Development of Methods for the Mapping of Outer Electron Charge Density Using Modern X-Ray Powder Diffraction Techniques

A method for constructing high accuracy electron density maps has been developed. This method uses a modified Rietveld analysis to determine structure factors from X -ray powder diffraction data. The modified Rietveld analysis determines measured, theoretical, and core structure factors. Fourier inversions of measured, measured minus theoretical, and measured minus core structure factors are used to construct electron density, electron difference, or outer electron density maps. This method was designed to measure the small electron densities associated with bonding. To measure these densities, electron difference and outer electron density maps must be constructed. Constructing accurate maps is extremely exacting and complicated work. To better understand this method, each step of the process is thoroughly discussed. Both theoretical and practical aspects of the method are covered, including several examples. Finally, measurements were made for CoAl, NiAl, SiC, and Mo. Due to diffractometer limitation, the accuracy of CoAl and NiAl measurements was very poor. The accuracy of the SiC and Mo measurements was much higher. Both SiC and Mo maps indicated regions of high electron density between the nearest neighbor atoms. The magnitude and location of these regions was consistent with an increase in electron density caused by covalent bonding.