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.