Determination of the Cation Distribution in NiFe₂(PO₄)₂ using Resonant X-ray and Neutron Powder Diffraction

The distribution of divalent iron and nickel over two metal sites of differing coordination geometry in NiFe₂(PO₄)₂, sarcopside, has been investigated by resonant X-ray and time-of-flight neutron powder diffraction. To assess the reproducibility of the X-ray technique, data have been collected from instruments X7A at Brookhaven National Laboratory and 8.3 at the Synchrotron Radiation Source, Daresbury Laboratory, England, using wavelengths λ_X1 = 1.7437 (3) Å and λ_X2 = 1.7434 (1) Å, respectively, close to the Fe²⁺ K edge determined by X-ray absorption near-edge structure. The real part of the anomalous-scattering correction for iron at each energy, f′(Fe)_X1 = −7.81 (9) and f′(Fe)_X2 = −10.16 (6), was determined experimentally by diffraction from Fe₃(PO₄)₂ under identical conditions. Occupancies obtained for iron at the M(1) site were found to be M(1)_X1 = 0.366 (6) and M(1)_X2 = 0.376 (3), compared with M(1)_N = 0.26 (15) from time-of-flight neutron powder diffraction.