Anharmonicity-induced phonon hardening and phonon transport enhancement in crystalline perovskite BaZrO₃

Abstract

We investigated the effects of cubic and quartic anharmonicity on lattice dynamics and thermal transport in highly anharmonic BaZrO3 crystal over a wide temperature range (300-2000 K) by combining the first-principles-based self-consistent phonon theory and a unified theory of thermal transport including population and coherence contributions. By considering the effects from bubble and loop diagrams, the contributions of both three-phonon (3ph) and four-phonon (4ph) interaction processes to phonon scattering rates and energy shifts were clarified. Anharmonic phonon renormalization is found to play a crucial role in determining the finite-temperature phonon energies and lattice thermal conductivity κL in BaZrO3. Specifically, the lattice anharmonicity induces significant low-frequency optical phonon hardening at elevated temperatures, which is correlated with the U-shaped potential energy surfaces for these modes. The low-frequency optical phonon hardening significantly suppresses phonon scattering rates by altering the phonon weighted phase space of both 3ph and 4ph interaction processes, thereby leading to significant enhancement in the κL and weaker temperature dependence of κL∼T-0.75 than traditional harmonic treatments. Moreover, although the coherent thermal transport channel is suppressed by anharmonic phonon renormalization, it is enhanced by the 4ph scattering processes. The coherence contribution becomes nonnegligible at elevated temperatures and may contribute up to 17.38% of the total κL at 1500 K. In this paper, we highlight the strong influence of the lattice anharmonicity on thermal conductivity in severely anharmonic systems and the importance of coherent thermal transport channel at elevated temperatures.