1. Key Laboratory of Transparent Opto-functional Inorganic Materials; Shanghai Institute of Ceramics, Chinese Academy of Sciences; 200050 Shanghai P.R. China
2. University of Chinese Academy of Sciences; 100049 Beijing P.R. China
3. Department of Materials Science; University of Milano-Bicocca; via Cozzi 55 20125 Milan Italy
4. Institute of Physics; ASCR; Cukrovarnicka 10 162 53 Prague 6 Czech Republic