Ice XVI

Ice XVI is the least dense (0.81 g/cm3)[1] experimentally obtained crystalline form of ice. It is topologically equivalent to the empty structure of sII clathrate hydrates. It was first obtained in 2014 by removing gas molecules from a neon clathrate under vacuum at temperatures below 147 K.[1] 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).[2][3]

Water phase diagram extended to negative pressures calculated with TIP4P/2005 model.[2]
Ice XVI. White edges mark the unit cell (~17 Å).

References
  1. Falenty, A.; Hansen, T. C.; Kuhs, W. F. (2014). "Formation and properties of ice XVI obtained by emptying a type sII clathrate hydrate". Nature. 516 (7530): 231. Bibcode:2014Natur.516..231F. doi:10.1038/nature14014. PMID 25503235.
  2. Conde, M.M.; Vega, C.; Tribello, G.A.; Slater, B. (2009). "The phase diagram of water at negative pressures: Virtual ices". The Journal of Chemical Physics. 131 (34510): 034510. Bibcode:2009JChPh.131c4510C. doi:10.1063/1.3182727. PMID 19624212.
  3. Jacobson, Liam C.; Hujo, Waldemar; Molinero, Valeria (2009). "Thermodynamic Stability and Growth of Guest-Free Clathrate Hydrates: A Low-Density Crystal Phase of Water". Journal of Physical Chemistry B. 113 (30): 10298–10307. doi:10.1021/jp903439a. PMID 19585976.
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