1. Contribution from the Centre de Recherche Paul Pascal, CNRS UPR 8641, avenue du Dr. A. Schweitzer, 33600 Pessac, France, and “Structural Ordering and Physical Properties”, PRESTO, Japan Science and Technology Corporation (JST) and Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji, Tokyo 192-0397, Japan