Structure of ice. V

Abstract

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 H_2O molecules in a monoclinic cell of dimensions ɑ = 9•22, b= 7•54, c = 10•35 Å, β = 109•2°, space group A2/ɑ. 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 Å (average, 2•80 Å), and the shortest next-nearest neighbor
\ndistance is 3•28 Å. Distortion from ideal tetrahedral coordination is rather large, bond angles (at oxygen)
\nranging from 84 to 128 °, with a r.m.s, deviation of 18° from 109•5°. There is no indication of bifurcated
\nhydrogen bonds. Proton ordering is not possible in the space group A2/ɑ indicated for the oxygen atoms.
\nA proton-ordered structure is possible in space group Aɑ, 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).