Na-Alternative to Tinsleyite Obtained under Hydrothermal Conditions: Crystal Structure and Comparative Crystal Chemistry

The synthesis and characterization of a new aluminophosphate, Na2Al2O(PO4)2·0.12H2O obtained as single crystals, is reported. Centrosymmetric tetramers built from AlO5 polyhedra sharing edges and vertices, represent the distinguished feature of the compound. These tetrameric units of AlO5 bipyramids are cross-linked by PO4 tetrahedra to form two-periodic slabs alternating with Na+ ions and a small amount of H2O molecules. The Na2Al2O(PO4)2·0.12H2O with an original crystal architecture is chemically and structurally related to the mineral tinsleyite, KAl2(PO4)2(OH)·2H2O. Similar clusters of Al-centered polyhedra are essential building blocks of both monoclinic structures. The main difference between them consists of the type of the Al coordination by O atoms: in tinsleyite, the clusters are designed from AlO4(OH)2 and AlO4(OH)(H2O) octahedra. In both cases, alkali Na or K atoms significantly distinct in size, act as structure regulating agents, determining the character of the developing crystal architecture. The flexibility of aluminophosphate constructions allows them to self-organize around structure-forming Na+ or K+ ions into anionic layers in Na2Al2O(PO4)2·0.12H2O or a framework (tinsleyite). The synthesis of sodium aluminophosphate under mild hydrothermal conditions and the topological resemblance of its structure with that of the mineral tinsleyite suggest a high probability of a mineral equivalent of the Na2Al2O(PO4)2·0.12H2O in nature.