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Monday, March 12, 2007
3:00 PM - 4:00 PM
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Discovery of the intermediate phase

Punit Boolchand
University of Cincinnati

A disordered network of polymeric chains, such as a Se glass, each atom having two neighbors (r = 2) is mechanically flexible. Upon cross-linking by group V and/or group IV additives, such networks progressively stiffen to become rigid when their connectivity or mean coordination number, , increases to a critical value = 2.40. A solitary elastic phase transition from a flexible to a stressed-rigid phase was predicted [1] from mean-field theory in the early 1980s. Starting in 1997, close examination of the vibrational behavior (in Raman scattering) and the nature of glass transitions (in modulated differential scanning calorimetry) of binary (Ge-Se, As-Se) and ternary (Ge-As-Se) covalent networks has revealed [2] two ( (1) and (2)) and not one ( ) elastic phase transition. Glasses bounded by these two thresholds define a new topological phase, an Intermediate phase (IP), with those at < (1) as flexible and at > (2) as stressed-rigid. Glass-compositions in the IP form ideal, space filling, and rigid but stress-free networks [3]. IP networks possess unusual functionalities including thermally reversing glass transitions that do not age much, features that are identified with self-organization of disordered networks. Recent experiments reveal IPs to be present in solid electrolyte glasses, and to have a close bearing to ionic transport in these materials.

1. J.C. Phillips and M.F. Thorpe, Solid State Commun. 53,699 (1985).
2. P.Boolchand, D.G.Georgiev, B.Goodman. J. of Optoelectronics and Adv. Materials, 3, 3, 703 (2001) (article in pdf format)
3 F. Wang, S. Mamedov, P.Boolchand, B. Goodman and M. Chandrasekhar, Phys. Rev. B 71, 174201 (2005) (article in pdf format).

Host: Turab Lookman, LANL/T-11