A REVIEW OF THE AVERAGE COORDINATION-NUMBER CONCEPT IN MULTICOMPONENTCHALCOGENIDE GLASS SYSTEMS

High purity chalcogenide glasses incorporating Ge, Sb, Se, As and Te w ere prepared by vacuum melting of previously distilled 5N to 6N pure r aw materials from which the surface oxide was also removed in some cas es. Molar volume and elastic properties of several isostructural chalc ogenide glasses prepared by similar processing techniques were obtaine d from the measured values of densities and the velocities of transver se and longitudinal acoustic waves respectively. Vickers hardness numb er (VHN) and indentation toughness were measured using microindentatio n technique. Glass transition temperatures (T(g)) were determined usin g differential scanning calorimetry (DSC). Property variations with th e average coordination number [r], both in the chalcogen rich and chal cogen deficient regions, were examined. In the chalcogen rich region, only the molar volume displayed a distinct minimum at [r] = 2.4, coinc ident with Phillips' 'percolation threshold' for each system studied. Elastic moduli, VHN, indentation toughness and the T(g), however, fail ed to show any dramatic changes at this threshold. An empirical relati onship between the average coordination number and Vickers hardness of a glass is proposed. A relationship between the average coordination number and glass transition temperature is also proposed by modifying the Gibbs-DiMarzio equation for glass transition of a crosslinked poly mer as a function of crosslink density. Physical properties appear to display extrema near the stoichiometric tie line indicative of the gla ss' preference for a chemically ordered covalent network (COCN) rather than to chance coordination predicted by the random covalent network (RCN) model. Subsequent changes in the magnitude of the physical prope rties in the chalcogen deficient region are presumably due to island s ubstructures separated by atomic arrangements of low connectivity.