Lattice thermal transport of strongly anharmonic crystals

Abstract

The prediction of lattice thermal conductivity via the Boltzmann transport equation and perturbation theory (PT) has been successful for many solid crystals. However, it is still challenging to use this method to reliably analyze strongly anharmonic materials with ultralow lattice thermal conductivities at high temperatures. Some basic issues must be addressed before precisely predicting the lattice thermal conductivities of strongly anharmonic materials, such as the temperature dependence of phonon frequency, higher-order phonon interactions and diffusive thermal transport behavior. Moreover, perturbation theory may partially break down for some materials with strong lattice anharmonicity.

In this thesis, we first use a two-dimensional material monolayer InSe as an example to investigate the anharmonic phonon frequency shift at finite temperatures. The results indicate that anharmonic phonon frequency renormalization from cubic and quartic lattice anharmonicity is non-negligible in the strongly anharmonic material InSe. We also find that tensile strain can effectively manipulate the lattice anharmonicity and thus the phonon frequency shift, which may help us to better alter the lattice thermal conductivity of two-dimensional materials under the strain field.

We further focus on the study of thermoelectric materials with ultralow lattice thermal conductivities at room temperature. Using Tl$_3$VSe$_4$ and BaAg$_2$Te$_2$ as examples, we investigate their lattice dynamics via advanced perturbation theory up to quartic phonon anharmonicity and perform MD simulations with machine-learning potentials at near first-principles accuracy. Strikingly, there are distinct differences in phonon linewidths calculated from perturbation theory and MD in the entire Brillouin zone. The comparison between the theoretical phonon linewidths and experimental results reveals that PT severely underestimates the phonon interactions at room temperature, even when the cubic and quartic phonon anharmonicity are considered. Our results demonstrate the limitations of state-of-the-art perturbation theory to explore the anharmonic lattice dynamics of strongly anharmonic materials over wide temperature ranges and provide new insight for more precisely modeling strongly anharmonic materials such as clathrates and perovskites.

Little is known about comprehensive scope of phonon diffusion in strongly anharmonic materials in the thermal transport realm. Therefore, using perovskite Cs$_2$PbI$_2$Cl$_2$ and chain-like TlXSe$_2$ (X=Tl, In, Al) as examples, we thoroughly investigate their diffusive thermal transport at room temperature. We find that diffusive thermal transport is crucial to determine the lattice thermal transport in perovskites, while it is negligible in chain-like TlXSe$_2$ materials, as the latter has strong phonon blocking.

In summary, the aim of this thesis is to accurately understand the phonon properties and lattice thermal transport of strongly anharmonic materials. Using these representative materials, we provide fundamental insights to access the lattice thermal conductivity of strongly anharmonic materials, which help accelerate the advancement of the understanding of condensed matter and phonon thermal engineering.