1. C.N.R.S., Laboratoire des Elements de Transition dans les Solides and Laboratoire Physique des Solides, 1, Pl. A. Briand, F‐92195 Meudon, France
2. L.U.R.E., Batiment 209 D, Centre Universitaire Paris‐Sud, F‐91405 Orsay, France
3. Helsinki University of Technology, Laboratory of Inorganic and Analytical Chemistry, SF‐02150 Espoo, Finland