1. Department of Solid State Physics, Risø National Laboratory, P.O Box 49, DK-4000 Roskilde, Denmark
2. Manchester Materials Science Centre, UMIST, Grosvenor Street, Manchester M1 7HS, U.K., and CCLRC Daresbury Laboratory, Warrington, WA4 4AD, U.K.
3. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55405-0132