Insilico investigations of the physical properties of hexagonal chalcogenide perovskites CsXS

Chalcogenide perovskites have attracted growing interest as inorganic materials with tunable electronic and optical properties and enhanced chemical stability compared to halide counterparts. In this work, we present a first-principles density functional theory (DFT) investigation of the structural, electronic, and optical properties of the cesium-based chalcogenide perovskites CsNbS3 and CsVS3. Structural optimizations are performed within the generalized gradient approximation (PBE-GGA) to establish equilibrium geometries within the ABX3 perovskite framework. The electronic properties are analyzed primarily using PBE-GGA to ensure internal consistency between band structures, density of states, and derived optical spectra. Both compounds exhibit narrow indirect band gaps at the PBE-GGA level, with valence-band maxima dominated by S-3p states and conduction-band minima arising mainly from Nb-4d and V-3d orbitals. Hybrid-functional (HSE06) calculations are additionally employed to assess the sensitivity of the electronic structure to exchange–correlation effects, revealing pronounced band-edge renormalization near the Fermi level rather than serving as a basis for electronic classification. Optical properties, including the dielectric function, absorption coefficient, reflectivity, and optical conductivity, are calculated consistently from the PBE-GGA electronic structures and show strong optical activity in the visible to near-infrared energy range, originating from S-p to transition-metal d interband transitions. A model-based thickness-dependent efficiency analysis is further presented to compare relative optoelectronic trends, without making predictive device-performance claims. Overall, this study provides a coherent and internally consistent description of the electronic and optical behaviour of CsNbS3 and CsVS3, clarifies the functional dependence of their electronic structures, and highlights the importance of careful methodological interpretation in first-principles studies of transition-metal chalcogenide perovskites.