1. Institute for High Pressure Physics, Russian Academy of Sciences, 108840 Troitsk, Moscow, Russia
2. Institut für Geologie und Mineralogie, Universität zu Köln, Cologne 50939, Germany
3. Consortium for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, United States
4. School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom