Charge density in anhydrite, CaSO4, from X-ray and neutron diffraction measurements

The room-temperature electron-density distribution in natural anhydrite, CaSO4, has been studied by both X-ray and neutron diffraction. Conventional refinements of the data sets yielded RXw -0.020 and RNw = 0.025. The X-ray intensities were also used for high-order refinements, a refinement based on a point-charge model for the representation of positive residual charge accumulations, and a multipole expansion refinement. All these refinements were performed to study and to reduce parameter bias arising from bonding effects. The multipole expansion model yielded the significantly improved R w = 0.0116. The resulting (X--X) and (X-N) dynamic deformation density maps showed features qualitatively in agreement. Special attention was given to the charge distribution within and around the [SO4] 2anion. The observed bond density distribution is different from earlier results obtained from comparable sulfate derivatives and from the [8206 ]2anion, indicating a considerable 3d population of the S atom.