Bonding anomalies in the rare earth sesquioxides

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Raman and IR vibrational spectra of C-type rare earth sesquioxides exhibit anomalous wavenumber decreases for europium (f6) and ytterbium (f13) oxide which have not been previously reported. Because of the large shifts observed for the Raman spectra, the IR band positions have been re-examined. Smaller, but definite decreases have been observed in the IR spectra as well. Scatter in the literature data had obscured these anomalies. The anomalies correlate with thermodynamic properties and indicate weaker bonding for these two oxides. It is suggested that these anomalies are caused by electron-phonon interaction, involving low-lying f-states which are below the Fermi level for europium and ytterbium. Therefore, these data provide another example of a system in which 4f electronic structure significantly influences chemical bonding. We suggest that it is due to the presence of the unique, low-lying electronic energy levels in these two elements, and not simply the result of the number of f electrons (half-full or full). Literature results suggest the anomalies are not due to "mixed valence" for Eu2O3 and Yb2O3; however, further research is needed to explain the origin more completely. Raman band positions are reported for the first time for several of the oxides. © 1987.

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Journal of Physics and Chemistry of Solids