Anharmonic lattice dynamics in large thermodynamic ensembles with machine-learning force fields: CsPbBr${}_3$ a phonon-liquid with Cs rattlers

Abstract

The phonon dispersion relations of crystal lattices can often be well-described with the harmonic approximation. However, when the potential energy landscape exhibits more anharmonicity, for instance, in case of a weakly bonded crystal or when the temperature is raised, the approximation fails to capture all crystal lattice dynamics properly. Phonon-phonon scattering mechanisms become important and limit the phonon lifetimes. We take a novel approach and simulate the phonon dispersion of a complex dynamic solid at elevated temperatures with Machine-Learning Force Fields of near-first-principles accuracy. Through large-scale molecular dynamics simulations the projected velocity autocorrelation function (PVACF) is obtained. We apply this approach to the inorganic perovskite CsPbBr${}_3$. Imaginary modes in the harmonic picture of this perovskite are absent in the PVACF, indicating a dynamic stabilization of the crystal. The anharmonic nature of the potential makes a decoupling of the system into a weakly interacting phonon gas impossible. The phonon spectra of CsPbBr${}_3$ show the characteristics of a phonon liquid. Rattling motions of the Cs${}^{+}$ cations are studied by self-correlation functions and are shown to be nearly dispersionless motions of the cations with a frequency of $\sim$0.8THz within the lead-bromide framework.

16 pages , 13 figures

References in corpus (8)

• On-the-fly machine learning force field generation: Application to melting points
• Phase transitions of hybrid perovskites simulated by machine-learning force fields trained on-the-fly with Bayesian inference
• Tuning of the Thermoelectric Figure of Merit of CH${}_3$NH${}_3$MI${}_3$ (M=Pb,Sn) Photovoltaic Perovskites
• Phonon anharmonicity, lifetimes and thermal transport in CH${}_3$NH${}_3$PbI${}_3$ from many-body perturbation theory
• Dynamic Shortening of Disorder Potentials in Anharmonic Halide Perovskites
• Assessing density functionals using many body theory for hybrid perovskites
• $\alpha$-$\beta$ phase transition of zirconium predicted by on-the-fly machine-learned force field
• phq: a Fortran code to compute phonon quasiparticle properties and dispersions

Cited by in corpus (7)

• Anharmonic electron-phonon coupling in ultrasoft and locally disordered perovskites
• Phase transitions in inorganic halide perovskites from machine learning potentials
• Limits of the phonon quasi-particle picture at the cubic-to-tetragonal phase transition in halide perovskites
• Revealing the free energy landscape of halide perovskites: Metastability and transition characters in CsPbBr${}_3$ and MAPbI${}_3$
• Tracing Ion Migration in Halide Perovskites with Machine Learned Force Fields
• Local structural disorder in crystalline materials
• Smooth Overlap of Spin Orientations: Machine Learning Exchange Fields for Ab-initio Spin Dynamics