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 valen ce in Ce2Fe14B, Ce2Fe14BHx, and Ce2Fe17. The crystal structures of the se compounds are isomorphic with those of technologically important pe rmanent magnet materials. Confirming other experiments, we find that t he Ce ion in Ce2Fe14B is in a strongly mixed-valent alpha-like state w hich is incompatible with a local 4f moment. Comparison of the spectro scopically determined valences of the three materials demonstrates a s hift toward a gamma-like cerium state, which supports a magnetic momen t, as the steric volume of the Ce site(s) increases. Our results stron gly suggest that the volume of the rare-earth site is the principal fa ctor controlling the Ce chemical valence in these systems.