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

Two novel aluminophosphates, Al₂(HPO₃)_2.26(HPO₄)_0.74 and Al₂(PO₄)₂(H₂O)₄, were synthesized using hydrothermal methods, while another polar aluminophosphite, Al[P₂O₃(OH)₃](H₂O), was obtained via a low-temperature flux method. They crystallized in the nonpolar space group P6₃/m and polar space groups of P2₁2₁2₁ and P6₃mc, respectively. The basic units of Al₂(HPO₃)_2.26(HPO₄)_0.74 consist of PO₄/PO₃ and AlO₆ groups, in which one AlO₆ octahedron coordinated with six PO₄ tetrahedra/PO₃ pyramids and one PO₄ tetrahedron/PO₃ pyramid linked three Al₂O₉ dimers. However, the fundamental units for Al₂(PO₄)₂(H₂O)₄ are PO₄ tetrahedra, AlO₆ octahedra, and AlO₅ trigonal bipyramids, in which one PO₄ tetrahedron is connected with three Al₂O₁₀ dimers and one Al₂O₁₀ dimer is linked with six PO₄ tetrahedra, forming its novel three-dimensional framework structure. Importantly, Al[P₂O₃(OH)₃](H₂O) and Al₂(PO₄)₂(H₂O)₄ 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 Al₂(PO₄)₂(H₂O)₄ and Al[P₂O₃(OH)₃](H₂O) are discussed.