Liquid-liquid phase transitions in glass-forming systems and their implications for memory technology

While it is broadly known that solid compounds may exist in distinct crystalline arrangements (polymorphs), the notion that some liquids may also adopt distinct phases with dissimilar structures and densities is much less widespread. One of the reasons is that these liquid-liquid (L-L) transitions often occur in the supercooled equilibrium regime below the melting line and can be challenging to observe experimentally. Glass-forming liquids that supercool over significant temperature ranges can therefore constitute useful systems for investigating these transitions. In this paper we review experimental evidence for L-L transitions in chalcogenide systems. In that respect, L-L transitions are found to be associated with transitions from fragile to strong viscous behavior in these glass-forming liquids. Moreover, they are signaled by extrema in multiple thermodynamic functions and sharp change in physical properties such as electrical conductivity. Finally, while the physical principles underlying these transitions are still unclear, they have been shown to play a critical role in the function of PCMs that are poised to transform fast computing technologies.