Ice-ten (Ice X)
As the pressure is raised and the O···O
distance contracts, ice-seven appears to undergo a
continuous transition into cubic ice-ten (ice X)
(still space group, a, b, c = 2.78 Å at
62 GPa, 300 K )
where the ice protons are equispaced (and equally
bonded) between the oxygen atoms
in a molecular crystal . Thus each oxygen atom has four equidistantly tetrahedrally placed nearest neighbor hydrogen atoms. The oxygen atoms are arranged
in a body-centered cubic arrangement (each has eight oxygen nearest neighbors arranged as a cube). The hydrogen atoms have 12 nearest neighbor hydrogen atoms arranged in a body-centered truncated
cube. It is likely that ice-ten is at least partially ionized with both hydrogen atoms and oxygen atoms fully ionized. Also, some protons are likely to occupy the octahedral cavities as this behavior has been seen to develop in ice-seven at lower pressures . These dissociations have not been shown in the cartoons here.
The melting curve
for ice-ten has been proposed at high temperatures
(1000-2400 K) [612a]. As ice X is approached with increasing pressure from very hot liquid water and the O-H energy minima draw near to each other, the protons rapidly swap positions giving many short-lived OH- and H3O+ ions [612b]. In the ice X, however, protons are less mobile than in ice VII as there is only one energy minimum for each O-H-O bond,
Ice-ten looks identical as seen from
the x, y or z direction.
Ice-ten has triple points with ice-seven
and ice-eight (100 K, 62 GPa) and ice-seven and water
(~1000 K, ~47 GPa) [612b].
Interactive Jmol structures are given.