Large optical anisotropy in noncentrosymmetric phosphate with pseudo 2D intercalated layer

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.