MICROSCOPIC ORIGIN OF THE GLASS-FORMING TENDENCY IN CHALCOHALIDES ANDCONSTRAINT THEORY

The glass forming tendency in two families of ternary glasses: the gro up IV-chalcohahdes (such as GeS-I) and the Group V-chalcohalides (such as As-S-I) is analyzed. Predictions of the extended constraint theory , which explicitly includes the role of 1-fold coordinated (OFC) halog en atoms is cast in the form of universal compositional pathways along which the mean constraints per atom ((n) over bar(c)) due to the near est-neighbor (nn) covalent forces equals 3. When compared to the estab lished glass forming compositions in more than 20 ternary glass system s, a correlation is observed with the predicted pathways. Notable exce ptions also occur and reside away from the predicted compositions. The present analysis reveals that optimally constrained random networks o ccur along and in the vicinity of the predicted pathways, while near-o ptimally constrained molecular clusters occur at specific stoichiometr ies residing away from the predicted pathways. Molecular structure of Te chalcohalides is reviewed and the role of OFC-halogen atoms on the glass forming tendency is commented upon within the context of extende d constraint theory. (C) 1998 Elsevier Science B.V. All rights reserve d.