How the temperature and size of A cation govern the structure and bandgap of Zr-based chalcogenide perovskites: Insights from ML accelerated AIMD

The effects of temperature and composition on the structural and electronic properties of chalcogenide perovskite (CP) materials AZrX3 (A=Ba, Sr, Ca; X=S, Se) in distorted perovskite (DP) phase have been investigated using ab-initio molecular dynamics accelerated by machine-learned force fields. Long-range van der Waals (vdW) interactions, incorporated into the Perdew-Burke- Ernzerhof (PBE) exchange correlation functional using the DFT-D3 scheme, are shown to be crucial for achieving correct preditions of structural parameters. An increase in the size of the A site cations is shown to reduce distortion of structure with respect to the parent cubic (C) phase, realised in the form of inter-octahedral tilting. The previously reported (J. Matter. Chem. C 2022, 10, 12032- 12042) gradual transformation of DP to C phase with increasing temperature is shown to be strongly composition dependent. The transformation temperature decreases with the size of cation A and increases with the size of anion X. Thus, within the range of temperatures considered here (300 - 1200 K), a complete transformation was observed only for BaZrS3 (∼600 K), SrZrS3 (∼1200 K) and BaZrSe3 (∼900 K). The computed band gap of CPs is shown to monotonically decrease with increasing temperature and the magnitude of this decrease is found to be proportional to the degree of thermally induced internal structure changes