Phase Transitions and Overdamped Dynamics in Halide Perovskites

Halide perovskites have emerged as one of the most interesting materials for photovoltaic and optoelectronic applications due to their favorable properties, often attributed to their dynamic softness. The soft nature of these materials arises mainly from a few specific phonon modes that drive the material to undergo several phase transitions. These soft modes are associated with strong anharmonicity, leading to overdamped phonons near the phase transitions.<br/><br/>Here, we present a study on phase transition and phonon dynamics in halide perovskites using molecular dynamics simulations (MD) with machine-learned potentials (MLP). The simulations capture the correct phase transition from the orthorhombic phase to the tetragonal phase and from the tetragonal to the cubic phase for the prototypical CsPbBr3 halide perovskite. These phase transitions are driven by the condensation of the in-phase and out-of-phase tilt modes of the PbX6 octahedra. The transition temperatures obtained are slightly underestimated compared to experimental studies but are in good qualitative agreement, indicating that the simulations capture the dynamics of these modes well.<br/>The dynamics of these modes are studied with phonon mode projections, and we find that they have a strongly anharmonic and overdamped character. We demonstrate this behavior and show that the overdamped region extends almost 200 K above the cubic to tetragonal phase transition temperature. Lastly, we simulate the quasi-elastic neutron scattering (QENS) spectra and obtain excellent agreement with experimental results. We show that these overdamped modes give rise to large intensities at low frequencies (0-2 meV), explaining why the spectra of the perovskite phases differ in this frequency range compared to the non-perovskite phases.