Two Metal Phosphate Nonlinear Optical Materials Simultaneously Exhibiting Ultraviolet Transparence and a Large Birefringence

For phosphate nonlinear optical (NLO) materials, how to improve their small birefringence is confronted with a great contradiction of their weak optical anisotropy of tetrahedral PO4 groups. Herein, by introducing La3+ with a closed-shell structure and Cd2+ with a d10 electronic configuration, two NLO materials with a large birefringence, namely, La­(PO3)3 (0.040@1064 nm) and β-Cd­(PO3)2 (0.059@1064 nm), have been synthesized by a high-temperature solution method. In particular, for β-Cd­(PO3)2, it possesses the largest birefringence among the known deep-ultraviolet (DUV) phosphates to date, which is attributed to cooperative effects of strong covalence of Cd–O groups and P–O pseudolayers similar to those of a plane. Meanwhile, β-Cd­(PO3)2 exhibits the shortest cutoff edge (<190 nm) among the reported Cd-based inorganic compounds and realizes a balance between DUV transparence and a large birefringence. This insight provides a new opportunity to design high-performance NLO materials using metal phosphates.