Split-vacancy defect complexes of oxygen in hcp and fcc cobalt

Abstract

One of the most ubiquitous and important defects in solids is oxygen. Knowledge about the solubility and diffusivity of oxygen in materials is crucial to understand a number of important technological processes, such as oxidation, corrosion, and heterogeneous catalysis. Density-functional theory calculations of the thermodynamics and kinetics of oxygen in cobalt show that oxygen diffusing into the two close-packed phases, namely $\ensuremath{\alpha}$ (hcp) and $\ensuremath{\beta}$ (fcc), strongly interacts with vacancies. We observe the formation of oxygen split-vacancy centers (V-${\mathrm{O}}_{\mathrm{i}}$-V) in both phases, and we show that this defect complex exhibits a similar migration energy barrier to the vacancy and oxygen interstitials. In contrast to the vacancy and oxygen interstitials, the oxygen split-vacancy centers exhibit an anisotropic strain field that couples to applied stress, making it possible to observe them through an internal friction experiment on quenched cobalt.