1. Department of Chemistry, University of Munich, Munich D-81377, Germany
2. School of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, P. R. China
3. Materials Science & Technology Division, Oak Ridge National Labortory, Oak Ridge, Tennessee 37831, United States
4. Key Laboratory of Polar Materials and Devices (MOE), East China Normal University, Shanghai 200241, P. R. China
5. Theoretical Materials Physics, Q-MAT, CESAM, University of Liège, B-4000 Liège, Belgium