First-principles total-energy electronic structure calculations based on the full-potential linear-muffin-tinorbital method have been used to study the electronic and mechanical properties of the L1 2-type ordered nickel-based intermetallics Ni 3 X ͑XϭMn, Al, Ga, Si, Ge͒. The calculated values for the equilibrium volume and elastic properties are generally in good agreement with experiments. The large shear anisotropy factor across the series is attributed to the anisotropy of the bonding charge density, which can be described by the combination of charge transfer from X to Ni and strong X p-Ni d ͑Mn d-Ni d in Ni 3 Mn͒ hybridization effect. The more pronounced directional bonding between the Ni and Si atoms compared to that between the Ni and Al atoms, and the small ͑large͒ redistribution of bonding charge in Ni 3 Al ͑Ni 3 Si͒ when the systems are under shear strain result in a stronger resistance to a shear for Ni 3 Si. The bonding charge densities for Ni 3 Ga and Ni 3 Ge are found to be similar to those for Ni 3 Al and Ni 3 Si, respectively. These results suggest that the addition of the extra p electron on the X atom increases the directionality of the bonding. The change of bonding charge directionality in Ni 3 Mn is due to the Mn d-Ni d hybridization. The calculated ratio of bulk to shear modulus of polycrystalline systems, B/G, proposed by Pugh to provide a simple rule of measuring the ease of plastic deformation, is found to correlate well with the absolute difference in the s-orbital electronegativity between the atomic constituents, and the difference in energy, E d ͑Ni͒ϪE p (X) ͓E d ͑Ni͒ϪE d ͑Mn͒ for Ni 3 Mn͔, across the series. ͓S0163-1829͑96͒01443-9͔

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The elastic constants of the L12-type ordered nickel-based intermetallics Ni3X (X = Mn, Al, Ga, Si, Ge), have been calculated by means of ab initio total-energy electronic structurecalculations based on the full-potential linear-muffin-tin-orbital (FLMTO) method. Theorigins in the electronic structure of the variation of the elastic constants, bulk and shearmoduli are investigated across the series, and the effects of the anisotropy of bonding chargedensity on the shear anisotropy factor and the degree of ductility is discussed.