1. Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki 305, Japan, Institute of Solid State Physics, Tokyo University, Roppongi, Minato-ku, Tokyo 106, Japan, National Institute for Resources and Environment, Tsukuba, Ibaraki 305, Japan, and National Institute of Material Chemistry, Tsukuba, Ibaraki 305, Japan