Quantum chemical modelling of the structure and properties of hypervalent defects in vitreous SiO2 and GeO2

A cluster approximation using a semiempirical MNDO-PM3 scheme is used to study the structure and properties of the defect structures which develop in vitreous SiO2 and GeO2 during the interaction of the previously discovered most probable defects (two-member cycles, fragments with double bonds O = A < (A = Si, Ge) and open chains) with valence-saturated segments of a fracture surface. During the interaction of open chains with such a surface, defects with a large dipole moment (up to 15-20 D) are formed, which may create anisotropic highly polarized regions in the glass. The bonds around hypervalent centers are weakened, and the characteristics of the newly formed and already existing bonds approach one another; that is, in a grouping of this sort, other decay paths may exist that change the direction of fracture. In structures formed by the interaction of O = A < defects and two-member cycles with the surface, hypervalent bonds are easily broken; that is, the hypervalent configuration is transformed into an ordinary one. In a number of cases, the potential surface contains two or three minima with similar energies, separated by low or moderate potential barriers. (C) 1999 American Institute of Physics. [S1063-7834(99)01408-2].