Phosphites: A Nonlinear Optical Materials System with a Wide Band Gap and Moderate Second-Harmonic Generation Effect

Ultraviolet (UV) nonlinear optical (NLO) materials have crucial practical applications in modern laser science and technology. Phosphates are promising candidates for searching UV NLO materials. However, small optical anisotropy of high-symmetrized (PO₄)^3- tetrahedron hinders phase matching (PM) in the UV wavelength range. Herein, a low-symmetrized phosphite anionic group (HPO₃)^2- is proposed as a new NLO-active group that exhibits a larger optical anisotropy and second-harmonic generation (SHG) coefficients than (PO₄)^3-. Accordingly, two phosphites, (C(NH₂)₃)₂Zn(HPO₃)₂ and [Al₂(HPO₃)₃(H₂O)₃]·H₂O, that contain (HPO₃)^2- groups are screened out and synthesized. The UV absorption edges of the two compounds are 6.18 and 6.23 eV, respectively. (C(NH₂)₃)₂Zn(HPO₃)₂ exhibited an SHG intensity of 1 times that of KDP at 1064 nm. This work proposes a new NLO-active fundamental building unit (FBU) and a new materials system for searching NLO materials.