Impurity effects on metallic cohesion: Lithium-row atoms in nickel clusters

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Abstract
We present results from a theoretical study of the localized bonding in atomic clusters consisting of octahedral nickel hosts and interstitial atoms from the lithium row of the Periodic Table. Trends in impurity binding energies and the effects of impurities on the elastic properties of the host are established by comparison of total energies and interatomic forces of the doped clusters with results for the bare host. Properties of the host cluster are strongly dependent upon impurity atom type. An orbital analysis identifies the microchemical mechanisms through which the elastic properties of the host respond to changes in impurity-host bond character, as we progress from strongly electropositive lithium to strongly electronegative fluorine. The tendency of Li and F to form ionic bonds destabilizes and weakens the Ni host cluster. A bonding mode which is predominantly covalent characterizes the remainder of the first-row series. A nonuniform variation of impurity-induced strain and elastic properties of the host is found, with midrow atoms B, C, and N effecting an appreciable strengthening of the host. Trends in the stability and strengthening character of the midrow series of atoms is a result of competing atom size and covalency effects. There is good qualitative correspondence between the trends and relative effects of different impurities in this study and the observed effects of first-row impurities on the cohesive properties of metallic host interfaces. This relationship suggests the microchemical properties of the cluster model may serve to identify atomic-level factors important to understanding macroscopic impurity effects in grain-boundary segregation.