Bond energies and defect forces around a vacancy in BCC transition metals

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Abstract
The authors present the tight-binding bond (TB) model, which provides a method of calculating bond energies and interatomic forces within a quantum-mechanical framework, and they apply it to the case of a single vacancy in BCC transition metals. Vacancy-induced changes in the strength of the bonds are described: the major changes are a marked strengthening of the bonds connecting the atom (111) to (222) and a lesser strengthening of the bonds surrounding the vacancy. This shows the importance of the effect of the bond charge redistribution induced by the vacancy. Defect forces obtained are negligibly small for the first neighbours of the vacant site, and inward and very large for the second and fifth neighbours. Lattice distortion around the vacancy to relax these forces evaluated using the experimental perfect-lattice Green functions is long-range, inward and radial except for the hardly displaced first-neighbour atoms. These results are quite different from those of the usual phenomenological models which do not account for the dominant effect of the bond charge redistribution. The results converge qualitatively well by including up to fifth moments in the calculation of the bond order.

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