1. Division of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, 7‐3‐1 Hongo, Bunkyo‐ku, Tokyo 113, Japan
2. Fundamental Research Center of Osaka Gas, Incorporated, 6‐19‐9 Torishima, Konohana‐ku, Osaka 554, Japan
3. Japan Fine Ceramics Center, 2‐4‐6 Mutsuno, Atsuta‐ku, Nagoya, Aichi 456, Japan