Li3[Al(PO4)2(H2O)1.5] and Na[AlP2O7]: from 2D layered polar to 3D centrosymmetric framework structures

Abstract

Two novel alkali metal aluminophosphates, namely, Li₃[Al(PO₄)₂(H₂O)_1.5] and Na[AlP₂O₇], were synthesized through low-temperature flux methods. They crystallized in the P4 and C2/c space groups, respectively. Both structures of Li₃[Al(PO₄)₂(H₂O)_1.5] and Na[AlP₂O₇] were built up by AlO₆ octahedra and PO₄ tetrahedra. The structure of Li₃[Al(PO₄)₂(H₂O)_1.5] was composed of novel two-dimensional (2D) aluminophosphate layers, which contained two types of eight-membered rings (8-MRs) along the c-axis. The structure of Na[AlP₂O₇] featured a unique 3D anionic framework composed of corner-sharing isolated AlO₆ octahedra and PO₄ tetrahedra. Three intersecting tunnels along the [001], [110], and [101] directions could be observed. Importantly, the material Li₃[Al(PO₄)₂(H₂O)_1.5] crystallized in the polar space group P4, showing a moderate second-harmonic generation (SHG) effect. Moreover, Na[AlP₂O₇] exhibited a wide transparency range with a short ultraviolet (UV) cut-off edge (∼190 nm), which indicated its potential application in the deep-UV region. Herein, the syntheses and topological structures of Li₃[Al(PO₄)₂(H₂O)_1.5] and Na[AlP₂O₇], as well as elemental analysis, thermal stability, infrared spectroscopy, UV-vis diffuse reflectance, structural properties, and nonlinear optical (NLO) properties are also discussed.