LOCAL-STRUCTURE OF SODIUM ALUMINOSILICATE GLASS - AN AB-INITIO MOLECULAR-ORBITAL STUDY

Ab initio molecular orbital theory was applied to investigate the local structure of sodium aluminosilicate, sodium silicate, and silica glasses. Several model clusters (H8Si2AlO10Na, H6Si3O10Na2, H7Si3O10Na, and H8Si3O10) were employed, and their geometries were optimized at the STO-3G level. Single-point calculations were then performed at the 3-21G+d(Si) and the 3-21G+d(Si,Al) levels by using the STO-3G geometries. The calculations have shown the following. (1) Even when the Na+ ions are introduced into glasses as charge compensators of the [AlO4]- tetrahedra, they can affect the local structure of the Si-O-Si network to increase the Si-O(b) (O(b) indicates bridging oxygen) bond length. (2) The. charge compensating Na+ ions interact symmetrically with the [AlO4]- tetrahedra, and the interaction is substantially ionic. (3) Al d orbitals are necessary to describe the realistic charge distribution of the cluster, suggesting that the Al-O bonds in the [AlO4]- tetrahedra have considerable pi-bond character as well as sigma-bond character. On the basis of the calculated results, the effects of the replacement of SiO2 by Al2O3 on the ionic conductivity and on the chemical durability of silicate glasses were discussed. We have also shown that the previous spectroscopic data such as X-ray photoionization, X-ray emission, and UV excitation energies for sodium aluminosilicate glasses are satisfactorily reproduced by the calculation and that the Si-29 chemical shifts can be interpreted in terms of the present results of a Mulliken population analysis.