Phase change alloy viscosities down to Tg using Adam-Gibbs-equation fittings to excess entropy data: A fragile-to-strong transition

Abstract

A method to enhance the data transfer rate of eutectic Sb-Te phase-change recording media A striking anomaly in the viscosity of Te 85 Ge 15 alloys noted by Greer and coworkers from the work of Neumann et al. is reminiscent of the equally striking comparison of liquid tellurium and water anomalies documented long ago by Kanno et al. In view of the power laws that are used to fit the data on water, we analyze the data on Te 85 Ge 15 using the Speedy-Angell power-law form, and find a good account with a singularity T s only 25 K below the eutectic temperature. However, the heat capacity data in this case are not diverging, but instead exhibit a sharp maximum like that observed in fast cooling in the Molinero-Moore model of water. Applying the Adam-Gibbs viscosity equation to these calorimetric data, we find that there must be a fragile-to-strong liquid transition at the heat capacity peak temperature, and then predict the "strong" liquid course of the viscosity down to T g at 406 K (403.6 K at 20 K min À1 in this study). Since crystallization can be avoided by moderately fast cooling in this case, we can check the validity of the extrapolation by making a direct measurement of fragility at T g , using differential scanning calorimetric techniques, and then comparing with the value from the extrapolated viscosity at T g . The agreement is encouraging, and prompts discussion of relations between water and phase change alloy anomalies. V C 2015 AIP Publishing LLC. [http://dx.

Glass Transitions, Semiconductor-Metal Transitions, and Fragilities in Ge−V−Te ( V=As , Sb) Liquid Alloys: The Difference One Element Can Make

“…A minimum fragility has been observed at an average bond coordination number (or bond density), <r>, higher than the rigidity percolation threshold at <r> = 2.4 [21,22]. Recently, the fragility of Ge 15 Te 85 near T g has been determined to be relatively strong compared to its high-temperature fragile liquid state [23]. A fragile-to-strong liquid transition was revealed by the Adam-Gibbs equation [24] fitting of viscosity data [23].…”

“…Fragilities for a series of compositions along the thick black arrow of Figure 1 were determined using the cooling rate dependence of the fictive temperature method, and also instrumentation (TA DSC), described in our previous work [21,22] on the fragility of Ge 15 As x Se 85-x, Ge 15 As x S 85-x , and also as one of the methods employed in the DSC study of Ge 15 Te 85 [23]. The fictive temperature is defined, for a given cooling rate, q c , as the temperature of onset of the heat capacity jump during upscan at the same rate i.e.…”

“…8c), meaning the SC-M transition has been pushed below the melting point and can no longer be observed due to fast crystallization. If the SC-M transition is sharp enough, then the accompanying heat capacity change will be sharp, and according to Adam-Gibbs theory, a fragile-to-strong transition (in which the viscosity changes more or less suddenly by some orders of magnitude) will occur during cooling [23] .…”

“…Recently, the fragility of Ge 15 Te 85 near T g has been determined to be relatively strong compared to its high-temperature fragile liquid state [23]. A fragile-to-strong liquid transition was revealed by the Adam-Gibbs equation [24] fitting of viscosity data [23]. It appears that the fragility of chalcogenide liquids can vary over a broad range from as strong as silica [8] (m=20) to as fragile as the simple ionic glass-former calcium potassium nitrate CKN (m=93, ref.…”

“…This is apparently a direct consequence of the structural fluctuations associated with an impending LL phase transition. The phenomenology of "hidden" LL phase transitions, and their relation to fast crystallization, in supercooled water has been the subject of exhaustive studies both by experiment and computer simulation [54], to which brief reference was made in our preceding paper [23]. In the water case, the response functions heat capacity, compressibility, and expansivity of water all show highly anomalous behavior with an apparent common divergence temperature just below the sharply defined homogeneous nucleation temperature (which can be measured for aqueous systems using small sample techniques).…”