Phase Stability and Transformations in Lead Mixed Halide Perovskites from Machine Learning Force Fields

TL;DR

This study uses machine learning force fields to perform large-scale molecular dynamics simulations of lead halide perovskites, revealing how A-site cations and halide distribution influence phase stability and structural transformations.

Contribution

Introduces an equivariant neural network potential combined with PDynA analysis to explore phase diagrams and dynamics of lead halide perovskites at large scales.

Findings

A-site cation type modulates octahedral tilt modes and phase pathways.

MA+ prevents beta-to-gamma transition in MAPbX3.

Halide composition and segregation affect tilt correlations and phase behavior.

Abstract

Lead halide perovskites (APbX$_3$) offer tunable optoelectronic properties but feature an intricate phase-stability landscape. Here we employ on-the-fly data collection and an equivariant message-passing neural-network potential to perform large-scale molecular dynamics of three prototypical perovskite systems: CsPbX$_3$, MAPbX$_3$, and FAPbX$_3$. Integrating these simulations with the PDynA analysis toolkit, we resolve both equilibrium phase diagrams and dynamic structural evolution under varying temperature and halide-mixing conditions. Our findings reveal that the A-site cation strongly modulates octahedral tilt modes and phase pathways: MA$^+$ effectively "forbids" the beta-to-gamma transition in MAPbX$_3$ by requiring extensive molecular rearrangements and crystal rotation, whereas the debated low-temperature phase in FAPbX$_3$ is best represented as an Im$\bar{3}$ cubic phase with $a^+a^+a^+$ tilts. Additionally, small changes in halide composition and arrangement $\unicode{x2013}$ from uniform mixing to partial segregation $\unicode{x2013}$ alter tilt correlations. Segregated domains can even foster anomalous tilting modes that impede uniform phase transformations. These results highlight the multi-scale interplay between cation environment and halide distribution, offering a rational basis for tuning perovskite architectures toward improved phase stability.