Force-Matching Based Polarizable and Non-Polarizable Force Fields for Perovskite and Non-Perovskite Phases of CsPbI$_3$

TL;DR

This study develops and compares polarizable and non-polarizable force fields for CsPbI3, revealing that explicit polarization is crucial for accurately modeling phase stability and interactions in perovskite polymorphs.

Contribution

The paper introduces first-principles-based polarizable and non-polarizable force fields for CsPbI3, demonstrating the importance of polarization in stabilizing specific phases.

Findings

Polarizable force field better reproduces structural details.

Explicit polarization stabilizes the δ phase at lower temperatures.

Insights into physical interactions governing phase stability.

Abstract

Lead halide perovskites have emerged as highly efficient solar cell materials. However, to date, the most promising members of this class are polymorphs, in which a wide-band gap $\delta$ phase competes with the photoactive perovskite $\alpha$ form, and the intrinsic physical interactions that stabilize one phase over the other are currently not well understood. Classical molecular dynamics simulations based on suitably parameterized force fields (FF) enable computational studies over broad temperature (and pressure) ranges and can help to identify the underlying factors that govern relative phase stability at the atomic level. In this article, we present a polarizable (pol) as well as a non-polarizable (npol) FF for the all-inorganic lead halide material CsPbI$_3$ as a prototype system exhibiting a $\delta$/$\alpha$ polymorphism. Both npol and pol FFs have been determined using a force-matching approach based on reference data from first-principles molecular dynamics simulations over a wide range of temperatures. While both FFs are able to describe the perovskite as well as the non-perovskite phase, finer structural details as well as the relative phase stability are better reproduced with the polarizable version. A comparison of these ab initio-derived interatomic potentials allows direct insights into the physical origin of the interactions that govern the interplay between the two competing phases. It turns out that explicit polarization is the essential factor that stabilizes the strongly anisotropic $\delta$ phase over the high symmetry perovskite $\alpha$ phase at lower temperatures. This fundamental difference between $\alpha$ and $\delta$ phase appears universal and might thus also hold for other perovskite compounds with $\delta$/$\alpha$ polymorphism and thus provide rational guidance for synthetic efforts to stabilize the photoactive perovskite phase at room temperature.