1. Institut Laue Langevin, 6 rue Jules Horowitz, B.P. 156, 38042 Grenoble Cedex 9, France, ISIS Facility, STFC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K., Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, U.K., Unilever Research and Development Laboratory, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, U.K., and CEA Saclay, Laboratoire Léon Brillouin, F-91191 Gif-sur-Yvette, France