Séminaire - Liquid-liquid transition and second critical point in dense liquids: the cases of sulfur and phosphorus - Frédéric Datchi

Lundi 8 mars 2021 à 10 h 30 en visioconférence.

Frédéric Datchi, DR CNRS, équipe PHYSIX à l'IMPMC

Abstract

The liquid-liquid transition (LLT) is a rare and intriguing phenomenon in which a single-component liquid transforms into another one via a first-order phase transition. Due to their counterintuitive nature, LLTs have intrigued scientists for several years and challenged our perception of the liquid state. They have been predicted from computer simulations of several systems, and heavily debated in the case of water. Experimental evidence remain scarce and mostly concern supercooled, i.e. metastable, liquids where the LLT often occurs simultaneously with crystallization, making it hard to separate the two phenomena. A liquid-liquid critical point (LLCP), similar to the gas-liquid critical point, has been predicted at the end of the LLT line in some cases, but so far never experimentally observed for any materials. This putative LLCP has for instance been invoked in the case of water to understand its many thermodynamic anomalies. Here I will present our recent discovery of a LLT in liquid sulfur. Using several x-ray based methods, we unambiguously demonstrate the first-order character of the transition, measure the density discontinuity and characterize the difference between the two liquid forms. The thorough investigation of the phase diagram in the domain 0-3 GPa, 300-1100 K enabled us to locate the transition line and demonstrate that it ends at a critical point, providing the first evidence for a liquid-liquid critical point in any system so far. I will also present some results of our on-going investigation of the LLT in phosphorus.