Fe_2.5[BP₂O₇(OH)₂][PO₃(OH)][PO₃(O_0.5OH_0.5)] · H₂O, a new phosphate-borophosphate with a microporous structure

A new phosphate-borophosphate Fe_2.5³⁺[BP₂O₇(OH)₂][PO₃(OH)][PO₃(O_0.5OH_0.5)] · H₂O, space group P12₁/n, is obtained under hydrothermal conditions. Blocks (Fe³⁺(PO₄)₆)¹⁵⁻ consisting of vertex-sharing (Fe1,Fe2)O₆ octahedra and six PO₄ tetrahedra are distinguished in the structure of the new phase, which was determined without preliminary knowledge of the chemical formula. Such blocks are known for many phosphates (borophosphates), germanates, gallates, and silicates. Blocks form layers connected by BO₄ tetrahedra into a framework with large pores reaching ∼9.5 Å, which are occupied by water molecules. The out-of-layer octahedral position of the Fe3 atom is split and occupied statistically. The anion radical is characterized as a phosphate-borophosphate: it consists of two isolated PO4 tetrahedra and a borophosphate soro group [BP₂O₇(OH)₂] first found in NaIn[BP₂O₈(OH)]. A layer of octahedra is characterized by higher local symmetry corresponding to the orthorhombic group Pm2₁n