Quantum-chemical simulation of the transformations of continuous random network in the vitreous GeO2статья
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Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.
Аннотация:The relative stability of cyclic structures in vitreous g-GeO2, resistance to breakage, and switching of bonds with the participation of hypervalent structures are studied within the cluster approach at the semiempirical MNDO-PM3 level. It is found that the specific stabilities of cycles with three and more GeO units are close to each other, and the six-membered rings possess the highest stability. The four-membered cycles exhibit the least resistance to the breakage due to the possibility of barrierless transformation into two two-membered rings. The destruction of larger cycles does not always give rise to the new bonds, because this requires a considerable deformation of the rigid environment, and the energy necessary for the cycle partitioning increases 2-3 times. Therefore, the four-membered rings are the weakest links in the g-GeO2 continuous random network. Upon heating, the network predominantly breaks down along the surfaces with the maximum concentration of four-membered cycles into the clusters surrounded mainly by two-membered cycles and Ge=O-type defects. The interaction of such fragments with each other and cycles of different sizes is possible without the overcoming of substantial barriers and leads to the ''sticking'' together of splinters. If the positions of the boundary atoms are not fixed in the cyclic structures, a minor energy needs to deform the cycle. This is accompanied by the formation of hypervalent regions with the threefold-coordinated oxygen atoms and fivefold- and sixfold-coordinated germanium atoms. The barrierless switching of bonds can occur in these regions, which brings about the transformation of the g-GeO2 structure.