Theory of bonding charge density in β′ NiAl

Abstract

Fox and Tabbernor [ Acta metall. mater. 39, 669 (1991)] have recently measured the four lowest structure factors F( G) of NiAl using highly accurate high energy electron diffraction. We present here a systematic comparison of their results with ab initio band theory, in the context of the local density formalism. We find very good agreement for the three of the four lowest measured structure factors, while our F(200) is ∼0.4 e/cell higher. We tentatively attribute this difference to uncertainties in the treatment of the temperature factors in non-monoatomic compounds. Indeed, comparing with experiment our calculation for the monoatomic Si crystal (where the temperature term factors out), we find that theory reproduces the measured structure factors to within a very small deviations of ∼0.02 e/atom. We have also examined the effect of high Fourier components that are not currently amenable to measurements on the ensuing NiAl deformation electron density distribution (DEDD) maps. We find that the truncation of the Fourier series after four structure factors misses the directional d-like charge lobes near the Ni sites. We show that static and dynamic DEDD give a similar picture of the bonding.