1. Contribution from the Anorganisch-chemisches Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland, LANSCE-LC, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Materials Research Laboratory, University of California, Santa Barbara, California 93106, and NIST Center for Neutron Research, Bldg 235, Gaithersburg, Maryland 20899