Effect of the tetrahedral groups on the optical properties of LaBRO₅ (R = Si and Ge): A first-principles study

As potential candidates for deep-UV nonlinear optical (NLO) crystals, borosilicates and borogermanates, which contain NLO-active groups such as B-O, Si-O, and Ge-O groups, have fascinated many material scientists' research enthusiasm. In this paper, the electronic structures and optical properties of two isostructural noncentrosymmetric crystals LaBRO₅ (R = Si and Ge) have been studied by the first-principles method. Combined with the analyses of the SHG-density and the localized electron-density difference, contributions of the constituent tetrahedra to the total NLO responses are investigated. Eventually, BO₄ and GeO₄ groups give nearly equal contributions to the SHG effect of LaBGeO₅, but for LaBSiO₅, SiO₄ groups express stronger SHG response than that of BO₄. Such interesting conclusion is consistent with the distortion index analyses and dipole moment