1. School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh, EH9 3JJ, U.K., Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, U.K., and CCLRC, Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, U.K.