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

Two novel aluminophosphates, Al 2 (HPO 3 ) 2.26 (HPO 4 ) 0.74 and Al 2 (PO 4 ) 2 (H 2 O) 4, were synthesized using hydrothermal methods, while another polar aluminophosphite, Al[P 2 O 3 (OH) 3 ](H 2 O), was obtained via a low-temperature flux method. They crystallized in the nonpolar space group P 6 3 /m and polar space groups of P 2 1 2 1 2 1 and P 6 3 mc, respectively. The basic units of Al 2 (HPO 3 ) 2.26 (HPO 4 ) 0.74 consist of PO 4 /PO 3 and AlO 6 groups, in which one AlO 6 octahedron coordinated with six PO 4 tetrahedra/PO 3 pyramids and one PO 4 tetrahedron/PO 3 pyramid linked three Al 2 O 9 dimers. However, the fundamental units for Al 2 (PO 4 ) 2 (H 2 O) 4 are PO 4 tetrahedra, AlO 6 octahedra, and AlO 5 trigonal bipyramids, in which one PO 4 tetrahedron is connected with three Al 2 O 10 dimers and one Al 2 O 10 dimer is linked with six PO 4 tetrahedra, forming its novel three-dimensional framework structure. Importantly, Al[P 2 O 3 (OH) 3 ](H 2 O) and Al 2 (PO 4 ) 2 (H 2 O) 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 Al 2 (PO 4 ) 2 (H 2 O) 4 and Al[P 2 O 3 (OH) 3 ](H 2 O) are discussed.