MODEL OF SILICOOXYGEN RING VIBRATIONS

In order to assign the bands in the IR spectra of silicates to the app ropriate normal vibrations, a vibrational model has been proposed, A c omplex silicooxygen ring is considered as a 'unit cell' composed of th e appropriate number of [SiO4](4-) tetrahedra. According to this model , in the ring silicates spectra we have to observe bands due to intern al vibrations of individual tetrahedra and bands corresponding exclusi vely to the ring structure. Change in the tetrahedra symmetry from Td (ideal tetrahedron) to C-2v (tetrahedron in a ring) and then to the ri ng symmetry: D-3h, D-4h and D-6h (ideal rings) with respect to reducib le representations makes it possible to differentiate between the band s 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 sym metric with respect to the axis of the highest fold, i.e. A(2)'' in th e 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 rin g silicates. (C) 1998 Elsevier Science B.V.