Ice XII is a metastable, dense, crystalline phase of solid water. It is a type of ice. 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 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 ice I_h at 77 K by rapid compression (0.81-1.00 GPa/min) or by warming high density amorphous ice at pressures between 0.8 and 1.6 GPa. The proton-ordered form of ice XII is ice XIV.

While it is similar in density (1.29 g/cm³ 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.

Ordinary water ice is known as ice I_h, (in the Bridgman nomenclature). Different types of ice, from ice II to ice XIV, have been created in the laboratory at different temperatures and pressures.

The crystal structure of ice XII

References

• C. Lobban, J.L. Finney and W.F. Kuhs, The structure of a new phase of ice, Nature 391, 268-270, 1998
• Chaplin, Martin (2007-07-01). Ice-twelve and ice-fourteen structures. Water Structure and Science. Retrieved on 2008-01-02.

v • d • e Crystalline forms of ice Ih • Ic • II • III • IV • V • VI • VII • VIII • IX • X • XI • XII • XIII • XIV • XV . XVI

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