Steric variation of the cerium valence in Ce2Fe14B and related compounds

Through measurements of the near-edge Ce L3 x-ray absorption structure we have investigated the influence of site volume on the cerium valence in Ce2Fe14B, Ce2Fe14BHx, and Ce2Fe17. The crystal structures of these compounds are isomorphic with those of technologically important permanent magnet materials. Confirming other experiments, we find that the Ce ion in Ce2Fe14B is in a strongly mixed-valent α-like state which is incompatible with a local 4f moment. Comparison of the spectroscopically determined valences of the three materials demonstrates a shift toward a γ-like cerium state, which supports a magnetic moment, as the steric volume of the Ce site(s) increases. Our results strongly suggest that the volume of the rare-earth site is the principal factor controlling the Ce chemical valence in these systems.