The most recently discovered ice structure – Ice XV

What does it look like?

Image generated by the VESTA (Visualisation for Electronic and STructural analysis) software http://jp-minerals.org/vesta/en/

What is it?

When it comes to the crystal structure of water ice, you probably are thinking 'well I'm sure they were all worked out long ago'. In that you would be wrong. Sorting out the crystal structures of water ice has been pretty complex – and even in recent years new phases have been discovered.

(a) The phase diagram of ice with some of the routes used to study the ordered form of ice, and (b) how the water molecule changes when going from a disordered to an ordered form of ice. Image taken from Salzmann et al., PRL, 2009.

As you can see in the figure above, we're now up to ice XV – yes, there are 15 known ways that the simple H₂O molecule can arrange itself as a solid. Ice XV was only discovered from observations and its structure worked out in 2009, though many had predicted that it would exist. It's an ordered form of ice, unlike the ice that you find in your freezer, where the molecules stay in one place and don't flip about. You can see how this works in (b) of the figure above.

Ice XV is the ordered form of the high-pressure Ice VI, which we've already featured on the blog.

Where did the structure come from?

The structure and discovery of Ice XV are described in this paper by Salzmann et al., which you can find on the arxiv pre-print server here.

Tags: high pressure   ice