Hydrogen diffusion coefficient in monoclinic zirconia in presence of oxygen vacancies

To better understand the hydrogen diffusion mechanisms in monoclinic zirconia that take place in fuel rod cladding during reactor operation, we calculate the diffusion paths of different defects involving hydrogen and oxygen vacancies using Density Functional Theory with hybrid functionals, and use them to obtain the hydrogen and oxygen diffusion coefficients. We find a hydrogen diffusion coefficient varying between 10^?10 to 10^?20 cm²?s^?1 at 600 K, strongly depending on the hydrogen to oxygen vacancy ratio. We find that the interstitial hydrogen atoms are the main diffusing species even though they are not the dominant configuration of hydrogen atoms. We confirm the existence of different huge trapping effects, which slow the hydrogen diffusion. The main mechanism is either the trapping of hydrogen atoms in oxygen vacancies or the formation of interstitial dihydrogen molecules depending on the hydrogen to oxygen vacancy ratio.