Vibrational spectrum of fresnoite (Ba2TiOSi2O7) and isostructural compounds

Two new isostructural compounds with fresnoite, Sr2TiSi2O8 and Ba2TiGe2O8, have been synthesized and investigated by X-ray diffraction and vibrational spectroscopy. The infrared (IR) and Raman spectra of fresnoite and fresnoite-like pyrosilicates are discussed on the basis of a factor group analysis and with the help of 28-30Si and 46-50Ti isotopic shifts. The short Ti-O bond participates to the 900-800 cm-1 region of the spectrum. However, because of an important mixing with some Si-O stretching modes having the same symmetry properties, no frequency is specifically related to the stretching of this bond. An assignment is also proposed for the remaining Si-O stretching frequencies. The existence or absence of a correlation between short Ti-O distances and high vibrational frequencies is briefly discussed. © 1981 Springer-Verlag.