Network Stiffening and Chemical Ordering in Chalcogenide Glasses: Compositional Trends of Tg in Relation to Structural Information From Solid and Liquid State NMR

Abstract

In summary the results of the present study illustrate the utility of solid- and molten-state NMR spectroscopy to provide valuable information on short- and medium-range ordering effects in non-oxide chalcogenide glasses. P-Se glasses with their efficient competition between homo- and heteropolar bond formation are the most ideal model systems for mean-field theory. Accordingly a dramatic change in dTg/d is observable at the percolation threshold =2.40. In contrast, As-Se and Ge-Se systems display a pronounced preference for heteropolar bond formation, resulting in chemical threshold effects that are superimposed upon the effects of physical percolation. These chemical threshold effects produce Tg maxima at compositions corresponding to R=1, where heteropolar bond formation is maximized. In ternary Ge-Se-X systems (X=P,As,Sb) these chemical threshold effects disappear because the formation of homopolar bonds is controlled by a strong secondary hierarchy. Over wide compositional regions of Se-deficient glasses this hierarchy serves to minimize the formation of Ge-Ge bonds and can be held responsible for the observation of universal Tg vs. dependences. Based on these results we predict that physical threshold behavior will in general be observable not only in those glass systems having no bonding preferences, but also in those ternary glass systems in which chemical ordering is observed, but where there is a clear secondary hierarchy of heteropolar bond formation. The most striking example is the Ge-P-Se glass system; other examples can be envisioned for tellurium-based chalcogenide glass systems Ge-X-Te. In contrast, no such secondary hierarchy is expected for sulfide-based glasses (such as Ge-X-S), where we envision heteropolar Ge-S and X-S bonding to dominate the structure to such an extent that no secondary hierarchy effects are expected. The experimental varification of these ideas is currently under investigation in our laboratories.