Na7Fe4(PO4)6: a mixed-valence iron phosphate containing a tetramer of edge-sharing FeO6 octahedra

Abstract

A novel mixed-valence iron phosphate, Na₇Fe₄(PO₄)₆, has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction and Mössbauer spectroscopy. The compound crystallizes in the trigonal space group Rc with a= 13.392(2), c= 17.858(3)Å, U= 2773.7(8)ų, Z= 6 and R= 0.0179. The structure consists of tetrameric units of edge-sharing FeO₆ octahedra, which are connected by corner-sharing PO₄ tetrahedra to form a three-dimensional framework structure, enclosing a network of cavities where the sodium cations reside. Mössbauer spectroscopy supports the presence of one Fe^II and three Fe^III in a tetrameric unit.