Études vibrationnelles des composés de type olivine-I. Les spectres i.r. et Raman des espèces isotopiques de Mg2SiO4

A group theoretical analysis has been carried out for the olivine structure, and the isotopic species of the type compound Mg2SiO4 have been investigated by Raman and i.r. spectroscopy. New assignments have been deduced from 24Mg-26Mg and 28Si-30Si isotopic shifts. The 1000-475 cm-1 region of the i.r. spectrum is essentially related to internal (stretching and bending) modes of the SiO4 tetrahedron whereas most of the lower frequency bands must be assigned to external modes. The SiO4 totally symmetric, stretching frequency (v1) is observed at 842 cm-1 in the i.r. spectrum where it is unequivocally identified through the nearly complete lack of a 28Si-30Si mass effect, whereas this mass effect appears in the v3 components by a 10-15 cm-1 shift. It has also been possible to evidence a number of vibrational interactions, such as the mixing of the symmetric and antisymmetric stretching, Raman-active, modes, the simultaneous participation of both Mg and Si atoms to some translational modes, and the mixing of some translational and rotational modes. The significance of force constant calculations is also briefly discussed. © 1973

Études vibrationnelles des composés de type olivine-II Orthophosphates, -arséniates et -vanadates AIBIIXVO4

The i.r. and Raman spectra of ABIIXO4 phosphates, arsenates and vanadates of olivine structure have been investigated, and their interpretation is discussed on both theoretical and experimental bases. Assignments have been deduced from the systematic investigation of pure compounds, solid solutions and isotopic species. For all the compounds investigated, the stretching frequencies of the tetrahedral XO4 anion may be considered as internal modes, but the corresponding bending frequencies can be considered as "internal" only for the phosphates and for the sodium compounds. For the arsenates and vanadates where A is lithium, the 6Li7Li isotopic shifts reveal important interactions between the Li translations and the AsO4 or VO4 bending vibrations. It has also been possible to evidence a number of vibrational interactions in the case of the external vibrations. Finally, we have further investigated to what extent the internal stretching vibrations of the complex anion are modified by the chemical nature of the bivalent cation BII and an original correlation with the second ionization potential of the BII cation is proposed. © 1974