Liquid-Liquid transtion, polymorphism and second critical point in dense liquids – LILI

The liquid-liquid transition (LLT) is a rare and intriguing phenomenon is which a single-component liquid transforms into another one via a first-order transition. Owing to its counterintuitive nature, the LLT has intrigued scientists for several years and challenged our perception of the liquid state, for which the notion of polymorphism was long considered impossible. LLTs have been predicted from computer simulations of several systems, and heavily debated in the case of supercooled water. However, our theoretical understanding remains relatively primitive, and there is no theory at present able to predict whether a given system will exhibit a LLT. This is why the experimental realizations reported so far remain scarce, have been made rather accidentally and are often controversial. A recent breakthrough was made by the present proposer consortium, with the experimental discovery of such a LLT in compressed liquid sulfur, and the first-ever evidence of a liquid-liquid critical point (LLCP) ending the transition line. Such a LLCP has long been searched in water but to date impossible to reach by experiment. Located at about 2.15 GPa-1035 K, the LLCP in sulfur can be easily approached by experiment, which opens brand new perspectives to the field.

The general objective of this project is to advance our understanding of LLTs, by obtaining accurate data sets from experiments and computer simulations that will form a solid basis to extract the systematics of LLTs, and aid the emergence of predictive theories. For this, we propose to study several systems which are representative of different types of materials, over a large range of pressure and temperature conditions (0-150 GPa, 300-3000 K), and using several x-ray and optical diagnostics available at the 3 partners’ sites. The first part of the project (Tasks 1 and 6) will focus on the two systems for which a LLT is now well established, phosphorus and sulfur. We will study for the first time the critical phenomena and universality class of the LLCP in sulfur using innovative small-angle x-ray scattering (SAXS) experiments in the diamond anvil cell (DAC). We will also determine whether a LLCP exists in phosphorus through x-ray measurements of the density jump along the LLT line. A better understanding of the microscopic nature of the low-density and high-density liquid phases and of the driving mechanism of the LLT in both S and P will be achieved through experiments and simulations. Finally, we will investigate their melting lines in the vicinity of the LLT and whether the two-state model is compatible with thermodynamic data for these two systems. In the second part of the project, we will extend our investigations to other systems which are promising candidates for a LLT, as suggested by computer simulations or previous experimental results. Specifically, we will focus on two network liquids (Task 2), B2O3 and AsS, the molecular liquids of nitrogen, carbon dioxide, hydrogen (Task 3), and the liquid alkali metals (Li, Na, K) (Task 4). For B2O3 and AsS, the LLT is expected below a few GPa and similar studies as those described above for sulfur and phosphorus will be carried out. For the molecular and alkali liquids, the expected location of the LLT resides at pressures from 20 to 150 GPa, which requires the use of smaller samples compressed in the DAC. The present proposers have developed new techniques in the framework of the ANR project MOFLEX which have made possible structural and vibrational studies of liquids composed of light elements in the DAC up to megabar pressures through x-ray diagnostics combined to Raman and Brillouin spectroscopies, which will be put to profit for this project. Additional technical developments (Task 5,) such as high P-T SAXS experiments, will be undertaken during the project to complement or enable new types of measurement under high P-T.