Properties of bcc metals by tight-binding total energy simulations

Monovacancies of six bcc d-transition metals V, Cr, Nb, Mo, Ta and W have been studied by tight-binding (TB) simulations using the NRL-TB method. This method satisfactorily reproduces the electronic properties, phonon frequencies, thermal expansion coefficients and atomic mean-squared displacements for pure bulk systems. The TB method shows that for group-V metals the atoms in the first neighbor shell of the vacancy relax inward by about 5% and lose charge from d orbitals, while second neighbor shell atoms relax outward by about 1%. For group-VI metals both first and second neighbor shells relax inward and gain d orbital charge, and for Mo and W the relaxation is in fact stronger for the second than the first neighbor shell. These results are not significantly affected by the use of charge self-consistent terms in the TB model. The structural and charge transfer changes are accompanied by new energy bands at selected k-points for group-VI while they cover the whole Brillouin zone (BZ) for the group-V metals. Comparison to first-principles calculations for one example, Nb, shows good agreement for the presence of new energy bands as well as the charge and structural relaxation of the first and second neighbor shells.