1. Department of Chemistry, The University of Liverpool, Liverpool L69 7ZD, U.K., ISIS Facility, CCLRC - Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OX11 0QX, U.K., and Ceramics Division, Filtronic Comtek, Enterprise Drive, Station Road, Four Ashes, Wolverhampton WV10 7DB, U.K.