Abstract
The electronic (350–900 nm), infrared and Raman (700–30 cm-1) spectra of polycrystalline samples of the mixed-valence (Ce3+/Ce4+) oxysulfide compound Ce4O4S3 were investigated. The compound is a low-gap semiconductor with intervalence transitions in the near-infrared (ca. 850 nm) and visible (ca. 540 nm) regions, so that resonance-enhanced Raman spectra were obtained in the 450–650 nm range. From some polarized Raman data and a comparison with the vibrational results for Ce2O2S and Ce2.0O2.5S reported in Parts I and II, tentative infrared and Raman assignments are proposed. These results allow one mainly to localize the totally symmetry Ag modes and to suggest that the higher wavenumber signals in the 580–410 cm-1 range are due to the stretching vibrations of the shortest Ce4+—O bonds. In addition, complete lattice dynamic calculations on the orthorhombic structure (D2h9, Z=2) of Ce4O4S3 were performed by using a valence force field potential function transferred from the force fields previously obtained for Ce2O2S and Ce2.0O2.5S. With these calculations one can satisfactorily reproduce the whole experimental wavenumbers, propose more confident vibrational assignments and confirm that the Ce4+—O bond strengths are definitively stronger than the Ce3+—O counterparts. Such conclusions are corroborated by the establishment of the Raman excitation profiles which maximize near 568 nm. The largest enhancements are obtained for the totally symmetric modes at 509, 412 and 350 cm-1, which contain important potential energy distributions in the Ce4+–O stretching force constants, and at 132 cm-1, which corresponds to a deformation or compressional motion (along b) within the (Ce4O4)6+ units. These vibrations are thus the more effective coupling modes in the intervalence charge-transfer mechanisms. © 1997 John Wiley & Sons, Ltd.
Published Version
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