Model of silicooxygen ring vibrations

In order to assign the bands in the IR spectra of silicates to the appropriate normal vibrations, a vibrational model has been proposed, A complex silicooxygen ring is considered as a 'unit cell' composed of the appropriate number of [SiO4](4-) tetrahedra. According to this model, in the ring silicates spectra we have to observe bands due to internal vibrations of individual tetrahedra and bands corresponding exclusively to the ring structure. Change in the tetrahedra symmetry from Td (ideal tetrahedron) to C-2v (tetrahedron in a ring) and then to the ring symmetry: D-3h, D-4h and D-6h (ideal rings) with respect to reducible representations makes it possible to differentiate between the bands due to ring structure (pseudo-lattice vibrations) and internal modes of tetrahedra. It has been established that in the case of all ideal rings there is only one IR active vibrational mode, namely the one symmetric with respect to the axis of the highest fold, i.e. A(2)" in the case of 3-membered rings and A(2u) in the case of 4- and 6-membered rings. The model proposed has been verified for different membered ring silicates. (C) 1998 Elsevier Science B.V.