Dielectric Anomalies in Crystalline Ice: Indirect Evidence of the Existence of a Liquid-Liquid Critical Point in H2O

TL;DR

This study presents dielectric anomalies in crystalline ice that provide indirect evidence supporting the existence of a liquid-liquid critical point in supercooled water, aligning with the LLCP theory and expanding understanding of water's complex phase diagram.

Contribution

It offers experimental evidence of dielectric anomalies linked to a hypothesized liquid-liquid critical point in supercooled water, supporting the LLCP theory.

Findings

Dielectric anomalies near 203 K and 212 K observed during warming.

Anomalies suggest existence of two supercooled water forms with different densities.

Critical point inferred near 33-50 MPa and above 212 K.

Abstract

The phase diagram of H2O is extremely complex, in particular, it is believed that a second critical point exists deep below the supercooled water (SCW) region where two liquids of different densities coexist. The problem however, is that SCW freezes at temperatures just above this hypothesized liquid-liquid critical point (LLCP) so direct experimental verification of its existence has yet to be realized. Here, we report two anomalies in the complex dielectric constant during warming in the form of a peak anomaly near Tp=203 K and a sharp minimum near Tm=212 K from ice samples prepared from SCW under hydrostatic pressures up to 760 MPa. The same features were observed about 4 K higher in heavy ice. Tp is believed to be associated to the nucleation process of metastable cubic ice Ic and Tm the transitioning of ice Ic to either ices Ih or II depending on pressure. Given that Tp and Tm are nearly isothermal and present up to at least 620 MPa and ending as a critical point near 33-50 MPa, it is deduced that two types of SCW with different density concentrations exists which affects the surface energy of ice Ic nuclei in the "no man's land" region of the phase diagram. Our results are consistent with the LLCP theory and suggest that a metastable critical point exists in the region of 33-50 MPa and Tc > 212 K.