1. Luminescence and Solid State Ionics Laboratory, Department of Physics, Bharathiar University, Coimbatore 641046, India
2. Materials Research Centre, Department of Physics, N. G. M. College, Pollachi, Coimbatore 642 001, India
3. Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka-5, 166 28 Prague 6, Czech Republic
4. Department of Physics, Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore 641 043, India
5. Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing 210094, China