Bandgap engineering in Ca x Ba 1-x ZrS 3 chalcogenide perovskites via A-site cation substitution: A first-principles study

Abstract Chalcogenide perovskites represent a promising lead-free alternative to halide-based photovoltaic absorbers, owing to their superior thermal and chemical stability. Despite the individual characterization of BaZrS3 and CaZrS3, no systematic first-principles study has covered the complete CaxBa1−xZrS3 solid-solution series. We report density functional theory (DFT) calculations, performed using QuantumATK, to investigate the composition (x = 0, 0.25, 0.50, 0.75, 1.0) and size-dependent structural, electronic, and optical properties of this series. Structural optimization with the LDA-PZ, GGA-PBE, and GGA-PBEsol functionals confirmed Vegard's law: the lattice parameters decrease monotonically with increasing Ca content, yielding an 11.7% reduction in unit cell volume from BaZrS3 to CaZrS3. GGA-PBEsol reproduced the experimental lattice parameters to within 1%, outperforming the other two functionals. Electronic structures computed with the HSE06 hybrid functional reveal a systematic bandgap increase from 1.70 eV (BaZrS3) to 1.95 eV (CaZrS3). Projected density of states analysis confirms that the valence band maximum and conduction band minimum are governed by S-3p and Zr-4d hybridization, respectively, while Ca²⁺ and Ba²⁺ behave solely as structural template agents with negligible direct electronic contributions. Optical calculations show that the absorption coefficients reach the order of 10⁵ cm⁻¹ across the visible spectrum, with Ca-rich compositions attaining peak values approaching 5.0×10⁵ cm⁻¹. Mixing-energy calculations reveal a positive enthalpy of A-site mixing throughout the series, corresponding to a regular-solution critical temperature of 2295–2527 K and to equilibrium solubility limits of x < 0.06 and x > 0.94 at 1473 K. The intermediate compositions are therefore predicted to lie within a miscibility gap over most of the reported synthesis range, with a small free-energy margin at its upper limit. CaxBa1−xZrS3 thus forms a tunable, non-toxic chalcogenide perovskite platform the 0.25 eV bandgap window achieved through A-site substitution alone positions this solid-solution series for thin-film photovoltaic design.