Embedded cluster models for electronic states of silicate glasses

The discrete variational (DV) Xα method is applied to the study of the electronic structure of silicate glasses in embedded model clusters. The effects of the cluster size, the size of embedded units, and the SiOSi bond angles on the electronic states are discussed. Embedding units drastically improve the description of the electronic state, when compared to the isolated SiO₄^4- cluster, which is the structural unit of silicate glasses; e.g., the Fermi energy for the embedded cluster becomes smaller when compared to that of the isolated cluster. Furthermore the Si and O ions both become more ionic in the embedded clusters. The electronic states localized around the center of the clusters, however, do not depend critically on either the size of the embedding units or size of the variational cluster. The range that the SiOSi bond angle takes leads to an energy shift and a broadening of each band which can be related to the spectroscopic peak broadening ( XPS, IR, Raman, X-ray and neutron diffraction, etc.) observed in silicate glasses.The ionicities of both the Si and O ions decrease and the SiO bond order increases with increasing SiOSi bond angle.