Vibrational studies of silicates and germanates-V. I.R. and Raman spectra of pyrosilicates and pyrogermanates with a linear bridge

The i.r. and Raman spectra of the pyro-silicates and -germanates of the type A2Pb2Si2O7 (A = K, Rb, Cs) have been investigated and compared with the spectrum deduced from a group theoretical analysis. The results are fully consistent with a true D3d symmetry of the Si2O7 or Ge2O7 group. The stretching frequencies of the Si2O7 and Ge2O7 groups have been assigned with the help of isotopic data on Si and Ge: these results, together with the comparison of i.r. and Raman spectra, show that the highest i.r. frequency must be assigned to the antisymmetric stretch of the Si-O-Si (or Ge-O-Ge) bridge, and not of the terminal SiO3 (or GeO3) group. The extension of these assignments to thortveitite Sc2Si2O7 and other pyro compounds with a linear bridge is briefly discussed. Reasonable assignments can also be proposed for the bending modes, but unexpected values of isotopic shifts suggest that some of them at least are strongly coupled with torsional and/or external modes. Finally, some of the external modes have been identified as arising from a translation of either the alkali cation, or of the lead atom. © 1973.