ELECTRON CHARGE-DISTRIBUTION OF CHROMIUM

A critical review of all the X-ray crystal structure factors of chromi um has been performed. This showed that the low-angle X-ray atomic sca ttering (form) factors have been most accurately measured by electron diffraction and that, with correct choice of Debye temperature and ana lysis procedure, these are in excellent agreement with the best theore tical calculations. These accurate form factors were found to be consi derably less than those obtained from free atom calculations and the i mplications of these differences for bonding in chromium was investiga ted by generating a (110) deformation electron density distribution. T his showed that bonding in chromium is achieved by the close-to-spheri cal depletion of electrons from atomic sites with redistribution such that there is a build-up of electrons in [100] and [110] between secon d and third nearest neighbour atoms with apparently very little charge density between nearest neighbour atoms in [111]. These observations are consistent with the fact that the directional component of bonding in chromium is dominated by the 3d electrons which can arrange themse lves so that the Cr atoms achieve closer packing in the b.c.c. structu re than if they were spherical.