Site Occupancy Determination in Th₂Zn₁₇- and TbCu₇-types Sm₂Fe_17–xCo_x Compounds using Synchrotron Resonant Diffraction

Abstract

Sm 2 Fe 17 compounds are high performance permanent magnets. Cobalt substitution allows to further improve their magnetic properties. Depending on the thermal treatment cobalt substituted compounds can be synthesized either in the TbCu 7 (disordered) or in the Th 2 Zn 17 (ordered) structure type. Rietveld refinement of the number of transition metal dumbbells replacing rare earth atoms from synchrotron powder diffraction data shows that the TbCu 7 disordered structure has the same composition as the ordered one (transition metal to rare earth ratio equal to 8.5). Then cobalt site occupancies have been determined in both structures using synchrotron resonant (anomalous) diffraction. Cobalt is found to be absent from the dumbbell sites. The diffraction results are confirmed by Mössbauer spectroscopy.