Glass transitions and first order liquid-metal-to-semiconductor transitions in 4-5-6 covalent systems

Abstract

In this paper the glass transition phenomenology for the 4-5-6 covalent system Ge-As-Se is reviewed and then it is argued that Ge-and Si-containing liquids should exhibit much of the phenomenology of the H 2O-based and SiO 2-based systems which dominate the natural world. In both systems, aqueous solutions and silica-based glassformers, the wide glass-forming ranges are preceded by H 2O-rich and SiO 2-rich composition ranges in which there is a liquid-liquid phase separation. For both the aqueous systems and the 4–6 (and 4-5-6) systems, it is argued that the liquid-liquid transitions are driven by a structural incompatibility of low temperature open network and high temperature, denser-packed structure (the latter being metallic in the cases of Ge and Si) which lead to first order phase transitions in the supercooling liquid. The argument is supported by showing that the phenomenologies of supercooled water and supercooled liquid silicon are very similar. For the latter phenomenology, we turn to molecular dynamics computer simulations of liquid silicon using the Stillinger-Weber potential which reproduces the melting point and many qualitative features of the real material. In the supercooled liquid state, S-W silicon shows, remarkably, all the anomalies in thermodynamic and diffusive properties known for supercooled water. These culminate in a directly observable, weak first order transition to the tetrahedral network amorphous phase, (as suggested, but not established, for water) which then nucleates the crystalline polymorph in accord with the experimental finding that liquid Ge can be supercooled to, but not below, the temperature estimated for the liquid-amorphous transition in liquid Ge.