Fragility transition in GeSe2–Se liquids

Abstract

The heat capacity and entropy of liquids in the GeSe 2 -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 GeSe 2 . The thermodynamics of liquid GeSe 2 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 GeSe 2 -Se glasses just above T g supports a fragility minimum near x ¼ 0.225, (x is x in Ge x Se 1Àx ) 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 GeSe 2 -Se. The transition is related to the onset of chemical disorder and the density minimum of GeSe 2 . The proposed fragile-to-strong transition of other tetrahedral liquids has also been linked to a first order liquidliquid transition, which recently also have been discussed for GeSe 2 .

Topological Ordering and Viscosity in the Glass-Forming Ge–Se System: The Search for a Structural or Dynamical Signature of the Intermediate Phase

“…For example, a least-squares parabolic fit to the m visc values of Senapati and Varshneya (1996) leads to a minimum at x = 0.196(2), whereas a similar fit to all of the m visc data points leads to a minimum at x = 0.223(2), consistent with the value x = 0.225 previously reported by Stølen et al (2002). The m DSC values of Gunasekera et al (2013) are smaller than other values of the fragility index and, for several intermediate phase compositions, are even smaller than the fragility index of silica m visc ≃ 21, where the latter was obtained by applying the MYEGA model to the viscosity data listed by Doremus (2002).…”

Relaxation and physical aging in network glasses: a review

“…An immediate inspection of this figure leads to the conclusion that such supercooled liquids behave very differently close to their glass transition. Some of them show a behavior η(T ) that follows an Arrhenius law of the form η=η ∞ exp[E A /T ], and typical examples are silica and germania [5] or GeSe 4 [6]. However, as one moves downwards in the figure, other liquids (e.g.…”

“…However, as one moves downwards in the figure, other liquids (e.g. B 2 O 3 , 2SiO 2 -Na 2 O, As 2 Se 3 or Se) now exhibit a viscosity behavior that shows an important bending [6] at intermediate values of T g /T , whereas organic glass-formers (OTP and toluene) display the most pronounced curvature and must involve a super-Arrhenius parametrization of the form η=η ∞ exp[E A (T )/T ] where the activation energy E A must now explicitely depends on temperature.…”

“…Uhlmann-Angell plot of viscosity rescaling the same data with respect to T g /T where T g is defined by η(T g )=10 12 Pa.s. Data taken from [5,6,7,8].…”

Superstrong nature of covalently bonded glass-forming liquids at select compositions

“…On a microscopic scale, local compositions, different from the nominal one, will appear and possess different fragility and diffusivity. At a macroscopic scale, it is well known that chalcogenide supercooled melts display a fragility minimum at certain compositions 13,14 identified with a flexible to rigid transition 15 . However, a more general correlation between homogenized liquids displaying no phase-separation (T > T g ) and corresponding glasses (T < T g ) upon viscous slow down and the onset of rigidity and stress transitions has never been established 16 .…”