Network Stiffening and Chemical Ordering in Chalcogenide Glasses

Abstract

Non-oxide chalcogenide glasses have attracted considerable scientific interest from the viewpoint of their exciting optical characteristicst [1, 2, 3, 4]. Wide glass forming regions offer ample opportunities for controlling desired physical properties by means of the chemical composition, making these materials attractive from a technological point of view [5, 6, 7]. Aside from these application aspects, non-oxide chalcogenide glasses represent interesting model systems for testing simple mean-field concepts used to discuss structure-property relations. It has been pointed out that certain physical characteristics of chalcogenide glasses, such as glass transition temperatures, melt viscosities, and molar volumes often depend non-linearly on their chemical composition [5, 6, 7, 8, 9]. An explanation has been offered on the basis of percolation theory, by considering the overall connectivity of the network [8, 10, 11, 12]. For any two- or three-dimensional network one can define an average coordination number $$ \left\langle r \right\rangle = \sum\limits_i {r_i n_i } /\sum\limits_i {n_i } $$ (1) where ni is the number of atoms of species i, rj is the number of bonds formed by it, and the summation extends over all of the different atomic species present in the network. Any covalent network constrained by bond-stretching and bond bending forces possesses a critical connectivity threshold at = 2.40. Glasses having average coordination numbers higher than this threshold value represent single infinite clusters with no residual zero-frequency modes [12–16].