Accurate structure analysis with synchrotron radiation. The electron density in Al2O3 and Cu2O

Abstract

Accurate room-temperature structure analyses have been carried out on the two well known structures of cuprite, Cu2O, and corundum, α-Al2O3, using synchrotron radiation, in order to assess the accuracy of single-crystal X-ray diffraction data that can be obtained with such a source. The two compounds were chosen since results can be rigorously cross checked against deposited data from careful X-ray tube measurements which have been analyzed in terms of electron-density distributions. The synchrotron data were collected on the five-circle diffractometer at HASYLAB in the dedicated mode of DORIS II (3.7 GeV) within normal beam time allowance at the same wavelengths as the tube experiments. The final Cu2O data set included 21 'forbidden' reflections whose intensities cannot be measured using tube radiation. The intensities of these latter reflections turned out to be predominantly determined by the anisotropic vibration of Cu. Refinements using multipole expansion, models yielded agreement indices R = 0.0173 and 0.0078 for Cu2O and Al2O3, respectively. Structure factors as well as static-model deformation properties including electric-field gradients are compared with the corresponding literature results. Most findings are in satisfactory agreement implying that high-quality diffraction data can be obtained with a synchrotron-radiation source within reasonable time, provided proper attention is given to the experiment and the data-reduction procedure. In particular, the use of synchrotron radiation allows recording of weak and very weak reflection intensities with an accuracy that could never be achieved in conventional tube experiments. An additional data collection on Cu2O in parasitic mode (5.3 GeV) shows that under less-favorable conditions data can also be collected at a synchrotron-radiation source with an accuracy sufficient for standard structure analyses.