Ice XVI

Ice XVI. White edges mark the unit cell (~17 Å).

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 I_c and further into ordinary Ice I_h . 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 (after Conde et al., 2009)

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 (034510). doi:10.1063/1.3182727.
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.