Structure and bonding in cerium oxysulfide compounds. III—Electronic, infrared and resonance Raman spectra of and lattice dynamics calculations on mixed-valence Ce4O4S3

✍ Scribed by C. Sourisseau; R. Cavagnat; M. Fouassier; R. Mauricot; M. Evain

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 446 KB
• Volume: 28
• Category: Article
• ISSN: 0377-0486
• DOI: 10.1002/(sici)1097-4555(199712)28:12<979::aid-jrs188>3.0.co;2-h

✦ Synopsis

The electronic (350-900 nm), infrared and Raman (700-30 cm-1) spectra of polycrystalline samples of the mixedvalence (Ce3'/Ce4') oxysulÐde compound were investigated. The compound is a low-gap semiconductor Ce 4 O 4 S 3 with intervalence transitions in the near-infrared (ca. 850 nm) and visible (ca. 540 nm) regions, so that resonanceenhanced Raman spectra were obtained in the 450-650 nm range. From some polarized Raman data and a comparison with the vibrational results for and reported in Parts I and II, tentative infrared Ce 2 O 2 S Ce 2.0 O 2.5 S and Raman assignments are proposed. These results allow one mainly to localize the totally symmetry modes A g 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'ÈO bonds. In addition, complete lattice dynamic calculations on the orthorhombic structure Z = 2) of were performed by using a valence force Ðeld potential function transferred from the (D 2h 9 , C e 4 O 4 S 3 force Ðelds previously obtained for and With these calculations one can satisfactorily repro-Ce 2 O 2 S Ce 2.0 O 2.5

S. duce the whole experimental wavenumbers, propose more conÐdent vibrational assignments and conÐrm that the Ce4'ÈO bond strengths are deÐnitively stronger than the Ce3'ÈO counterparts. Such conclusions are corroborated by the establishment of the Raman excitation proÐles 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 units. These vibrations are thus the more e †ective (Ce 4 O 4 )6' coupling modes in the intervalence charge-transfer mechanisms.