Phonon collapse, second-order phase transition, and glassy thermal transport in halide perovskites C s 3 B i 2 I 6 C l 3 and C s 3 B i 2 B r 9

Accurate prediction of phonon thermal properties and fundamental understanding of the multiphonon interactions in phase transition dynamics and glassy dynamics are significant but challenging in halide perovskites. Herein, we investigate the anharmonic lattice dynamics and phase transition mechanism in crystalline halide perovskites $\mathrm{C}{\mathrm{s}}_{3}\mathrm{B}{\mathrm{i}}_{2}{\mathrm{I}}_{6}\mathrm{C}{\mathrm{l}}_{3}$ and $\mathrm{C}{\mathrm{s}}_{3}\mathrm{B}{\mathrm{i}}_{2}\mathrm{B}{\mathrm{r}}_{9}$ based on ab initio phonon dynamics. In calculating the phonon self-energy, both the bubble and the loop diagrams are considered by including the second-order perturbation term. Results indicate that (i) the transverse soft phonon mode in the Brillouin-zone boundary collapses dramatically on cooling due to the liquidlike dynamic disorder of the halogen atoms, and its phonon linewidths approach the damping limit at the critical phase transition point; (ii) the high-order anharmonicity induces phonon blueshift in the low-frequency region, leading to a qualitative competition between the enhanced phonon population and the reduced phonon scattering strength as $T$ increases, yielding a glassy thermal-transport behavior; and (iii) the giant overdamped four-phonon linewidths are responsible for the resonance scattering between neighbor nesting phonon branches, whereas the three-phonon scattering does not capture and describe accurately the dynamics behavior due to the critical scattering rules associated with the energy- and quasimomentum conservation. Taking into account the anharmonic phonon renormalization, which considers the nuclear quantum anharmonic effect, our calculated critical temperatures ${T}_{\mathrm{c}}$'s of the monoclinic-to-trigonal second-order phase transition are at 98.7 and 102 K for $\mathrm{C}{\mathrm{s}}_{3}\mathrm{B}{\mathrm{i}}_{2}\mathrm{B}{\mathrm{r}}_{9}$ and $\mathrm{C}{\mathrm{s}}_{3}\mathrm{B}{\mathrm{i}}_{2}{\mathrm{I}}_{6}\mathrm{C}{\mathrm{l}}_{3}$, respectively. These results are in excellent agreement with the experimental value of 95 K for $\mathrm{C}{\mathrm{s}}_{3}\mathrm{B}{\mathrm{i}}_{2}\mathrm{B}{\mathrm{r}}_{9}$. Involving the four-phonon scattering and the Wigner thermal-transport model, we recover well the experimental results of the lattice thermal conductivity and supply reasonable descriptions of it both in magnitude and $T$ dependence. This work demonstrates the effects of the $T$ evolutions of the phonon thermal properties induced by the multiphonon interactions on structural phase transitions and heat transport, deepening the underlying physical understanding of the phonon dynamics in disordered crystals with strong phonon anharmonicity.