Neutron diffraction study of the cation distributions in the systems Fe7 –xMx(PO4)6(M = Mn or Co)

Abstract

The cation distributions in the mixed-metal phosphate systems Fe_{7 –x}M_x(PO₄)₆[(1) M = Mn, x= 2; (2) M = Co, x= 3] have been studied by Rietveld analysis of powder neutron diffraction data. Both systems are synthesised hydrothermally, with subsequent low- temperature (500 °C) annealing. They adopt the Fe₇(PO₄)₅ structure type, which provides four crystallographically distinct metal sites, three of which are octahedral and one five-co-ordinated. Iron is found to be preferentially oxidised over the dopant cation in both cases, giving idealised compositions ‘Fe^IIFe^III₄Mn^II₂(PO₄)₆’ and ‘Fe^III₄Co^II₃(PO₄)₆’. The cation sites are found to be strongly ordered, with Fe³⁺ preferentially occupying the two most distorted octahedral sites, rather than the more regular octahedral site and the five-co-ordinated (trigonal bipyramidal) site, in both cases. These observations may be rationalised in terms of a balance of coulombic, ligand-field, and other considerations.