First-Principles Calculations to Investigate the Ground State, Mechanical Stability, Electronic Structure, and Optical Properties of Tl2SnX3 (X = S, Se, Te)

In this work, the Density Functional Theory (DFT) analysis of the Tl₂SnX₃ series (X = S, Se, Te) is performed, and the ground states are confirmed by the calculation of the elastic constant Cij. Based on the DFT calculation, the Tl₂SnX₃ structures are direct-gap semiconductors with bandgaps of 1.434, 1.181, and 0.907 eV, respectively. Chalcogen substitution significantly impacts their electronic structures, notably increasing the Density of States (DOS) width in the valence band from sulfur to tellurium, and shifting the dielectric function's real part, ε₁(ω), toward lower energies. This change means that the optical activity and response to electric fields are better, with Tl2SnTe3 showing the best polarization response and light-matter interaction abilities. Optical tests show that Tl₂SnTe₃ has very high optical absorption, peaking at ≈17 × 10⁴ cm⁻¹ along [010], and reflectivity levels above 90%, marking its suitability for high-reflectivity applications. Moreover, loss energy function analysis also shows that Tl₂SnTe₃ has a strong electron loss resonance at lower energies, which means it has strong interactions with electrons.