How the Temperature and Composition Govern the Structure and Band Gap of Zr-Based Chalcogenide Perovskites: Insights from ML Accelerated AIMD

Ab initio molecular dynamics in the NPT ensemble is employed to investigate the structure andelectronicproperties of chalcogenide perovskites AZrX(3) (A = Ca, Sr,Ba; X = S, Se) over the temperature range of 300-1200 K. Theband gap of these materials is found to monotonously decrease butthe magnitude of this change is shown to be strongly composition-dependent.Various factors influencing this trend, including charge transfer,volume expansion, and distortion of the ideal cubic framework, areanalyzed. The effects of temperature and composition on the structuralandelectronic properties of chalcogenide perovskite (CP) materials AZrX(3) (A = Ba, Sr, Ca; X = S, Se) in the distorted perovskite (DP)phase are investigated using ab initio molecular dynamics (AIMD) acceleratedby machine-learned force fields. Long-range van der Waals (vdW) interactions,incorporated into the Perdew-Burke-Ernzerhof (PBE) exchange-correlationfunctional using the DFT-D3 scheme, are found to be crucial for achievingcorrect predictions of structural parameters. Our calculations showthat the distortion of the DP structure with respect to the parentcubic (C) phase, realized in the form of interoctahedral tilting,decreases with the increasing size of the A cations. The tendencyfor a gradual transformation of the DP-to-C phase with increasingtemperature is shown to be strongly composition-dependent. The transformationtemperature decreases with the size of cation A and increases withthe size of anion X. Thus, within the range of the temperatures consideredhere (300-1200 K), a complete transformation is observed onlyfor BaZrS3 (& SIM;600 K) and BaZrSe3 (& SIM;900K). The computed band gap of CPs is shown to monotonically decreasewith increasing temperature, and the magnitude of this decrease isfound to be proportional to the extent of the thermally induced changesin the internal structure. Diverse factors affecting the magnitudeof band gaps of CP materials are analyzed.