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Ice XVI is the least dense (0.81 g/cm) experimentally obtained crystalline form of ice. It is topologically equivalent to the empty structure of sII 
Ice XVI is the least dense (0.81 g/cm) experimentally obtained crystalline form of ice. It is topologically equivalent to the empty structure of sII clathrate hydrates
Clathrate hydrates, or gas hydrates, clathrates, hydrates, etc., are crystalline water-based solids physically resembling ice, in which small non-polar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped ...
. It was first obtained in 2014 by removing gas molecules from a neon clathrate
Neon compounds are chemical compounds containing the element neon (Ne) with other molecules or elements from the periodic table. Compounds of the noble gas neon were believed not to exist, but there are now known to be molecular ions containing ne ...
under vacuum at temperatures below 147 K. The resulting empty water frame, ice XVI, is thermodynamically unstable at the experimental conditions, yet it can be preserved at cryogenic temperatures. Above 145–147 K at positive pressures ice XVI transforms into the stacking-faulty ice Ic and further into ordinary ice Ih. Theoretical studies predict ice XVI to be thermodynamically stable at negative pressures (that is under tension
Tension may refer to:
Science
* Psychological stress
* Tension (physics), a force related to the stretching of an object (the opposite of compression)
* Tension (geology), a stress which stretches rocks in two opposite directions
* Voltage or el ...
).
References
Water ice {{inorganic-compound-stub