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Insights into the Water Conundrum from Systems with Many Variables - and High Tech Applications.

C. Austen Angell

School of Molecular Sciences, Arizona State University, Tempe AZ, 85287

            While much progress has been made on resolving the source of the much-discussed “anomalies of water”, much remains uncertain. One reason is that water is unique among molecular liquids, and hides it secrets behind a curtain of fast crystallization kinetics to its ice phases.  Several strategies have been developed for penetrating this curtain, with different degrees of success. We will review briefly, the thermodynamic extrapolation approach, including the two state modelling approach, the crystallization kinetics-to-diffusivity approach, and the second component ideal solution approach. 

            Then we will go into some detail with an “analog systems” approach in which there are many composition variables to play with. We study systems that exhibit higher order structural transitions - transitions involving density maxima and even density minima, within the stable liquid state (sometimes near the upper limit of the liquid state, LH panel below). These are increasingly water-like, the lower the Tm-scaled temperature at which they occur. Their characteristic temperatures can be correlated using a new parameter that we call the Pauling metallicity MP. (the inverse of the atomic fraction-averaged Pauling electronegativity for the liquid alloy).  As the metallicity increases, the characteristic temperatures of the transitions, which can be identified as metal-to semiconductor (M-SC) transitions, sharpen and decrease in temperature relative to their melting points. At MP = 0.46, they submerge below the melting points as shown in the RH panel below, and become inaccessible to further study, except by ultrafast probe techniques.

Interestingly enough, the compositions of all the known “phase change materials” (under active development for high capacity ultrafast digital memory devices [Intel chip of 16 GB available from Amazon]) fall within the shaded zone and indicate a characteristic temperature of about 0.85 of the melting point. The fast crystallization of these compositions aided by the M-SC/( fragile-to-strong liquid) transition, is regarded as a key to the success of  the PCM technology. We note, without further comment, that the temperature at which the water divergences occur is, by a number of estimates, 228K, or 0.84 of the ice Ih melting point.

References. S. Wei, P. Lucas, C.A. Angell, J. Appl. Phys. 118, 034903 (2015); Phys. Rev Appl. 7, 034035 (2017),  Bull. Mater. Res. Soc., special issue (in press – source of the above figures.)