1. Inst. Solid State Chemistry, University of Augsburg, Universitätsstr. 1, D-86159 Augsburg, Germany, CNRS−IMP, Bp-5 Odeillo, F-66125 Font-Romeu Cedex, France, Department General Energy Research, Paul Scherrer Institute, CH-5232 Villigen, Switzerland, and Department of Mechanical and Process Engineering, ETH-Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland