Large Optical Anisotropy in Noncentrosymmetric Phosphate with Pseudo 2D Intercalated Layer

On account of the high Td 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 the planar and tetrahedral motifs. The arrangements of (C3H5N2)+ and (H2PO4)− 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.