CHANGES IN THE STRUCTURE OF ALKALI-METAL SILICATE-GLASSES WITH THE TYPE OF NETWORK MODIFIER CATION - AN ABINITIO MOLECULAR-ORBITAL STUDY

The structure of alkali-metal silicate glasses has been investigated by ab initio molecular orbital calculations on clusters H6Si2O7 and H4Si2O7M2 (M = Li, Na, K). The optimized geometries at the 3-21G* level are in good agreement with the experimental values obtained by diffraction techniques. Our calculations have further revealed that for a given alkali-metal concentration the silicon-bridging oxygen bond length increases while the silicon-nonbridging oxygen bond length decreases in the order Li, Na, K. Spectroscopic data such as photoelectron, X-ray emission, and UV excitation energies are satisfactorily reproduced by the calculations, and Si-29 NMR chemical shifts are interpreted in terms of charge distributions calculated for the clusters. Both Si and O d functions contain substantial populations although the O d orbitals hardly contribute to the Si-O overlap populations. Activation energies for conduction of alkali-metal cations are estimated on the basis of the calculated charges and optimized geometries. We also discuss the mechanism of the charge transfer from alkali-metal to network-forming atoms in the course of the reaction between alkali-metal oxides and the Si-O-Si network.