Various crystallographic phases of solid oxygen occur under different conditions. Six are known, but the exact structure of one phase that exists at pressures above 10 GPa has eluded scientists. X-ray diffraction, spectroscopic and theoretical studies have failed to uncover its configuration.

Growing single crystals in this phase is difficult, but Paul Loubeyre and his colleagues at the French Atomic Energy Commission in Bruyères-le-Châtel found a way to do it. They then teamed up with Malcolm McMahon, a physicist at the University of Edinburgh, UK. He and his PhD student, Lars Lundegaard, had perfected a single-crystal diffraction technique on elements such as rubidium and tellurium at the Daresbury Synchrotron Radiation Source in Chesire, UK. Together, they hoped to resolve the long-standing question about oxygen's structure.

McMahon found that four O2 molecules come together in a rhombohedral shape, probably as a result of weak chemical bonds (see page 201). “The breakthrough was combining our French colleagues' ability to grow a single oxygen crystal with our speciality of single-crystal diffraction techniques,” he says.

These first-time collaborators recently received a grant to continue their work on crystals at even higher pressures.