1. Osaka National Research Institute, 1‐8‐31 Midorigaoka, Ikeda, Osaka 563‐8577, Japan
2. Universite de Paris‐XI Orsay Laboratoire de Chimie des Solides, Batiment 414‐91405 Orsay Cedex, France
3. Faculty of Science, Kobe University, Nada Kobe, Hyogo 657, Japan