1. Department of Materials Chemistry, The Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden, Université de Toulouse, CIRIMAT UMR CNRS 5085, 118 rte de Narbonne, 31062 Toulouse Cedex 9, France, and Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden