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 transition. While LLTs have been predicted from computer simulations of several systems, experimental evidence remains scarce and often controversial as it mostly concerns metastable liquids where the LLT occurs simultaneously with crystallization, making it hard to separate the two phenomena.

The main objective of this project is to significantly advance our understanding of liquid polymorphism and, by extension, of the liquid state itself, by providing accurate datasets from which the systematics of LLT can be extracted, and eventually will aid the emergence of theories from which predictions can be made.

To reach our objective, we propose to study elemental sulfur and phosphorous, over a large range of P-T conditions combining experimental results, from x-ray and optical diagnostics, with theoretical studies using state-of-the-art ab initio computer simulations and machine learning techniques that will provide information at the microscopic and thermodynamic levels.

In particular, machine learning has emerged in recent years as a powerful tool for the description of complex chemical systems, offering an alternative approach for the representation of potential-energy surfaces by fitting large data sets from electronic structure calculations. This allows to extend the reach of simulations both in time and space, thanks to the low cost of evaluating ML models while preserving an accurate and precise physical description of the interaction between atoms, characteristic of first-principles calculations.

If time allows, other systems, representative of various types of liquids (network, molecular and metallic liquids), will also be investigated, in parallel with experiments.


 PhD student: Sonia SALOMONI
PhD supervisors: A. Marco Saitta, Frédéric Datchi, Arthur France-Lanord 
Research laboratory: IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie