1. Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 84536 Bratislava, Slovakia, Fax: +421‐(0)2‐59410444
2. Institute of Materials Science, Darmstadt University of Technology, Petersenstrasse 23, 64289 Darmstadt, Germany
3. Centre de Recherche sur les Matériaux à Haute Température, CRMHT‐CNRS, 1D avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France