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

摘要

The phase diagram of H2O is extremely complex, in particular, it is believedthat 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-liquidcritical point (LLCP) so direct experimental verification of its existence hasyet to be realized. Here, we report two anomalies in the complex dielectricconstant during warming in the form of a peak anomaly near Tp=203 K and a sharpminimum near Tm=212 K from ice samples prepared from SCW under hydrostaticpressures up to 760 MPa. The same features were observed about 4 K higher inheavy ice. Tp is believed to be associated to the nucleation process ofmetastable cubic ice Ic and Tm the transitioning of ice Ic to either ices Ih orII depending on pressure. Given that Tp and Tm are nearly isothermal andpresent 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 concentrationsexists 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 theoryand suggest that a metastable critical point exists in the region of 33-50 MPaand Tc > 212 K.