We develop a theoretical electronic structure based approach suitable for the study of complex silicas, and apply it to three different silica polymorphs. The method is first principles but simplified to cope with large systems. Charge transfer between ions is included in a self-consistent fashion. The method is found to be adequate to describe the relative energies and the electronic structure of silica and is used to construct a simplified model energy for the structural transition of high cristobalite. In addition we have studied the moderately complex silica, melanophlogite. Melanophlogite, although a clathrate structure belonging to the group of silica polymorphs known as clathrasils, is found to be only 10 kJ/mol above α-quartz, in accord with the recent thermochemical study of the stability of zeolites.

• Received 14 March 1995

DOI:https://doi.org/10.1103/PhysRevB.52.1618