1. Department of Material Science and Engineering, ENG445, Seoul National University, San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, Korea, and Chimie Molécolaire et Organization du Solide, UMR 5637 Case 007, Université Montpellier 2, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France