Band structures, chemical bonding, and frequency-dependent optical properties of nonlinear optical crystals HgGa2S4 and Hg0.5Cd0.5Ga2S4

Band structures, density of states, chemical bonding properties, and frequency-dependent linear and nonlinear optical properties have been investigated in terms of the density functional theory and an anharmonic oscillator model for HgGa2S4 (HGS) and Hg0.5Cd0.5Ga2S4 (HCGS) crystals. The results obtained show that the top valence bands (VBs) are contributions from Ga-S bonding states and the bottom conduction bands (CBs) are mostly due to GaS antibonding states for the HGS crystal, and the top VBs mostly originates from the S-3p states and the bottom CBs are mainly composed of the Ga-S antibonding states for the crystal HCGS. The population analysis shows that both crystals have mixed ionic-covalent bonds. The interactions between Ga and S atoms mostly appear as covalent character, and the Hg-S and Cd-S bonds have substantially ionic characters in HGS and HCGS crystals, respectively. The calculated refractive indices of HGS are compared with the experimental ones, and are found to be in agreement with the experiment data in the low-energy region. It is also found that the band gap of HGS is smaller than that of HCGS, and that the second-order susceptibilities of HGS are larger than those of HCGS.