1. Functional Clay Materials Group, Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
2. Solid-State NMR Group, Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan