1. Research Center, Asahi Glass Co., Ltd., 1150 Hazawa‐cho, Kanagawa‐ku, Yokohama 221, Japan
2. Institute of Industrial Science, University of Tokyo, 7‐22‐1 Roppongi, Minato‐ku, Tokyo 106, Japan
3. Department of Materials Science and Technology, Science University of Tokyo, Noda, Chiba 278, Japan
4. Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831