X. Z. Liao,1 J. Zou, 1 D. J. H. Cockayne, 1 R. Leon, 2 and C. Lobo3 1Australian Key Centre for Microscopy & Microanalysis, The University of Sydney, Sydney NSW 2006, Austral 2Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 911093Department of Electronic Materials Engineering, The Australian National University, Canberra ACT 0200, Austra (Received 17 November 1998 )