Phase Relations in Water-Ice

A starting point to understanding the inner structure of an icy satellite like Titan, is a full understanding of the phase diagram of H2O-ice. Methane clathrates are also important but H2O-ice is a more general and abundant system. The low temperature phases are perfectly crystalline but the hydrogen sub-lattice disorders with increasing T before melting. The molecular phases consist of one single hydrogen-bond network, while the high pressure ones consist of two interpenetrating networks. A common low temperature phenomenon is amorphization under pressure or decompression. The reconstruction of one single into two interpenetrating networks, or vice-versa, is not straightforward and occurs in steps, with perhaps more than one amorphous and metastable phases intervening before the full reconstruction occurs. We wish to understand two things: 1) the phase boundaries between the low temperature (i.e., ~70 K at 0 GPa, ~300K at 10 GPa), high pressure phases, including hydrogen disordered phases. These boundaries are still unknown. 2) Since metastability is the norm in H2O-ice, rather than the exception, we wish to predict energy barriers and transition rates, for structural transitions. Transition rates are important to determine the lifetime of metastable phases, which might be very abundant in these objects. To address the first problem, i.e., the ordering of H and "empty sites", we will use techniques similar to those used to investigate solid solutions. For the second problem we need an effective technique to search for the "transition path" and energy barriers. A key issue will be to include quantum mechanical tunneling in these low-temperature processes. (Truhlar, Wentzcovitch, and Baroni)

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