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.

中文翻译：

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两种金属磷酸盐非线性光学材料同时表现出紫外透明和大双折射

对于磷酸盐非线性光学(NLO)材料，如何提高其小双折射与其四面体PO ₄基团的弱光学各向异性存在很大矛盾。在此，通过引入具有闭壳结构的La ³⁺和具有ad ^{10电子构型的Cd 2+}，两种具有大双折射的非线性光学材料，即La(PO ₃ ) ₃ (0.040@1064 nm)和β-Cd( PO ₃ ) ₂ (0.059@1064 nm)，采用高温溶液法合成。特别是对于 β-Cd(PO ₃ ) ₂，它具有迄今为止已知的深紫外（DUV）磷酸盐中最大的双折射，这归因于Cd-O基团的强共价性和类似于平面的P-O伪层的协同作用。同时，β-Cd(PO ₃ ) ₂在已报道的Cd基无机化合物中表现出最短的截止边缘（<190 nm），实现了DUV透明度和大双折射之间的平衡。这种见解为使用金属磷酸盐设计高性能 NLO 材料提供了新的机会。