Reflexprofilanalyse zur Bestimmung der Elektronendichteverteilung in Kristallen

Accurate electron density maps for the study of chemical bonding and thermal motions in crystals can be obtained only if the integral Bragg intensities are corrected for absorption, extinction, thermal diffuse scattering etc. This can be done by careful studies of the photographically observed relative course of the line profiles, using a modified oscillation method of James and Bosanquet in combination with a stationary film. The analysis of the profiles by folding integrals simultaneously offers 4 parameters: the amount (β) of one-phonon scattering in the Bragg reflexion, the absorption coefficient μ (including secondary extinction) and the parameters n, R₀ of the shape factor of the polydisperse assembly of mosaic blocks. Thus some important correction factors are obtained for the structure factor measured in the Geiger-counter goniometer. The whole extinction coefficient is obtained directly from the one profile analysis without grinding, polishing, etching or using a set of platelets. A small amount (γ) of crystalline powder can now no longer disturb experimental results. Grinding with 1 μ carborundum powder, γ-values up to 10% were observed on NaCl single crystals. The amounts β of one-phonon scattering are 10.2% and 16% for the NaCl Bragg reflexions 600 and 800 respectively at 20°C.