Maria Grazia Izzo — Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca' Foscari #
The interplay between liquid-liquid and ferroelectric phase transitions in supercooled water #
A liquid-liquid (LL) phase transition below water's freezing point, so-called supercooled regime, has emerged as a credible hypothesis for explaining its equilibrium state anomalies. The LLPT has nowadays been observed in several Molecular Dynamics (MD) simulations, with supporting evidence from experiments, albeit not directly observed. MD simulations provided evidence for a phase transition between a High Density Liquid (HDL) and a Low Density Liquid (LDL) [1]. Speculations on possible ferroelectric phase transition in water and, more generally, in polar liquids have persisted for some time. However, the two scenarios, namely LL and ferroelectric phase transitions, have never been correlated. Reanalyzing the water MD simulations in Ref. [1], a distinct correlation between density and total polarization emerges: while HDL retains paraelectric characteristics, the trend of LDL polarization suggests a ferroelectric character. In light of this result, we developed a classical density functional theory in a mean field approximation, with dipolar interaction treated perturbatively, that, starting from the microscopic potential interaction of a polar liquid, is able to describe the concomitant occurrence of ferroelectric and LL phase transitions. The peculiarity of our approach is the inclusion of the density-polarization coupling.  which is inherent to dipolar interaction. Covering the critical point, the Widom and first-order LL phase transition lines, we compared the density and polarization phase diagrams of water obtained from MD simulations and theory, revealing significant alignment. As further confirmation of ferroelectric order existence in LDL, collective propagating modes, stemming from the spontaneous breaking of the continuous rotational symmetry group O(3), have been identified in MD results. Possible developments of the present theory involving the potential for improper ferroelectric transitions or glassy transition in dipolar degrees of freedom will be analyzed.  [1] P. G. Debenedetti, F. Sciortino, and G. H. Zerze. Second critical point in two realistic models of water. Science 369, 289 (2020)