Vibrational spectrum of akermanite-like silicates and germanates

The i.r. and Raman spectra of A112B11X2O7 pyro-silicates and -germantes having the akermanite, or an akermanite-like, structure are reported and interpreted with the help of a group theoretical analysis, and 24Mg-26Mg and 28Si-30Si isotopic shifts. The stretching vibrations of the Si2O7 or Ge2O7 groups are generally free from interactions with the lattice frequencies whereas some of the bending vibrations of these groups are strongly mixed up with vibrations of the B11O4 tetrahedra. The existing data on the vibrations of these tetrahedral groups have been confirmed and extended. The XOX bridge frequencies of the pyro groups have been identified; the value of the bridge angle deduced from these frequencies is in satisfactory agreement with the literature values obtained by X-ray diffraction. © 1979.