706 Structure of Ice V

Ice V, the high-pressure ice phase stable at pressures of about 3 to 6 kbar, density 1.23 g.cm -3, has a structure involving 28 HzO molecules in a monoclinic cell of dimensions a= 9.22, b= 7.54, c= 10.35 A, p= 109"2 °, space group A2/a. The structure is a single tetrahedral framework, rather than a 'self clathrate' as occurs in the denser forms ice VI and VII. Each oxygen atom is hydrogen bonded to 4 near neighbors at distances of 2.76-2.87 A (average, 2.80 A), and the shortest next-nearest neighbor distance is 3.28 A.. Distortion from ideal tetrahedral coordination is rather large, bond angles (at oxygen) ranging from 84 to 128 °, with a r.m.s, deviation of 18 ° from 109.5 °. There is no indication of bifurcated hydrogen bonds. Proton ordering is not possible in the space group A2/a indicated for the oxygen atoms. A proton-ordered structure is possible in space group Aa, but is considered unlikely on the basis of comparison with X-ray evidence for proton order in ice II. For the ice V structure to remain proton disordered on quenching to 120 °K (the experimental conditions), the ordering energy must be less than 0.14 kcal.mole-1.