ON LEAST-SQUARES ESTIMATION OF EXTINCTION CORRECTIONS

Extinction corrections from published studies of the electron density in alpha-Al2O3 (corundum) do not show the crystal-size and wavelength dependence expected of extinction, as is now confirmed by values determined for small crystals. Corrections evaluated by minimizing differences between observed and calculated structure factors are sensitive to the weights of the observations to a degree that precludes accurate results. That sensitivity helps to obscure strong dependence of the extinction corrections on the structure-factor models used in structure refinements. A model-independent determination indicates that model-dependent corrections can exaggerate the effects of extinction on the measured structure factors.