Construction of a model for amorphous tetrahedral materials using ordered units

Abstract

A model for amorphous tetrahedral materials is constructed using 14-atom ‘structural units’ of tetrahedral shape and with diamond-cubic symmetry. These tetrahedral modules (TMs) are joined by planes of eclipsed bonds along {111} faces—that is, TMs are packed face-to-face. Since this method of packing involves five TMs with common vertices along 〈110〉 directions, the model is not space-filling. Large assemblies therefore contain significant amounts of strain and there is no long-range periodicity. Radial distribution functions are calculated which are in good agreement with experimental data for a-Ge and represent a major improvement over the fit to experimental rdfs obtained from microcrystallite models, where little or no attempt has been made to specify the interfacial structure between crystallites. These ‘polytetrahedral’ structures have features in common with each of the three extreme conceptual models for amorphous materials; namely, the micro-crystallite, continuous random network and amorphous cluster hypotheses. There is also considerable similarity with ‘multiply-twinned’ non-crystallographic clusters which have been observed in the early stages of growth of particles of several metals and semiconductors.