Melting a solid below the freezing point

Figure caption: When a crystal structure of bismuth (right) is decompressed from 3.2 GPa to 1.2 GPa, it melts into a liquid at about 2.3 GPa (middle), and then recrystallizes at 1.2 GPa (left). The so-called metastable liquid produced by this decompression occurs in a pressure-temperature range similar to where the supercooled bismuth is produced.
Figure caption: When a crystal structure of bismuth (right) is decompressed from 3.2 GPa to 1.2 GPa, it melts into a liquid at about 2.3 GPa (middle), and then recrystallizes at 1.2 GPa (left). The so-called metastable liquid produced by this decompression occurs in a pressure-temperature range similar to where the supercooled bismuth is produced.

A metastable liquid may exist under supercooling, sustaining the liquid below the melting point such as supercooled water and silicon. One important question is whether a crystalline solid may directly melt into a sustainable metastable liquid. A research group utilizing HPCAT's facility observed first experimental evidence of creating a metastable liquid directly by a different approach: melting a high-pressure solid crystal of the metal bismuth via a decompression process below its melting point. When a crystal structure of bismuth is decompressed from 3.2 GPa to 1.2 GPa at 489 K, it melts into a liquid at about 2.3 GPa (middle), and then recrystallizes at 1.2 GPa. The decompression-induced metastable liquid can be maintained for hours in static conditions, and transform to crystalline phases when external perturbations, such as heating and cooling, are applied. It occurs in the pressure–temperature region similar to where the supercooled liquid Bi is observed. Akin to supercooled liquid, the pressure-induced metastable liquid may be more ubiquitous than we thought. More details in Lin et al, Nature Comm. 8, 14260, 2017.