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Iridium crucibles, examined after being used in the extraction of gold and silver from residues remaining after zinc manufacture, showed an anomalously high permeability to substitutional components and impurities, such as gold. To discover the cause of this high permeability, the volume diffusion of gold into nominally pure single crystals of iridium (single-Ir), annealed under ultrahigh and technical grade vacuums, UHV and TGV, respectively, was studied at temperatures from 1300 to 2000 K. The coefficients of volume diffusion of gold into singleIr were measured by secondary ion mass spectrometry. The activation enthalpies measured for volume diffusion of the gold were: (Q Au Ir )UHV = (423 ± 5) kJ mol –1 and (Q Au Ir )TGV = (193 ± 27) kJ mol –1 in UHV and TGV, respectively, for annealing single-Ir. On annealing in TGV, interstitial impurities formed vacancy-impurity complexes (VICs). The binding energy of the VIC components (EVIC)Ir = (116 ± 16) kJ mol –1 . In TGV, the gold diffusion was determined by ‘extrinsic’ vacancies dissociating from the VICs. At typical working temperatures for iridium the concentration of the ‘extrinsic’ vacancies was several thousand times larger than the concentration of the equilibrium ‘intrinsic’ vacancies. The ‘extrinsic’ vacancies are responsible for the high coefficient of volume diffusion and the corresponding anomalously high mass transfer of gold into iridium during TGV annealing.

Mechanisms of Volume Diffusion of Gold into Single Crystal Iridium