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

Glass-transition temperatures (T-g) 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 a more covalent system. The comparison of T-g 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 T-g 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 observation 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 is controlled by liquid-liquid (LL) and SC-M transitions that have become submerged below the liquidus surface. In the case of the Ge-Te binary, a crude extrapolation toGeTe stoichiometry indicates that the SC-Mtransition lies about20% belowthe melting point, suggesting a parallel with the intensely researched hidden liquid-liquid transition in supercooled water. In the water case, superfast crystallization initiates in the high-fragility domain some 4% above the LL transition temperature (T-LL) which is located at approximately 15% below the (ambient pressure) melting point.