Fourteen different polymorphs of water are known experimentally. However, the search of new solid phases remains a very active area of investigation. Two principal routes exist for locating new solid phases of water. The first is the search (at low temperatures) of proton ordered analogous of the high temperature proton disordered ices. This is the research that lead to the discovery of ices ice VIII, ice IX, ice XI, ice XIII and ice XIV. Ice VI being probably the only proton disordered ice for which the proton ordered ice has not yet been found. Some authors label this unprepared ice phase as ice XV. The second route is the search of new ices at high pressure and/or high temperature. This route is being object of an intensive activity, both experimentally and theoretically. Recently, a plastic crystal phase of water has been found by molecular simulation for some water potential models (SPC/E, TIP4P, TIP5P and TIP4P/2005) . This plastic crystal has yet to be prepared experimentally, but the authors have labelled it as ice XV. The phases of ice have been labelled with the Roman numerals I-XIV in the approximate order in which they were produced experimentally. Therefore, one will have to wait for which of these phases is prepared first to know what ice XV really is.
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