Fragile-strong behavior in the As _xSe _1-x glass forming system in relation to structural dimensionality

A series of physical properties have been measured throughout and above the glass transition for the whole As _xSe _1-x system, including the activation for viscous flow E _η, the activation energy for enthalpy relaxation E _H, and the activation energy for structural relaxation E _a obtained by specific heat spectroscopy. All properties show a double minimum at an average coordination number 〈r〉 = 2.3 and 〈r〉 = 2.5 with a local maximum at 〈r〉 = 2.4. This is in stark contrast to the physical properties previously measured on the same samples at room temperature and which instead show a single minimum centered at 〈r〉 = 2.4. The observed trend is consistent with the dimensionality of the network derived from structural data obtained by nuclear magnetic resonance. An analysis of the complex heat capacity also reveals a bimodal relaxation process in As-rich glasses, which explains why they are kinetically fragile but appear thermodynamically strong. Finally, these results demonstrate that previous observations of an "intermediate phase" in As _xSe _1-x glasses near 〈r〉 = 2.3 is associated with the high temperature behavior of the glassy network and should be interpreted in terms of the temperature dependence of structural constraints rather than the number of constraints in the room-temperature glass.