to systematically investigate anharmonic phonon renormalization, phase transitions, and thermal transport in crystalline KCaF_3. Compared to other anharmonic phonon renormalization techniques, the SSCHA method allows the relaxation of the internal atomic coordinates, providing a more accurate representation of finite-temperature phonons with the quantum nuclei fluctuation. Therefore, the SSCHA is an ideal tool to investigate the temperature-dependent phonon properties of the light-element perovskite KCaF_3, even in the presence of large atomic displacements at finite temperatures. Additionally, our calculations not only give the finite-temperature effective harmonic force constants but also the temperature-dependent higher-order force constants, which is essential for accurately modeling thermal transport in strongly anharmonic compounds. Finally, using the unified theory of thermal transport incorporating both phonon propagation and coherence, we find an ultralow lattice thermal conductivity of the light-element perovskite KCaF_3, which makes it a highly promising material for thermoelectrics and thermal barrier coatings.