  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.
