A MULTIPLE WAVELENGTH STRUCTURE ANALYSIS OF KTaO3. H.-G. Krane1, A. Kirfell, K. F. Fischer2, lInstitut fur Mineralogie, University of Wurzburg, 2FR Kristallographie, University of Saarbrücken, Germany
Synchrotron radiation provides the opportunity to vary the radiation energy over a considerable range and thus to assess the reliability of structure parameters derived from least-squares refinements and to study the influence of absorption and extinction, particularly on the thermal displacement parameters.
Four data collections (4-Circle-Diffractometer at beamline D3, HASYLAB) have been carried out between the K- and L-absorption edges of Ta ([[lambda]]) = 0.71 Å, [[lambda]] = 0.56 Å, [[lambda]] = 0.40 Å, [[lambda]] = 0.237 Å) on a sphere (r = 0.065 mm) of cubic KTaO3 (a = 3.9885(2) Å, Pm3m). The measurements at [[lambda]] = 0.237 Å were performed using an untuned Si-monochromator and suppressing the 111-reflection by 2 mm Aluminium. The four data sets comprised between 1243 ([[lambda]] = 0.71 Å, smax = 1.315 Å-1) and 2071 ([[lambda]] = 0.40 Å, smax = 1.978 Å-1) reflections yielding 163 to 474 unique reflections, all internal agreement indices Rint(F2) being between 0.019 and 0.029. The data processing included spherical absorption and TDS corrections.
Conventional refinements (6 parameters), both for all-data and high-order reflections (s > 0.9 Å-1, without extinction), and a multipole refinement (14 parameters) for set III ([[lambda]] = 0.40 Å) converged between 0.0087 <= R <= 0.0165. The refined thermal displacement parameters u(K), u(Ta), u11(O) and u22(O) were found systematically dependent on both wavelength and absorption coefficient. While the relative distribution of the u-values along the bonds remained unchanged, the individual values showed an approximately linear positive correlation with the absorption coefficient varying between u = 426 cm-1 and u = 21.5 cm-l. At present, there is no obvious explanation for this behaviour. Results of a detailed analysis being under progress will be presented.