From ND LISGARTEN
The article ‘All about Ice’ (18 December) mentions the crystalline
structures of ordinary hexagonal ice and refers to Kepler’s understanding
‘that the most efficient way to pack spheres is in a hexagonal array’.
There is a closely related cubic arrangement which is, geometrically, just
as efficient. The arrangement of the oxygen atoms for hexagonal ice is,
however, not based on close packing of equal spheres; the structure of hexagonal
ice is considerably more open than that of hexagonal close packing.
Mention is also made of ‘a form of ice with a different crystal structure
altogether’. This is associated with an ice structure in which the oxygen
atoms are arranged in a diamond cubic form; ice with this arrangement is,
in fact, cubic and is, structurally, closely related to ordinary hexagonal
ice. The geometry of both ice crystal forms can be regarded as suitably
arranged, similar hexagonal layers of oxygen atoms.
Strong evidence for the cubic form of ice was produced in Germany more
than fifty years ago. The cubic form was confirmed both here and in Japan
in 1956. It was produced by condensing water vapour on to a cold (-120 °C)
surface, the cubic arrangement determined by electron diffraction. Since
then a great deal of work has been done on various aspects of cubic ice.
N. D. Lisgarten Imperial College, London
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