Extra-Dense Glassy Ice

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)