Optoelectronic properties of XIn₂S₄ (X = Cd, Mg) thiospinels through highly accurate all-electron FP-LAPW method coupled with modified approximations

We report the structural, electronic and optical properties of the thiospinels XIn₂S₄ (X = Cd, Mg), using highly accurate all-electron full potential linearized augmented plane wave plus local orbital method. In order to calculate the exchange and correlation energies, the method is coupled with modified techniques such as GGA+U and mBJ-GGA, which yield improved results as compared to the previous studies. GGA+SOC approximation is also used for the first time on these compounds to examine the spin orbit coupling effect on the band structure. From the analysis of the structural parameters, robust character is predicted for both materials. Energy band structures profiles are fairly the same for GGA, GGA+SOC, GGA+U and mBJ-GGA, confirming the indirect and direct band gap nature of CdIn₂S₄ and MgIn₂S₄ materials, respectively. We report the trend of band gap results as: (mBJ-GGA) > (GGA+U) > (GGA) > (GGA+SOC). Localized regions appearing in the valence bands for CdIn₂S₄ tend to split up nearly by ≈1 eV in the case of GGA+SOC. Many new physical parameters are reported that can be important for the fabrication of optoelectronic devices. Optical spectra namely, dielectric function (DF), refractive index n(ω), extinction coefficient k(ω), reflectivity R(ω), optical conductivity σ(ω), absorption coefficient α(ω) and electron loss function are discussed. Optical's absorption edge is noted to be 1.401 and 1.782 for CdIn₂S₄ and MgIn₂S₄, respectively. The prominent peaks in the electron energy spectrum situated between 15 eV and 23 eV for the herein studied materials indicate a transition from metallic to the dielectric character.