Electronic structure of SiO2:: Charge redistribution contributions to the dynamic dipoles/effective charges of the infrared active normal modes

This article applies ab initio calculations at the (i) Hartree-Fock self-consistent field single determinant and (ii) configuration interaction multideterminant expansion levels to study the diagonal components of the dynamic dipoles, Deltamu(i)/Deltaq(j) with i = j, or equivalently infrared effective charges, e(i)(*) associated with asymmetric. bond stretching nu(y), symmetric bond stretching nu(z), and out-of-plane bond rocking nu(x), normal mode infrared active vibrations of noncrystalline SiO2. The normal mode dynamic dipoles (hereafter, Deltamu/q(i)) are decomposed into equilibrium charge density (ionic) and orbital variation (charge redistribution) contributions. The calculations are based on small clusters in which Si-O-Si groups are connected through O atoms to embedding Si atom terminators Si* that emulate the connectivity of these Si-O-Si groups to the SiO2 continuous random network. Values of Deltamu/Deltaq(i) have been determined as a function of the Si-O-Si bond angle a at the bridging O-atom sites, and agree with values obtained from analysis of infrared spectra. Finally, the ab initio calculations are extended to noncrystalline silicon-carbon alloys and silicon nitride, and values of Deltamu/Deltaq(i) are determined for infrared active vibrations associated with N- and C-atom asymmetric stretching normal mode motions. The normalized equilibrium charge density or ionic contributions of the O, N, and C atoms follow trends expected on the basis of their relative Pauling electronegativities in bonds with Si. (C) 2002 American Vacuum Society.