Solid-state synthesis and characterization of novel aluminophosphates, A<inf>3</inf>Al<inf>2</inf>P<inf>3</inf>O<inf>12</inf> (A = Na, K, Rb, Tl): Influence of A⁺ Ions on the coordination of aluminum

Four aluminophosphates, A3Al2P3O12 (A = Na, K (1), Rb (2), Tl (3)), have been synthesized by solid-state reactions and characterized by X-ray diffraction and NMR and IR spectroscopic techniques. Aluminum has trigonal bipyramidal coordination in the thallium compound and tetrahedral coordination in the others. Potassium, rubidium and thallium analogues have been structurally characterized by single-crystal X-ray diffraction and found to possess three-dimensional (Al2P3O12)3- anionic frameworks with channels occupied by A+ countercations. These frameworks are built from corner connections of PO4 tetrahedra with AlO4 tetrahedra in 1 and 2 and with AlO5 trigonal bipyramids in 3. Pertinent crystal data are as follows: for 1, orthorhombic space group Pna21, a = 8.685(2) Å, b = 16.947(2) Å, c = 8.458(3) Å, Z = 4; for 2, orthorhombic space group Cmc21, a = 17.164(2) Å, b = 8.6270(6) Å, c = 8.8140(14) Å, Z = 4; for 3, orthorhombic space group Pna21, a = 6.1478(15) Å, b = 10.396(3) Å, c = 17.787(5) Å, Z = 4. Compound 3 is a rare example of an oxide possessing aluminum exclusively in trigonal bipyramidal coordination.