STRUCTURE AND BONDINGS IN CERIUM OXYSULFIDE COMPOUNDS .1. ELECTRONIC,INFRARED AND RESONANCE RAMAN-SPECTRA OF CE2.0O2.5S

As a first stage, the electronic (350-900 nm), infrared and Raman (700 -10 cm(-1)) spectra of polycrystalline samples of the oxysulfide compo und Ce2.0O2.5S were investigated and tentative vibrational assignments are proposed from a comparison with the simpler Ce2O2S parent compoun d, Great changes in the vibrational patterns demonstrate that the two oxysulfides are not isostructural and that the oxidized form cannot be described as a Ce3+-containing peroxide or superoxide system, As a se cond stage, the resonance Raman spectra of Ce2.0O2.5S were recorded wi thin the contour of the first electronic transitions (650-450 nm) on t he absorption edge, The Raman excitation profiles maximize at near 490 nm and exhibit large enhancements for most of the vibrational modes, in particular the higher wavenumber nu Ce-O equatorial stretching mode s at 584 and 548 cm(-1). This shows that the resonant electronic trans ition is more likely an excitonic type and involves a large electronic delocalization, particularly over the cerium-oxygen bonds in the equa torial planes, In conclusion, the Ce2.0O2.5S compound does not look li ke as a mixed-valence sample with distinct Ce3+ and Ce4+ constituent i ons, Class II in Day's notation, as already known in Ce4O4S3; it is be tter described as a Ce4+-containing broadband semiconductor in which l ow-energy excitonic-type transitions implying the intermediate 4f leve ls have to be considered and in which valences do not appear to be fir mly trapped, (C) 1997 John Wiley & Sons, Ltd.