Water structure and science
(http://www1.lsbu.ac.uk/water/ice_xvii.html)

Ice-seventeen (Ice XVII)

Ice-XVII crystal structure,

from [2796]

ice-XVII, from [2796]

Molecular hydrogen (H2) forms several different clathrates as the H2 molecules can fit in most cages with multiple H2 in the larger cages (see elsewhere). At high pressures (≈ 400 MPa, 280 K) a different structure is formed (an intercalated filled ice rather than a clathrate) with the composition (H2O)2H2 and consisting of interpenetrating spiral chains of water molecules showing topological similarity to the mineral quartz [2773]. All the hydrogen can be released by warming under vacuum for about one hour at a temperature of 110-120 K to give a new metastable hexagonal ice structure with space group P6122 with cell dimensions 6.3305 Å, 6.3305 Å, 6.0580 (a, b, c; 90°, 90°, 120°, 6 molecules, 75 K). The hydrogen-bonding is random, following the 'ice rules'.

Ice-XVII crystal structure,

from [2796]

ice-XVII, from [2796]

Above 130 K, the sample transforms into ice Ih. It is microporous, due to its hexagonal helical pores (with 6.10 Å. channel diameter from the center of the oxygen atoms leaving a space of ≈ 3 Å minimum diameter) and will re-adsorb molecular H2(≈ 1.8 Å diameter) if exposed to it. The structure of the heavy water version of this ice has been determined by neutron diffraction [2796].

Ice XVII consists of sheets of pentamers of hydrogen-bonded water molecules made up from (H2O)12 partial dodecahedra (see left, the xz-plane down the y-axis) that form hexagonal helices down the z-axis (see above right). This xz sheet-like structure is also formed in the [CoII(cyclam)Cl2].3H2O (cyclam =1,4,8,11-tetraazacyclotetradecane) clathrate [1871].

Confinement of Ne atoms or O2 molecules inside the spiral hexagonal channels of the deuterated ice XVII is possible at mild pressure (~100 kPa) and low temperature (~40 K). These are filled ices rather than clathrates. The molar ratios are H2O:Ne = 6 mol:1.27 mol (unit cell) and H2O:O2 = 6 mol:1.36 mol (unit cell) [3632]. CO2-filled ice XVII has been found by molecular dynamics simulations to be more stable than the CO2-filled sI clathrate at pressures ranging from 0.6 GPa to 1.8 GPa and up to about 308 K [3749]. Ice XVII may be used as a cheap, useful, and environmentally friendly, microporous material for the storage of CO2 as (H2O)3.5CO2. [3059]. Ice XVII can also be filled with N2 forming an sX clathrate hydrate, (H2O)2.6N2 [3750].

H2 molecules rotate almost freely in these narrow channels [3172]. Confinement of different mixtures of ortho-H2 and para-H2 molecules within the channels allows the assignment of the H2 motion spectral bands to rotational and center-of-mass translational transitions of either para-H2 or ortho-H2 [2796].

D2O ice XVII may be converted to pure D2O cubic ice without stacking defectsunder dynamic vacuum to remove the contained H2 [3857] .

For an interactive Figure, see Jmol.

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(c) Martin Chaplin 9 February, 2020
(printed 25 May 2020)