Neutron diffraction study of the cation distributions in the systems Fe_7-xM_x(PO₄)₆ (M = Mn or Co)

The cation distributions in the mixed-metal phosphate systems Fe_7-xM_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)₆ 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^IIFe₄^IIIMn₂^II(PO ₄)₆' and 'Fe₄^IIICo₃^II(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.