Anharmonic electron-phonon coupling in ultrasoft and locally disordered perovskites

TL;DR

This paper develops a first-principles approach to account for anharmonicity and local disorder in electron-phonon coupling in perovskites, revealing their impact on vibrational dynamics and band gap behavior near phase transitions.

Contribution

It introduces a unified method using the special displacement approach to incorporate anharmonicity and disorder in electron-phonon interactions in perovskites, advancing theoretical understanding.

Findings

Polymorphism significantly alters vibrational dynamics in halide perovskites.

Band gap changes are driven by vibrational effects near phase transitions.

Accurate band gap predictions require considering disorder, spin-orbit coupling, and electron-phonon interactions.

Abstract

Anharmonicity and local disorder (polymorphism) are ubiquitous in perovskite physics, inducing various phenomena observed in scattering and spectroscopy experiments. Several of these phenomena still lack interpretation from first-principles since, hitherto, no approach is available to account for anharmonicity and disorder in electron-phonon couplings. Here, relying on the special displacement method, we develop a unified treatment of both and demonstrate that electron-phonon coupling is strongly influenced when we employ polymorphous perovskite networks. We uncover that polymorphism in halide perovskites leads to vibrational dynamics far from the ideal noninteracting phonon picture and drives the gradual change in their band gap around phase transition temperatures. We also clarify that combined band gap corrections arising from disorder, spin-orbit coupling, exchange-correlation functionals of high accuracy, and electron-phonon coupling are all essential. Our findings agree with experiments, suggesting that polymorphism is the key to address pending questions on perovskites' technological applications.