Electronic Structure and Band Gap Modulation of Ba–S Compounds via Tuned Hybrid Functional Calculations

Abstract

This work presents a combined theoretical and experimental investigation of optimized crystal structural and electronic properties of Ba-S compounds (BaS, BaS₂, BaS₃). Films were synthesized via e-beam evaporation followed by sulfurization, yielding phase pure Ba-S compounds as confirmed by XRD. Optical measurements revealed a systematic reduction in band gap from BaS (≈3.8 eV) to BaS₂ (≈2.9 eV) and BaS₃ (≈2.6 eV), consistent with increasing sulfur coordination and the formation of S–S bonds. To interpret these results, DFT calculations were performed using the HSE06 hybrid functional, with systematic tuning of the HFSCREEN parameter to improve agreement with experiment. The results reveal a progressive narrowing of the band gap across the Ba–S series, following the trend BaS (≈3.8 eV) > BaS₂ (≈2.7 eV) > BaS₃-tetragonal (≈2.7 eV). This reduction is attributed to enhanced S–S bonding, polysulfide chain formation, and increased contributions from S 3p orbitals, which introduce new states near the Fermi level. The HFSCREEN (μ) parameter was systematically varied and selected to reproduce the experimental band gaps while maintaining consistency with expected screening trends across the Ba–S series. Overall, HFSCREEN-enhanced hybrid functional calculations provided more accurate band gap predictions than the GGA/PBE method, successfully capturing observed trends and reflecting key structure–property relationships within the Ba–S compounds.