1. Contribution from the Materials Research Laboratory, University of California, Santa Barbara, California 93106, Manuel Lujan Jr. Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Solid State Physics, Risø National Laboratory, DK-4000 Roskilde, Denmark, Department of Physics, Brookhaven National Laboratory, Upton, New York 11973, and Department of Chemistry, The University of Chicago, Chicago, Illinois 60637