STRUCTURE AND BONDING IN CERIUM OXYSULFIDE COMPOUNDS .2. COMPARATIVE LATTICE-DYNAMICS CALCULATIONS ON CE2O2S AND CE2.0O2.5S

Using the previously reported vibrational results (infrared, Raman and resonance Raman spectra) for the Ce2O2S and Ce2.0O2.5S solid compound s (see Part I), comparative lattice dynamics calculations were perform ed, These calculations allow one to reproduce satisfactorily the exper imental wavenumbers and to propose more confident vibrational assignme nts for the statistically disordered Ce2.0O2.5S, on the assumption of a D-3 symmetry crystal structure, In particular, it is shown that the cerium to oxygen bonding forces are distinct in the two oxysulfides: t he f(Ce-Ol) stretching force constants increase significantly from 90 N m(-1) in the Ce3+-containing Ce2O2S phase to 110 and 140 N m(-1) for the (Ce-Ol) axial and equatorial forces, respectively, in the Ce2.0O0 .2S oxidation product. The fact that the latter value (140 N m(-1)) co mpares well with that estimated in cerium dioxide CeO2 supports previo us conclusions from the optical study, i.e. that Ce2.0O2.5S is more li kely described as a Ce4+-containing sample. (C) 1997 John Wiley & Sons , Ltd.