1. SiAlON Group, Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
2. Solid-State NMR Group, Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0003, Japan
3. Frontier Research Center for Applied Atomic Sciences, Ibaraki University, 162-1 Shirakata, Tokai, Naka, Ibaraki 319-1106, Japan
4. NMR Station, Research Network and Facility Services Division, National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0003, Japan