1. Laboratory for High Performance Ceramics and Laboratory for Analytical Chemistry, Empa, Swiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland, Institute for Ceramics, Glass and Construction Materials, Technical University Bergakademie Freiberg, Agricolastrasse 17, 09596 Freiberg, Germany, and Center for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstrasse 8-10, 1040 Vienna, Austria