Theoretical prediction of the structural, electronic and optical properties of SnB2O4 (B = Mg, Zn, Cd)

The structural, electronic and optical properties of the cubic spinels SnB₂O₄, with B = Mg, Zn and Cd, were studied by means of the full-potential (linear) augmented plane wave plus local orbitals method within the local density and generalized gradient approximations for the exchange-correlation potential. The Engel-Vosko form of the generalized gradient approximation (EV-GGA), which better optimizes the potential for the band structures, was also used. The results of bulk properties, including lattice constants, internal parameters, bulk moduli and their pressure derivatives are in good agreement with the literature data. The band structures show a direct band gap (Γ-Γ) for the three compounds. The computed band gaps using the EV-GGA show a significant improvement over the more common GGA. All the calculated band gaps increase with increasing pressure and fit well to a quadratic function. Analysis of the density of states revealed that the lowering of the direct gap (Γ-Γ) from SnMg₂O₄ to SnZn₂O₄ to SnCd₂O₄ can be attributed to the p-d mixing in the upper valence band of SnZn₂O₄ and SnCd₂O₄. We present calculations of the frequency-dependent complex dielectric function ?(ω). We find that the values of zero-frequency limit ?₁(0) increase with decreasing the energy band gap. The origin of the peaks and structures in the optical spectra is determined in terms of the calculated energy band structures.