Peculiarities of defect structure and mechanical properties of iridium: Results of ab initio electronic structure calculations

The nature of the unusual mechanical properties of Ir in comparison with other fcc metals (brittle failure after a long stage of plastic deformation) is investigated on the basis of ab initio total energy local density calculations. The structure and energy characteristics of defects (vacancies, dislocation cores, stacking faults) as well as decohesion curves are calculated for Ir, and their peculiarities are discussed in comparison with results for a typical ductile metal Au. The chemical bonding of Ir in comparison with Pt and Au is considered based on the calculated charge density maps and tight-binding parameters. It is shown that the appearance of a "pseudocovalent" component of the chemical bonding in Ir under shear deformation leads to unusually large values of its shear moduli, which is the final cause of its brittleness, according to the standard criteria of brittle-ductile behavior.