STRUCTURE AND BONDING IN CERIUM OXYSULFIDE COMPOUNDS .3. ELECTRONIC, INFRARED AND RESONANCE RAMAN-SPECTRA OF AND LATTICE-DYNAMICS CALCULATIONS ON MIXED-VALENCE CE4O4S3

The electronic (350-900 nm), infrared and Raman (700-30 cm(-1)) spectr a of polycrystalline samples of the mixed-valence (Ce3+/Ce4+) oxysulfi de compound Ce4O4S3 were investigated, The compound is a low-gap semic onductor with intervalence transitions in the near-infrared (ca, 850 n m) and visible (ea. 540 nm) regions, so that resonance-enhanced Raman spectra were obtained in the 450-650 nm range. From some polarized Ram an data and a comparison with the vibrational results for Ce2O2S and C e2.0O2.5S reported in Parts I and II, tentative infrared and Raman ass ignments are proposed, These results allow one mainly to localize the totally symmetry A(g) modes and to suggest that the higher wavenumber signals in the 580-410 cm(-1) range are due to the stretching vibratio ns of the shortest Ce4+-O bonds. In addition, complete lattice dynamic calculations on the orthorhombic structure (D-2k(9), Z = 2) of Ce4O4S 3 were performed by using a valence force field potential function tra nsferred from the force fields previously obtained for Ce2O2S and Ce2. 0O2.5S. With these calculations one can satisfactorily reproduce the w hole experimental wavenumbers, propose more confident vibrational assi gnments and confirm that the Ce4+-O bond strengths are definitively st ronger 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 sym metric modes at 509, 412 and 350 cm(-1), which contain important poten tial energy distributions in the Ce4+-O stretching force constants, an d at 132 cm(-1), which corresponds to a deformation or compressional m otion (along b) within the (Ce4O4)(6+) units, These vibrations are thu s the more effective coupling modes in the intervalence charge-transfe r mechanisms. (C) 1997 John Wiley & Sons, Ltd.