Site-selective substitutions and magnetic structures in ( Fe , Co ) 2 ( P , Si ) compounds probed by x-ray linear dichroism and neutron diffraction

We investigate the crystallographic site occupancies and magnetic structures of Fe_1.93-xCoₓP_1-ySiy 0≤x≤0.6 and 0≤y≤0.4 compounds using a combined x-ray absorption/linear dichroism (XAS/XLD) and neutron powder diffraction approach. Co K-edge XLD, supported by fdmnes simulations, provides direct spectroscopic evidence that Co preferentially occupies the tetrahedral $3f$ site of the hexagonal Fe₂P-type structure. In contrast, P K-edge XLD and neutron diffraction show that Si is distributed over both $1b$ and $2c$ metalloid sites, with at most a weak preference for $2c$, rather than the commonly assumed exclusive $2c$ occupation. Neutron diffraction further establishes that the hexagonal Fe₂P-type compounds adopt a collinear ferromagnetic structure with ordered moments along the hexagonal c axis, whereas the orthorhombic Co₂P-type phase observed at high Co content exhibits reduced and canted magnetic moments. Across the hexagonal series, Si substitution enhances the total magnetic moment through contributions from both $3f$ and $3g$ sites, whereas Co substitution suppresses it primarily via a reduction of the $3f$ site moment. These results provide an experimental basis for understanding why global structural parameters such as c/a alone are insufficient to rationalize the magnetic trends in (Fe,Co)₂(P,Si) and demonstrate that site-selective occupancy must be included in structure-property relationships in Fe₂P-derived hard-magnetic materials.