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Ice XII is a metastable, dense, crystalline phase of solid water. It was first reported in 1996 by C. Lobban, J.L. Finney and W.F. Kuhs and after initial caution was properly identified in 1998. It had been predicted by computer modelling some time before its discovery.

It was first obtained by cooling liquid water to 260 K at a pressure of 0.55 GPa, it was discovered existing within the phase stability region of ice V. Later research showed that ice XII could be created outside that range. Pure ice XII can be created from HDA ice under high compression, 0.81-1.00 GPa/min, and temperatures below 77 K. It transforms at temperatures above 135 K to cubic ice (Ic).

While it is similar in density (1.29 g/cm-3 at 127 K) to ice IV (also found in the ice V space) it exists as a tetragonal crystal. Topologically it is a mix of seven- and eight-membered rings, a 4-connected net (4-coordinate sphere packing) -- the densest possible arrangement without hydrogen bond interpenetration.