Design, Synthesis, and Improvement of Nonlinear Optical Properties from Aluminophosphite to Aluminophosphate.

Two novel aluminophosphates, Al(HPO)(HPO) and Al(PO)(HO), were synthesized using hydrothermal methods, while another polar aluminophosphite, Al[PO(OH)](HO), was obtained via a low-temperature flux method. They crystallized in the nonpolar space group 6 and polar space groups of 222 and 6mc, respectively. The basic units of Al(HPO)(HPO) consist of PO/PO and AlO groups, in which one AlO octahedron coordinated with six PO tetrahedra/PO pyramids and one PO tetrahedron/PO pyramid linked three AlO dimers. However, the fundamental units for Al(PO)(HO) are PO tetrahedra, AlO octahedra, and AlO trigonal bipyramids, in which one PO tetrahedron is connected with three AlO dimers and one AlO dimer is linked with six PO tetrahedra, forming its novel three-dimensional framework structure. Importantly, Al[PO(OH)](HO) and Al(PO)(HO) demonstrate moderate second-harmonic generation effects in the UV region, with ∼0.15 and ∼0.68 × KDP, respectively. The syntheses and topological structures, compositional analysis, thermal behavior, FT-IR spectral characterization, Raman spectral analysis, and UV-vis diffuse reflectance along with the NLO performances of Al(PO)(HO) and Al[PO(OH)](HO) are discussed.