Faiq Umar, Sikander Azam, Saleem Ayaz Khan
A first-principles framework is presented for band-edge and optical-response control in the stannate chalcogenides Ca₂SnX₄ (X = S, Se, Te). All-electron APW+lo calculations (WIEN2k) are performed on PBEsol-relaxed structures, with electronic and optical properties evaluated using the Tran–Blaha modified Becke–Johnson potential including spin–orbit coupling. Across the S→Se→Te series, the band gap narrows systematically (≈2.5 eV, ≈1.8 eV, and ≈1.3 eV, respectively), leading to a red shift of the absorption edge from the near-UV to the visible/near-IR region, accompanied by an enhancement of the low-frequency dielectric response and refractive index. The calculated optical functions including the dielectric tensor (ε₁, ε₂), refractive index n(ω), reflectivity R(ω), energy-loss function Im[–1/ε], and absorption coefficient α(ω) exhibit well-defined interband features governed by compositional changes in σ/π Sn–X hybridization. Real-space charge density and electron localization function analyses reveal a mixed iono-covalent bonding framework, where Ca acts as an electron donor to a covalently bonded (SnX₄) network, with bond-center localization progressively broadening from S to Te. Mechanical and thermodynamic parameters (bulk and shear moduli, Debye temperature) confirm lattice robustness suitable for optical applications, while semiclassical transport behavior and defect energetics identify favorable growth conditions that minimize sub-gap losses and enable luminescent activation. These results establish Ca₂SnSe₄ and Ca₂SnTe₄ as promising high-index materials for visible–near-IR optoelectronic applications, whereas Ca₂SnS₄ emerges as a low-loss dielectric suitable for UV-transparent optical components.