Fragility transition in GeSe2–Se liquids

Abstract

The heat capacity and entropy of liquids in the GeSe₂–Se system are reported. The thermodynamics of the system is evaluated and discussed in light of earlier reported viscosities and recent structural studies of liquid GeSe₂. The thermodynamics of liquid GeSe₂ suggest that extensive structural disordering takes place in the liquid on heating and points to a fragile melt at high temperatures whereas the thermodynamic characteristics of the system close to the glass transition temperature suggests a much less fragile character. A consistent picture is evident from earlier viscosity data; the viscosity of GeSe₂–Se glasses just above T_g supports a fragility minimum near x = 0.225, (x is x in Ge_xSe_1−x) while a monotonically increasing fragility is evident from the viscosity in the stable liquid and non-viscous region. We propose that the above discrepancy between the behaviour of the melts close to T_g and at high temperatures can be explained by a fragile-to-strong transition in supercooled GeSe₂–Se. The transition is related to the onset of chemical disorder and the density minimum of GeSe₂. The proposed fragile-to-strong transition of other tetrahedral liquids has also been linked to a first order liquid–liquid transition, which recently also have been discussed for GeSe₂.