Determination of the cation distribution in Fe₂Ni(PO₄)₂ using isotopic substitution and powder neutron diffraction

The distribution of divalent iron and nickel over the two metal sites with differing coordination geometry in Fe2Ni(PO4)2, sarcopside, has been investigated using time-of-flight powder neutron diffraction of nickel isotopically substituted materials. Data from four separate samples were collected using HRPD at ISIS, containing natNi, 58Ni, 60Ni and 62Ni, under identical conditions. The occupancy of iron on the M(1) site was found to be 0.290 (1) from a combined-data-set Rietveld refinement of the three isotopically substituted samples, compared with 0.26 (4) and 0.26 (15) respectively from this and a previous time-of-flight powder neutron diffraction study using natural-abundance nickel, and 0.366 (6) and 0.376 (3) using anomalous X-ray scattering techniques. A critical comparison of isotope substitution neutron diffraction and anomalous X-ray scattering methods for distinguishing nickel and iron from powder data is presented.