The unusual mechanical properties of Ir, such as brittle failure after a long plastic deformation stage, were investigated by means of ab initio total-energy local density calculations. The structural and energetic characteristics of defects (vacancies, dislocation cores, stacking faults), as well as decohesion curves, were calculated and their peculiarities were compared with the results for the typical ductile metal, Au. The cores of the partial dislocations in Ir were significantly narrower than those in Au and other face-centered cubic metals. However, this difference was not of critical importance with regard to the peculiar mechanical properties of Ir. Due to a high dislocation mobility, the deformation mechanisms in Ir did not differ essentially from those in other face-centered cubic metals. The chemical bonding of Ir, as compared with Pt and Au, was considered on the basis of calculated charge-density maps and tight-binding parameters. It was shown that the appearance of a so-called pseudo-covalent component of the chemical bonding in Ir, under shear deformation, led to unusually large values of the shear moduli. This then led to the observed brittleness; according to standard brittle-ductile theory. The same component was also suggested to be the reason for the unusually high vacancy-migration energy in Ir. In addition, small solubilities of interstitial impurities were expected in Ir as compared with other face-centered cubic metals.

Peculiarities of Defect Structure and Mechanical Properties of Iridium - Results of ab initio Electronic Structure Calculations. Y.N.Gornostyrev, M.I.Katsnelson, N.I.Medvedeva, O.N.Mryasov, A.J.Freeman, A.V.Trefilov: Physical Review B, 2000, 62[12], 7802-8