Microscopic origin of the glass forming tendency in chalcohalides and constraint theory M. Mitkova 1 , P. Boolchand * Department of Electrical and Computer Engineering, and Computer Science, University of Cincinnati, Cincinnati, OH 45221-0030, USA Received 16 February 1998; received in revised form 13 May 1998 Abstract The glass forming tendency in two families of ternary glasses: the group IV-chalcohalides (such as Ge±S±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) halogen atoms is cast in the form of universal compositional pathways along which the mean constraints per atom  n c  due to the nearest-neighbor (nn) covalent forces equals 3. When compared to the established glass forming compositions in more than 20 ternary glass systems, a correlation is observed with the predicted pathways. Notable exceptions also occur and reside away from the predicted compositions. The present analysis reveals that optimally constrained random networks occur along and in the vicinity of the predicted pathways, while near-optimally constrained molecular clusters occur at speci®c stoichiometries 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 extended constraint theory. Ó 1998 Elsevier Science B.V. All rights reserved. Journal of Non-Crystalline Solids 240 (1998) 1±21 Glossary  r: mean coordination number  r c : critical mean coordination nn: nearest neighbor  n c : mean constraints per atom  n c  3: optimally constrained network  n c > 3: overconstrained network  n c < 3: underconstrained network T ` : liquidus temperature T g : glass transition temperature OFC: one-fold coordinated atoms; in chalcohalides this represents a halo- gen atom r bonded to one nearest- neighbor atom. n 1 =n: fraction of OFC atoms in a network GFT: glass forming tendency MDSC: temperature modulated dierential scanning calorimetry Group IV chalco- halides: A x B 1xy C y ternary; A  tathogen, B  chalcogen; C  halogen;  n c  3 requires y 10x  2=3 (line PQ) Group V chalco- halides: D x B 1xy C y ternary; D  pnictide, B  chalcogen, C  halogen;  n c  3 requires y 5x  23 (line RS) if D is 3-fold, and y 7x  2=3 (line TU) if D is quasi-4-fold coordinated * Corresponding author. Tel.: +1-513 556 4758; fax: +1-513 556 7326; e-mail: pboolcha@ececs.uc.edu. 1 Permanent address: Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, So®a, Bulgar- ia. 0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 7 4 4 - 3