From silicates to oxonitridosilicates: improving optical anisotropy for phase-matching as ultraviolet nonlinear optical materials.

Oxonitridosilicates, in which O atoms in SiO4 are partially substituted by N atoms, are proposed to improve optical anisotropies of silicates as UV NLO materials. The optical properties calculation showed that the heteroleptic SiOxN4-x (x = 1-3) tetrahedra have strong polarizability anisotropy and large hyperpolarizability. Accordingly, nine noncentrosymmetric (NCS) oxonitridosilicate crystals collected in the inorganic crystal structural database (ICSD) are evaluated by using the first principles method. Finally, Si2N2O and LiSiON are screened out owing to wide band gaps (6.49 and 6.95 eV), large birefringences (0.102 and 0.060 at 1064 nm), and large SHG coefficients (3.3 and 2.2 times that of d36(KDP)). More importantly, the cation selection and structural characteristics that are beneficial for enhancing the band gap and birefringence are identified. This study provides a novel strategy to design and find UV NLO crystals.