Crystal Structure Prediction and Phase Stability in Highly Anharmonic Silver-Based Chalcohalide Anti-Perovskites

TL;DR

This study uses first-principles calculations to predict crystal structures and phase stability of silver-based chalcohalide anti-perovskites, revealing new stable phases and temperature-induced transitions that impact their potential energy applications.

Contribution

It introduces novel predicted ground-state structures and phase transition insights for CAP materials, advancing understanding of their stability and properties.

Findings

Identified a new cubic P2_13 structure as the stable phase for S-containing CAP.

Proposed orthorhombic and cubic structures as ground states for Se-containing CAP.

Revealed temperature-induced phase transformations and stability issues in CAP.

Abstract

Silver-based chalcohalide anti-perovskites (CAP), Ag$_{3}$BC (B = S, Se; C = Cl, Br, I), represent an emerging family of energy materials with intriguing optoelectronic, vibrational and ionic transport properties. However, the structural features and phase stability of CAP remain poorly investigated to date, hindering their fundamental understanding and potential integration into technological applications. Here we employ theoretical first-principles methods based on density functional theory to fill this knowledge gap. Through crystal structure prediction techniques, ab initio molecular dynamics simulations, and quasi-harmonic free energy calculations, we unveil a series of previously overlooked energetically competitive phases and temperature-induced phase transitions for all CAP. Specifically, we identify a new cubic $P2_{1}3$ structure as the stable phase of all CAP containing S both at zero temperature and $T \neq 0$ K conditions. Consequently, our calculations suggest that the cubic $Pm\overline{3}m$ phase identified in room-temperature X-ray diffraction experiments is likely to be metastable. Furthermore, for CAP containing Se, we propose different orthorhombic ($Pca2_{1}$ and $P2_{1}2_{1}2_{1}$) and cubic ($I2_{1}3$) structures as the ground-state phases and reveal several phase transformations induced by temperature. This theoretical investigation not only identifies new candidate ground-state phases and solid-solid phase transformations for all CAP but also provides insights into potential stability issues affecting these highly anharmonic superionic materials.