Tuning Einstein Oscillator Frequencies of Cation Rattlers: A Molecular Dynamics Study of the Lattice Thermal Conductivity of CsPbBr₃

The pure CsPbBr₃ perovskite is an archetypal example of a strongly anharmonic crystal that poses a major challenge for computational methods to describe its thermodynamic properties. Its lattice dynamics exhibits characteristics of a phonon liquid: mode coupling, low lifetimes, and “rattlers”. To study the thermal conduction in this crystal, including the effect of dynamic disorder introduced by the Cs rattlers, we applied large-scale molecular dynamics (MD) simulations combined with machine-learning interatomic potentials. We simulate its ultralow lattice thermal conductivity in the cubic phase and obtain phonon spectra by measuring velocity autocorrelation functions. The thermal conductivity at 500 K is computed to be 0.53 ± 0.04 W/m·K, which is similar to that of demineralized water under normal indoor conditions. MD-based insight into the heat transport mechanism of halide perovskites is presented. In the analysis, the Cs cations are interpreted as damped Einstein oscillators. The phonon band structure of a system with artificially raised Cs masses demonstrates an increased interference of the Cs rattling with the acoustic phonon modes. We show that the thermal conductivity of the CsPbBr₃ perovskite can still be slightly decreased by tuning the cation rattling frequency into the range of the low-lying acoustic modes.