1. Department of Subatomic and Radiation Physics, Radiation Physics Group, Gent University, Proeftuinstraat 86, B-9000 Gent, Belgium, Member of IBITECH; Eenheid Algemene Chemie (ALGC), Free University of Brussels, Pleinlaan 2, B-1050 Brussels, Belgium, and Laboratory for Crystallography and Study of the Solid State, Gent University, Krijgslaan 281-S1, B-9000 Gent, Belgium