Ice Ih

Ice Ih (hexagonal ice) is a proton disordered ice phase having the space group P6₃/mmc. Ice Ih has the following lattice parameters at 250 K: a=4.51842 Å, $b=a{\sqrt {3}}$, and c=7.35556 Å with four molecules per unit cell (table 3 in Ref. 2). The proton ordered form of ice Ih is known as ice XI, which (in principle) forms when ice Ih is cooled to below 72K (it is usually doped with KOH to aid the transition).

References

1. Linus Pauling "The Structure and Entropy of Ice and of Other Crystals with Some Randomness of Atomic Arrangement", Journal of the American Chemical Society 57 pp. 2674 - 2680 (1935)
2. K. Röttger, A. Endriss, J. Ihringer, S. Doyle and W. F. Kuhs "Lattice constants and thermal expansion of H2O and D2O ice Ih between 10 and 265 K", Acta Crystallographica Section B 50 pp. 644-648 (1994)
3. Carlos Vega, Carl McBride, Eduardo Sanz and Jose L. F. Abascal "Radial distribution functions and densities for the SPC/E, TIP4P and TIP5P models for liquid water and ices Ih, Ic, II, III, IV, V, VI, VII, VIII, IX, XI and XII", Physical Chemistry Chemical Physics 7 pp. 1450 - 1456 (2005)
4. Jose L. F. Abascal and C. Vega "The melting point of hexagonal ice (Ih) is strongly dependent on the quadrupole of the water models", PCCP 9 pp. 2775 - 2778 (2007)
5. E. G. Noya, C. Menduiña, J. L. Aragones, and C. Vega "Equation of State, Thermal Expansion Coefficient, and Isothermal Compressibility for Ices Ih, II, III, V, and VI, as Obtained from Computer Simulation", Journal of Physical Chemistry C 111 pp. 15877 - 15888 (2007)