Anomalous liquids on a new landscape: From water to phase-change materials

A liquid that is cooled below its melting temperature, referred to as a supercooled liquid, can solidify into an amorphous rigid state (i.e., glass), if cooling is fast enough and crystallization is avoided. The phenomenology of supercooled liquids has been in general established. However, there are pronounced exceptions (e.g., water) which do not fall into the class of ‘normal’ liquids but exhibit a transition behavior in their liquid states. The latest advances connect the unusual aspect of liquids to the properties of phase-change materials (PCMs) that are the basis for non-volatile memory and neuromorphic technologies. In this article, we review the liquid anomalies in the alloys based on group-IV, V, VI elements including technologically important compositions. Their different behaviors are rationalized in terms of liquid–liquid (metal-semiconductor, and fragile-strong) transitions. We discuss their implications for understanding unusual phase switching behaviors in these materials. Lastly, unsolved problems and new opportunities are outlined.