1. College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, P.R. China
2. Baotou Research Institute of Rare Earths, Baotou 014020, P.R. China
3. China Northern Rare Earth High-Tech, Baotou 014030, P.R. China
4. College of Chemistry and Environmental Science, Inner Mongolia Normal University, Hohhot 010010, P.R. China