CsFe 3 (PO 4 ) 3 (OH 2 ) 2 : a mixed-valence iron phosphate containing trimeric units of iron–oxygen polyhedra

A mixed-valence iron phosphate, CsFe3(PO4)3(OH2)2, has been synthesized hydrothermally and characterized by single-crystal X-Ray diffraction and Mössbauer spectroscopy. The compound crystallizes in the monoclinic space group C2/c with a= 13.435(2), b= 10.488(1), c= 9.024(2)Å, β= 108.60(1)°, U= 1205.1(3)Å3 and Z= 4. The structure consists of a three-dimensional framework of vertex-sharing FeIIIO5 trigonal bipyramids, FeIIO4(OH2)2 octahedra and PO4 tetrahedra, enclosing a network of cavities, where the caesium cations reside. Mössbauer spectroscopy confirms the presence of one FeII and two FeIII.