The reversibility and first-order nature of liquid-liquid transition in a molecular liquid

Liquid-liquid transition is an intriguing phenomenon in which a liquid transforms into another liquid via the first-order transition. For molecular liquids, however, it always takes place in a supercooled liquid state metastable against crystallization, which has led to a number of serious debates concerning its origin: liquid-liquid transition vs. unusual nano-crystal formation. Thus, there have so far been no single example free from such debates. Here we show the first firm experimental evidence that the transition is truly liquid-liquid transition and not nano-crystallization for a molecular liquid, triphenyl phosphite. We kinetically isolate the reverse liquid-liquid transition from glass transition and crystallization with an extremely high heating rate of flash differential scanning calorimetry, and prove the reversibility and first-order nature of liquid-liquid transition. Our finding not only deepens our physical understanding of liquid-liquid transition but also will initiate a new phase of its research from both fundamental and applications viewpoints.