Unveiling the oxygen-mediated hopping and defect effects of H+ in monoclinic ZrO2: A DFT+KMC study

During the aqueous corrosion of zirconium alloy cladding, the diffusion behavior of hydrogen within the oxide layer is a critical factor governing the hydrogen uptake rate. As the migration mechanisms of hydrogen in monoclinic zirconia (m-ZrO2) and the influence of defects on its diffusion remain unclear, there is still a significant discrepancy between the calculated diffusion coefficients of hydrogen in pure m-ZrO2 and the experimental values obtained from zirconium cladding oxide layers. To address this critical issue, this study systematically elucidates the atomic-scale diffusion mechanisms of H+ and the influence of defects by combining density functional theory and kinetic Monte Carlo (KMC) simulations. The research reveals that the microscopic diffusion of H+ in m-ZrO2 occurs through an alternating-hopping mechanism between two distinct stable sites, mediated by three fundamental elementary pathways. KMC calculations demonstrate that the H+ diffusion coefficient in the pure crystal significantly bridges the gap between previous theoretical predictions and experimental values. Further defect studies identify oxygen vacancies as having a strong trapping effect on H+ (capture radius of 2.8 Å), constituting the primary defect responsible for the lower experimental diffusion coefficients. Among alloying elements, Nb notably increases the H+ migration barrier and reduces jump frequency, suppressing H+ diffusion by up to an order of magnitude at temperatures below 600 K, whereas Sn exhibits minimal influence. Electronic structure analysis reveals that the anisotropy in H+ diffusion is caused by the asymmetric distribution of O2- ions acting as bridging sites in the mediated diffusion mechanism. The impact of Nb on the H+ migration barrier is achieved through a synergistic mechanism: enhancing the covalency of O-H bonds and providing a uniform delocalized electron environment that stabilizes H+. This study advances the understanding of the microscopic mechanisms governing hydrogen diffusion during corrosion, provides key kinetic parameters for multiscale modeling, and offers theoretical guidance for optimizing zirconium cladding performance.