* Astronomy

A new evolutionary crystallography algorithm predicts the structure of crystals under a range of extreme pressure and temperature conditions on the basis of the chemical composition alone. One of these crystals would be a form of red-coloured oxygen. Predicting crystal structures is difficult even for simple solids, partly because of the task of sorting among the astronomical number of possible ways given atoms can compose a basic repeatable unit cell.

Artem Oganov, a scientist at the Swiss Federal Institute of Technology (ETH) in Zurich, in Switzerland, and Colin Glass, a Ph.D. student, approach the problem by combining electronic structure calculations and a specifically developed evolutionary algorithm. In exploring the myriad atomic arrangements, they proceed in a step-by-step, continual-optimisation fashion that avoids configurations less likely to succeed. This makes the algorithm very efficient and allows the researchers to make certain specific predictions.

One example is calcium carbonate (CaCO3) at very high pressures. Oganov's team for the first time predicted two new stable structures for this mineral. By now, both structures have been confirmed in experiments by Japanese colleagues. Oganov and Glass have also solved the structures crystalline oxygen at high pressure.

Oxygen is unique from the chemical point of view. The only magnetic molecular element known, under pressure it loses its magnetism and turns red. The structure of red oxygen, which remained unknown for a long time, seems to be finally solved and turns out to be unique; that is, it does not manifest itself in any other element. At even higher pressure oxygen is known to turn black in colour and become superconducting, which happens because of the increased interactions between the O2 molecules. The ETH researchers also predict a new stable phase of sulphur and several new metastable forms of carbon.

Read more (PDF)