The charge density in copper

Copper is a face-centered cubic (fee) metal. Band structure calculations show the valence bands to be copper d bands and hybrid bands of sd, pd, and sp character. The hybridization is essential for the conductivity of copper, as some of the bands cross the Fermi surface and are thus only partially occupied (K. Schwarz, private communication). 

Schneider, Hansen, and Kretschmer (SHK) (1981) have measured the 19 reflections with sin 0/A 0.7 eA 3 with 0.03 A y-radiation. Deformation densities based on these reflections show a small accumulation of charge of height 0.19 eA 3 at 1/4 1/4 0 and equivalent positions, which is between nearest neighbors located along the [110] directions, as well as an accumulation of similar height, but somewhat more extended, in the voids between the atoms at 1/4 1/4 1/2. This seems fully compatible with a hybrid bonding model. 

Much of the available experimental information on intermetallic compounds comes from high-energy electron diffraction (HEED) measurements. In electron diffraction, the electron beams interact with the electrostatic potential in the crystal. The electron structure factor is therefore directly dependent on this 

The alloy /J NiAl is a solid with a CsCl-type structure in which one atom is located at the corners, and the second atom at the center of the unit cell. The valence-electron to atom ratio is often quoted as 1.5, using a counting scheme in which the transition metal has zero valence and the A1 is considered as trivalent. 

In such models, the bonding is considered to be partially ionic with a charge transfer from A1 to the Ni 3d valence band. To explain the properties of /J NiAl at a more sophisticated level, Fox and Tabernor (1991) measured four low-angle structure factors by the HEED critical-voltage technique. The deformation density based on these four reflections shows a depletion of density around both the Ni and A1 atoms, and a buildup of about 0.13 eA-3 along the [111] direction halfway between Ni and A1 nearest neighbors. 

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