1. The State Key Laboratory for Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China, Intense Pulsed Neutron Source Division, Argonne National Laboratory, Argonne, Illinois 60439, Electron Microscopy Laboratory, Department of Physics, Peking University, Beijing 100871, P. R. China, and Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P. R. China