Ice XV is a crystalline form of ice, a partially proton-ordered form of ice VI. It is created by cooling hydrochloric-acid-doped ice VI to around 130 K at 1 GPa (9820 atm). ^[1]

Predictions and the experimental discovery

Although the parent phase ice VI was discovered in 1935, corresponding proton-ordered forms (ice XV) had not been observed until 2009. Theoretically, the proton ordering in ice VI was predicted several times; for example, density functional theory calculations predicted the phase transition temperature is 108 K and the most stable ordered structure is antiferroelectric in the space group Cc, while an antiferroelectric P2₁2₁2₁ structure were found 4 K per water molecule higher in energy. ^[2]

On 14 June 2009, Christoph Salzmann and colleagues at the University of Oxford reported having experimentally reported an ordered phase of ice VI, named ice XV, and say that its properties differ significantly from those predicted. In particular, ice XV is antiferroelectric rather than ferroelectric as had been predicted. ^{[1] [3]}

Phase transition mechanism and crystal structure

In detail, ice XV has a smaller density (larger unit-cell volume) than ice VI. This makes the VI-to-XV disorder-to-order transition much favoured at low pressures. Indeed, differential scanning calorimetry by Shephard and Salzmann revealed that reheating quench-recovered HCl-doped ice XV at ambient pressure even produces exotherms originating from transient ordering, i.e. more ordered ice XV is obtained at ambient pressure. Being consistent with this, the ice VI-XV transition is reversible at ambient pressure. ^[4] It was also shown that HCl-doping is selectively effective in producing ice XV while other acids and bases (HF, LiOH, HClO₄, HBr) do not significantly enhance ice XV formations. ^[5]

Based on powder neutron diffraction, the crystal structure of ice XV has been investigated in detail. Komatsu et al. suggested that, in combination with density functional theory calculations, none of the possible perfectly ordered orientational configurations are energetically favoured, suggesting that there are several energetically close configurations that coexist in ice XV. They proposed 'the orthorhombic Pmmn space group as a plausible space group to describe the time-space averaged structure of ice XV. ^[6] Salzmann et al. argued that P-1 model is still the best (with the second best candidate of P2₁), whereas Rietveld refinement using the Pmmn space group only works well for poorly ordered samples. The lattice parameters, in particular b and c, are good indicators of the ice XV formation. Combining density functional theory calculations, they successfully constructed fully ordered model in P-1 and showed that experimental diffraction data should be analysed using space groups that permit full hydrogen order while the Pmmn model only accepts partially ordered structures. ^[7]

References

• Chaplin, Martin (2007-11-11). "Ice-six structure". Water Structure and Science. Retrieved 2008-01-02.