1. Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland, and Ecole Polytechnique, Mecanique Energetique, Technopole de Chateau-Gombert, 5 rue Enrico Fermi, 13453 Marseille Cedex 13, France