Structure determination by use of pattern decomposition and the Rietveld method on synchrotron X-ray and neutron powder data; the structures of Al₂Y₄O₉ and I₂O₄

High-resolution synchrotron powder X-ray data have been used to solve the structures of I₂O₄ and Al₂Y₄O₉ by profile decomposition into individual structure amplitudes, followed by application of standard Patterson and direct methods. The data were obtained from a series of spectra recorded with a small linear position-sensitive detector, and some of the errors arising from this approach are discussed. The final structures were obtained by Rietveld refinement, and R_p values for the profile intensities are 10.9 and 9.2% for Al₂Y₄O₉ and I₂O₄ respectively. For Al₂Y₄O₉ good agreement was found with the known isomorphous compound Al₂Eu₄O₉, while I₂O₄ data were compared with high-resolution neutron powder data. This analysis, together with other recently reported studies, shows that structure determination from powder data will be feasible even for rather complicated structures.