X-ray-absorption Spectral Study of the R₂Fe₁₇₋ₓMₓ Solid Solutions (R=Ce, Nd and M=Al, Si)


The x-ray-absorption near-edge structure (XANES) spectra obtained at the cerium LIII edge of the Ce2Fe17-xAlx solid solutions and Ce2Fe14Si3, show two absorption peaks characteristic of the 4f1 and 4f0 configurations of cerium, peaks which indicate that cerium is in a mixed valent state in these compounds. All the XANES spectra have been consistently and excellently fit with one sigmoidal function and two Gaussianbroadened Lorentzian functions. The cerium spectroscopic valence obtained from the relative areas of the two peaks decreases from 3.64 to 3.43 between x=0 and 9 in Ce2Fe17-x,Alx, and correlates linearly with the cerium site volume This correlation confirms that the cerium valence is strongly dependent upon steric effects. In contrast the cerium valence obtained from the XANES spectrum of Ce2Fe14Si3 is not determined by stenc effects and indicates, in agreement with other measurements and calculations, that silicon is more covalently bonded with its near-neighbor cerium atoms than is aluminum. The neodymium LIII,-edge XANES spectra of the Nd2Fe17-x Alx solid solutions, where x is 0, 3, and 8, reveal the presence of only trivalent neodymium and an increase of the empty 5d state density when aluminum is substituted in place of iron. XANES measurements at the iron K edee of the Ce2Fe17-xAlx, and Nd2Fe17-1Alx, solid solutions show changes m the relative intensity of the multiple scattering peaks, changes which are related to the changing composition of the first three neighbor shells with increasing aluminum content.



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