1. Department of Natural Resources, Faculty of Applied Sciences, Sabaragamuwa University of Sri Lanka, Buttala 91100, Sri Lanka,
2. Department of Material Science, Interdisciplinary Faculty of Science and Engineering, Shimane University, Matsue, Japan and
3. Japan Atomic Energy Research Institute, Tokai‐mura, Naka‐gun, Ibaraki 319‐1195, Japan