Low-angle atomic scattering factors and charge density of aluminum

Abstract

All the published experimental and theoretical atomic scattering (form) factors for aluminum have been carefully reviewed and only the low angle 111, 200 and 220 crystal form factors appear to be significantly different from the best free atom values. The best powder X-ray diffraction and high accuracy 111 and 200 electron diffraction form factors for Al are in good agreement with each other and with the best theoretical calculations. The equivalent scattering factors measured by X-ray Pendellösung methods seem somewhat high and are not a good representation of the charge density of aluminum. A (110) deformation charge density map based on the best experimental form factors indicates that bonding in aluminum is achieved by the spherical depletion of electrons from atomic sites and redistribution so that there is a build-up between nearest-neighbor (n.n.), second n.n. atoms, etc; this, of course, is the traditional plcture of a metallic bond. A comparison of the aluminum (110) deformation density map with the equivalent map for copper shows that the bonding schemes for the two elements follow an almost identical pattern. This indicates that even though copper and aluminum have large differences in atomic number and valency, their bonding schemes are primarily determined by the face-centered-cubic crystal structure.