Li3[AlP2O7F(OH)](H2O)0.5 and A[Al2(PO4)2F(H2O)](A = K, Rb): Counter Cation Templates Derived Three Polar Alkaline Metal Fluoroaluminophosphates

Three novel alkali metal fluoroaluminophosphates, Li₃[AlP₂O₇F(OH)](H₂O)_0.5 and A[Al₂(PO₄)₂F(H₂O)](A = K, Rb), were designed and synthesized by using low-temperature flux methods. They crystallized in polar space groups P₄ and P2₁2₁2₁, respectively. Li₃[AlP₂O₇F(OH)](H₂O)_0.5 features a unique two-dimensional layered structure of fluoroaluminophosphate [AlP₂O₇F(OH)]^3–_n, composed of alternately connected AlFO₅ octahedra and PO₄ tetrahedra. In contrast, with counter cations from Li⁺ to K⁺/Rb⁺, two new crystals of A[Al₂(PO₄)₂F(H₂O)](A = K, Rb) have been obtained. They possess a distinct three-dimensional anionic framework, [Al₂(PO₄)₂F]⁻_n, consisting of corner-sharing AlO₅ triangular bipyramids, AlFO₅ octahedra, and PO₄ tetrahedra, revealing intersecting open tunnels along the [010], [001], and [111] directions. Notably, both Li₃[AlP₂O₇F(OH)](H₂O)_0.5 and K[Al₂(PO₄)₂F(H₂O)] exhibit moderate second harmonic generation effects. Besides, porous material of K[Al₂(PO₄)₂F(H₂O)] exhibited remarkable ionic exchange properties with Cs⁺, implying its potential utility in the remediation of radioactive waste and offering a promising solution for managing nuclear contaminants. This research reports on their syntheses, topological structures, elemental analysis, thermal stability, IR Raman spectroscopy, UV–Vis diffuse reflectance spectroscopy, ion exchange property, and nonlinear optical characteristics.