Crystallographic Site-Occupancy Refinements in Thin- Film Oxides by Channelling-Enhanced Microanalysis

Specific site-occupation determinations of rare-earth (RE) additions in thin-film garnets of nominal composition Y 1.7 Sm 0.6 Lu 0.7 Fe 5 O 12 based on the orientation dependence of electron-induced characteristic X-ray emissions are described. The application of this technique called ‘channelling-enhanced microanalysis’ to a general non-layered crystal structure requires a theoretical prediction of the characteristic X-ray production as a function of incident-beam orientation because the specific-site-sensitive orientations cannot be determined by inspection of the crystal structure. Hence, a real-space formulation considering flux loss from the incident beam and under the assumption of highly localized inner-shell excitations has been described for the characteristic X-ray production in thin crystals. Applying this theory, a g = 121 systematic orientation was predicted to be the orientation that is specific-site sensitive for these thin-film garnets. Experimentally observed data were then refined, using a constrained least-squares analysis to give probabilities for the occupation of RE additions in the different crystallographic sites. Thus, it has been shown that in these compounds, Lu 3+ and Sm 3+ additions prefer octahedral occupation with a probability ≥95%. Assumptions, limitations and future potentials of this novel crystallographic technique are also discussed.