On account of the high T_d symmetry of the optically active [PO₄] motif, the birefringence of ultraviolet (UV) nonlinear optical (NLO) phosphates is extremely small. Here, two UV-transparent phosphates (C₄H₇N₂)(H₂PO₄) and (C₃H₅N₂)(H₂PO₄) exhibiting pseudo two-dimensional (2D) intercalated layers were successfully synthesized by simultaneously introducing planar and tetrahedral motifs. The arrangements of planar and tetrahedral motifs within the pseudo 2D intercalated layers change from the inverse pairing mode to the uniform pairing mode, resulting in the structural evolution from centrosymmetric (CS) (C₄H₇N₂)(H₂PO₄) to noncentrosymmetric (NCS) (C₃H₅N₂)(H₂PO₄). Compared with the CS phosphate (C₄H₇N₂)(H₂PO₄) (0.12 at 546 nm, 5.21 eV), the NCS (C₃H₅N₂)(H₂PO₄) exhibits larger birefringence (0.15 at 546 nm), a blue-shifted band gap (5.41 eV), and a phase-matching second harmonic generation. Structural analysis and first-principles calculations indicate that the large birefringence in the (C₃H₅N₂)(H₂PO₄) crystal is caused by the closely antiparallel arrangement between adjacent pseudo 2D intercalated layers, in which the planar (C₃H₅N₂)⁺ motifs play a dominant role in optical properties.

中文翻译：

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非中心对称磷酸盐中的大光学各向异性，具有伪 2D 插层


由于光学活性 [PO₄] 基序的高 T_d 对称性，紫外 （UV） 非线性光学 （NLO） 磷酸盐的双折射非常小。在这里，通过同时引入平面和四面体图案。平面和四面体基序在伪 2D 插层内的排列从逆配对模式变为均匀配对模式，导致结构从中心对称 （CS） （C₄H₇N₂）（H₂PO₄） 演变为非中心对称 （NCS） （C₃H₅N₂）（H_{阿拉伯数字}PO₄）。与 CS 磷酸盐 （C₄H₇N₂）（H₂PO₄）（546 nm 处为 0.12,5.21 eV）相比，NCS （C₃H₅N₂）（H₂PO₄） 表现出更大的双折射（546 nm 处为 0.15）、蓝移带隙 （5.41 eV） 和相位匹配的二次谐波代。 结构分析和第一性原理计算表明，（C₃H₅N₂）（H₂PO₄） 晶体中的大双折射是由相邻的伪二维插层之间的紧密反平行排列引起的，其中平面 （C₃H₅N₂）⁺基序在光学特性中起主导作用。