Crystal structure and Mossbauer spectroscopy of a new iron phosphate Mg sub(2.88)Fe sub(4.12)(PO sub(4)) sub(6)

A new mixed valence iron phosphate Mg sub(2.88)Fe sub(4.12)(PO sub(4)) sub(6) has been prepared as single crystals by the flux method and as a powder by solid state reaction and its crystal structure has been determined by X-ray diffraction. This compound crystallises in the triclinic system with space group P1 and a = 6.325(5) A, b = 7.911(3) A, c = 9.271(3) A, [alpha] = 104.62(1)[degrees], [beta] = 108.41(1)[degrees] and [gamma] = 101.24(1)[degrees] and Z = 1. Its structure is similar to that of the mineral howardevansite. It consists of Fe sub(2)O sub(10) bioctahedral units and MgO sub(5) trigonal bipyramids sharing edges to form infinite zigzag chains. Such chains are linked to each other by MO sub(6) (0.88 Mg super(2+) + 0.12 Fe super(2+)) octahedra and PO sub(4) tetrahedra resulting in a 3D covalent framework. Magnetic measurements indicated two successive transitions at 46 K and 22 K. Mossbauer spectroscopy confirmed the mixed valence of iron and gave accurate information about its local environment.