Vibrational studies of olivine-type compounds-II Orthophosphates, -arsenates and -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.