1. Department of Chemistry; Aarhus University; Langelandsgade 140 8000 Aarhus C Denmark
2. Laboratorie Léon Brillouin; CEA Centre National de la Recherche Scientifique, CE-Saclay; Gif-sur-Yvette 91191 France
3. Australian Centre for Neutron Scattering; Australian Nuclear Science and Technology Organisation; Locked Bag 2001 Kirrawee DC NSW 2232 Australia
4. Beijing National Laboratory for Molecular Sciences; State Key Laboratory of Rare Earth Materials Chemistry and Applications; College of Chemistry and Molecular Engineering; Peking University; Beijing 100871 P. R. China