Scientists have worked out the structure for so-called very high density
amorphous ice (VHDA). The density of this ice is 1.25 g/cm3,
compared to 0.92 g/cm3 for ordinary ice and 1.0 g/cm3
for liquid water (at sea level and at a temperature of 4 C). This means
that VHDA ice would sink in water, not float like regular ice.
Most solids are denser than their corresponding liquids. In this respect
water is unusual, and this has made all the difference in the world
when it comes to the meteorological, chemical, and biological look of
things on Earth. Trying to understand why water is so unusual is why
physicists have spent so much time squeezing and freezing water in so
many ways.
To date, 13 different forms of crystalline water ice (each varying,
to some degree, in its internal structure) have been identified (http://www.cmmp.ucl.ac.uk/people/finney/jlf.html).
As for amorphous ices, in which the molecules don't adopt a regular
array, a fifth type was recently discovered. This last species, VHDA,
is notable since it retains its structure even at ambient pressure (although
it is made at a pressure of 14 kilo-bar), at liquid nitrogen temperatures,
77 K.
The team (University College London, Rutherford Appleton Lab, University
of Innsbruck) that has now worked out the structure for VHDA by diffracting
a beam of neutrons from the material suggests that VHDA may be a candidate
structure for the hypothetical second kind of liquid water whose existence
some think is necessary to explain the important anomalies of water.
However, their work also raises problems for the two-liquid scenario
by implying that rather than there being a single high-density structure,
a potentially large number of them might exist. (Finney
et al., Physical Review Letters, 11 November 2002;
contact John Finney at 44 20 7679 7850, j.finney@ucl.ac.uk; background
article, Mishima and Stanley, Nature,
26 Nov 1998, p. 329)
