1. The Guyuan Museum of Ningxia, Guyuan 756000, P. R. China
2. Key Laboratory of Macromolecular Science of Shaanxi Province, College of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710062, P. R. China
3. The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China