1. Contribution from the Departments of Inorganic Chemistry and Condensed Matter Physics, University of Zaragoza−C.S.I.C., Plaza San Francisco s/n, E-50009 Zaragoza, Spain, Department of Chemistry, University of Tasmania, Box 252-75, Hobart, Tasmania 7001, Australia, and Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, Illinois 60439-4814