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 Fe7 –xMx(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 ‘FeIIFeIII4MnII2(PO4)6’ and ‘FeIII4CoII3(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.