Effect of functional on structural, elastic stability, optoelectronic and thermoelectric characteristics of semiconducting MgX₂Se₄ (X = Lu, Y) spinels

Effect of functional on structural, elastic, optoelectronic and thermoelectric characteristics of semiconducting MgX₂Se₄ (X = Lu, Y) spinels has been realized by WIEN2k code. The lattice constant of MgY₂Se₄ is slightly greater than that of MgLu₂Se₄, and these quantities are slightly deviated from the experimental values, where the error does not exceed 1.3%. The cohesive energy proves that both spinels are chemically stable in the normal case, and this stability is more pronounced in MgLu₂Y₄. The large ionic radius of Lu compared to Y explains the high bulk modulus of MgY₂Se₄ as well as its hardness. The spinels under study have Г → Г direct band gap located between 1.178 and 1.4 eV for all the functionals, proving their semiconductor nature. Se-s, Y-d, Lu-d states in MgY₂Se₄ and MgLu₂Se₄ dominate the upper valence band, while the first conduction band located between Fermi level and 1.5 eV is empty. There is a strong coupling between Se-p–Lu-p sites for MgLu₂Se₄ and Se-p–Y-p states in MgY₂Se₄, which reflects their hybridization. The high absorption in the ultraviolet range, the band gap between 1 and 2.4 eV and the refractive index in the range of 2.29–2.61 favour these spinels as absorbers in solar cells. Peaks of all the optical quantities studied relating to the mBJ–GGA functional are shifted to the right compared with GGA and GGA+SO approximations. The thermoelectric parameters were investigated as a function of photon energy and temperature using GGA+SO functional.