1. School of Materials Science, Japan Advanced Institute of Science and Technology, 15 Asahidai, Tatsunokuchi, Ishikawa 923-12, Japan, Department of Chemistry, University of Florence, Via Gino Capponi, 7, 50121 Florence, Italy, and Department of Soil Science and Plant Nutrition, University of Florence, P.le delle Cascine, 28, 50144 Florence, Italy