LOW-FREQUENCY VIBRATIONAL-SPECTRA OF CRYSTALLINE AND GLASSY METABORATES BA2MII(B3O6)2 (MII = MG, CA, NI, CO, CD)

The low frequency vibrational spectra (Raman and IR) of crystalline me taborates Ba2M(II)(B3O6)2 (M(II) = Mg, Ca, Ni, Co, Cd) are interpreted on the basis of a factor group analysis and with the help of Mg-24-Mg -26 and Ca-40-Ca-44 isotopic shifts. The far-IR spectra of the glasses appear as the envelope of the spectra of the corresponding crystallin e compounds for M(II) = Mg, Ca and Cd, but they exhibit an additional broad absorption centred near 400 cm-1 for M(II) = Co, Ni and Zn. This absorption is assigned to a fourfold coordination of part of the M(II ) cations, the remaining being in sixfold coordination (as in the crys tal) and responsible for a broad absorption in the 300-200 cm-1 region . The possible existence of correlations between coordination and vibr ational frequencies is discussed for either alkali and alkaline-earth cations, or transition elements.