Anharmonic lattice dynamics in large thermodynamic ensembles with machine-learning force fields: CsPbBr3, a phonon liquid with Cs rattlers

The phonon dispersion relations of crystal lattices can often be\nwell-described with the harmonic approximation. However, when the potential\nenergy landscape exhibits more anharmonicity, for instance, in case of a weakly\nbonded crystal or when the temperature is raised, the approximation fails to\ncapture all crystal lattice dynamics properly. Phonon-phonon scattering\nmechanisms become important and limit the phonon lifetimes. We take a novel\napproach and simulate the phonon dispersion of a complex dynamic solid at\nelevated temperatures with Machine-Learning Force Fields of\nnear-first-principles accuracy. Through large-scale molecular dynamics\nsimulations the projected velocity autocorrelation function (PVACF) is\nobtained. We apply this approach to the inorganic perovskite CsPbBr$_{3}$.\nImaginary modes in the harmonic picture of this perovskite are absent in the\nPVACF, indicating a dynamic stabilization of the crystal. The anharmonic nature\nof the potential makes a decoupling of the system into a weakly interacting\nphonon gas impossible. The phonon spectra of CsPbBr$_{3}$ show the\ncharacteristics of a phonon liquid. Rattling motions of the Cs$^{+}$ cations\nare studied by self-correlation functions and are shown to be nearly\ndispersionless motions of the cations with a frequency of $\\sim$0.8THz within\nthe lead-bromide framework.\n