1. Georg-August-Universität Göttingen, GZG, Abt. Kristallographie, Goldschmidtstr. 1, 37077 Göttingen, Germany
2. The University of Edinburgh, School of Chemistry and Centre for Science at Extreme Conditions, West Mains Road, Edinburgh EH9 3JJ, UK
3. SRS, Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK; and The University of Manchester, The School of Materials, Materials Science Centre, Oxford Road, Manchester M13 9PL, UK