Theoretical Study of a Novel Infrared Nonlinear Optical Crystal of BaGa4S7

The first-principles calculation based on density functional theory, the electronic structure and optical properties of BaGa4S7 (BGS) were systematically investigated by using generalized gradient approximation (GGAPBE) and hybrid functional method (HSE06). The results showed that the theoretical results from the HSE06 method coincided well with the experimental values. Geometry optimization showed that the theoretical lattice parameters of the BGS were also in agreement with the experimental values. Furthermore, the results of the electronic structure showed that the BGS is a nonlinear optical crystal with a wide direct bandgap energy value, as the bandgap width obtained by the HSE06 method was 3.54 eV, which was in accordance with the experimental values. The band structure and density values of state calculations showed that the top of the valence band was mainly composed of S-3p orbital and Ga-4s, 4p orbital electron contribution. On the other hand, the bottom of the conduction band was mainly composed of Ga-4s, 4p, S-3p, and Ba-5d orbital electron contribution, showing that the orbital coupling between Ga and S atoms determined the optical properties of the BGS, while the contribution of Ba atoms to the optical properties was small. The optical properties obtained from the calculation results showed that the crystal material had strong absorption and reflection characteristics in the ultraviolet band, good transmittance in the infrared area, average static dielectric constant, and an average refractive index of 2.873, 1.69, respectively. Moreover, the static double refractive index was 0.07, showing that BGS crystal materials had excellent phase matching performance in a wider range of wavelengths, with a high laser damage threshold. These results proved that the BGS could be a promising material for IR nonlinear optical crystals.