1. The ISIS Pulsed Neutron Scattering Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, U.K.; Department of Materials, University of Oxford, Oxford, OX1 3PH, U.K.; The H. Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland; and ExxonMobil Research and Engineering Co., Annandale, New Jersey 08801