1. Department of Physical Chemistry, Plovdiv University ``Paisii Hilendarski'', 24 Tzar Asen Str., 4000 Plovdiv, Bulgaria
2. Department of Physics and Oxide Research Center, Hankuk University of Foreign Studies, Yongin 449-791, South Korea
3. Retired from: Department of Inorganic and Physical Chemistry, University of Food Technologies, 26 Maritsa Blvd., 4000 Plovdiv, Bulgaria