Glass transitions, semiconductor-metal (SC-M) transitions and   fragilities in Ge-V-Te (V=As, or Sb) liquid alloys: the difference one   element can make
  Glass transition temperatures (Tg) and liquid fragilities are measured along a line of constant Ge content in the system Ge-As-Te, and contrasted with the lack of glass-forming ability in the twin system Ge-Sb-Te at the same Ge content. The one composition established as free of crystal contamination in the latter system shows a behavior opposite to that of more covalent system. Comparison of Tg vs bond density in the three systems Ge-As-chalcogen differing in chalcogen i.e. S, Se, or Te, shows that as the chalcogen becomes more metallic, i.e. in the order S<Se<Te the bond density effect on Tg becomes systematically weaker, with a crossover at <r> = 2.3. When the more metallic Sb replaces As at <r> greater than 2.3, incipient metallicity rather than directional bond covalency apparently gains control of the physics. This leads us to an examination of the electronic conductivity and, then, semiconductor-to-metal (SC-M) transitions, with their associated thermodynamic manifestations, in relevant liquid alloys. The thermodynamic components, as seen previously, control liquid fragility and cause fragile-to-strong transitions during cooling. We tentatively conclude that liquid state behavior in phase change materials (PCMs) is controlled by liquid-liquid (SC-M) transitions that have become submerged below the liquidus surface. In the case of the Ge-Te binary, a crude extrapolation to GeTe stoichiometry indicates that the SC-M transition lies about 20% below the melting point, suggesting a parallel with the intensely researched "hidden liquid-liquid (LL) transition", in supercooled water. In the water case, superfast crystallization initiates in the high fragility domain some 4% above the TLL which is located at ~15% below the (ambient pressure) melting point. 
Shuai Wei, Garrett J. Coleman, Pierre Lucas, and C. Austen Angell
cond-mat.mtrl-sci