APPLICATION OF THE DV-X-ALPHA CLUSTER METHOD TO CALCULATIONS OF THE ELECTRONIC-STRUCTURE OF SILICATE AND PHOSPHATE-GLASSES

The electronic structure of simple silicate and phosphate clusters, which are assumed to exist in rapidly quenched Li2O-SiO2 and Li2O-P2O5 glasses, has been calculated by the discrete variational (DV)-Xalpha molecular orbital method. The calculations have been carried out for several sizes of clusters, namely, tetrahedral units (SiO44- and PO43-), dimers (Si2O76- and P2O74-), and trimers (Si3O96- and P3O93-). For both systems the valence electronic state is characterized by an occupied valence band, with O 2p components, and an unoccupied band of Si or P orbitals, separated by an energy gap. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases with increasing cluster size. A Mulliken population analysis shows that the negative effective charge on the non-bridging oxygens is decreased with decreasing energy gap. The bond order of the Si-O and P-O bonds, and thus the strength of these bonds, is increased with increasing cluster size.