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

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 T-p = 203 K and a sharp minimum near T-m = 210 K from ice samples prepared from SCW under hydrostatic pressures of up to 760 MPa. The same features were observed about 4 K higher in heavy ice. T-p is believed to be associated with the nucleation process of metastable cubic ice Ic, and T-m, is the transitioning of ice Ic to either ice Ih or II depending on the pressure. Given that T-p and T-m, are nearly isothermal, present up to at least 620 MPa, and end as a critical point near 33-50 MPa, it is deduced that two types of SCWs with different density concentrations exist, 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 T-c >= 210 K.