Metalophosphates with mixed-polyhedral framework structures: syntheses and characterization

Abstract

Three new microporous compounds with octahedral-pentahedral-tetrahedral framework structures (so called OPT materials) with metalophosphate composition were prepared under solvothermal conditions and characterized by X-ray diffraction and Mössbauer spectroscopy. Ironalumino- and irongallophosphates are violet and brown-violet crystals with NH₄[Fe_1.15Al_1.85P₃O₁₂](H₂O)₂ and NH₄[Fe_1.05Ga_1.95P₃O₁₂] (H₂O)₂ composition. Nickelgallophosphate NH₄[Ni_1.0Ga_2.0P₃O₁₂](H₂O)₂ forms light yellow lozenges. All products have isotype crystal structures in space group C2/c. The framework structures are built of corner- and edge-sharing distorted octahedra, trigonal bipyramids and phosphate tetrahedra. Iron occurs in two different valence states and occupies different polyhedra types in the structures, confirmed by Mössbauer spectroscopy. The compounds have three-dimensional channel systems which are filled by ammonium groups and water molecules, forming a strong hydrogen bonding system. Upon heating, the frameworks collapse during deammonization at temperatures below 573 K. A deformation analysis of the different coordination polyhedra in comparable compounds was performed and the structural relationship to a similar ammonium gallophosphate is discussed.