Zr vacancies and their complexes with hydrogen in monoclinic zirconia: formation energies and positron lifetimes

Abstract

Cation vacancies in metal oxides have high formation energies and, thus, are not as abundant as their oxygen-related counterparts. Nonetheless, they can be readily created during non-equilibrium processes. Positron-annihilation spectroscopy (PAS) is a well suited technique of probing such defects since negatively-charged lattice vacancies are deep potential wells which can act as positron traps. The present study reports first-principles calculations of positron trapping at Zr monovacancies in monoclinic zirconia. The binding of positrons and associated lifetimes for these defects were obtained within two-component density-functional theory under different approximations for the electron–positron correlation. The role of hydrogen was also explored. Low-energy vacancy–hydrogen defect complexes were determined for one and two hydrogen atoms bound to a single Zr monovacancy. Hydrogen decoration of the vacancy affected the localization of the positron leading to an appreciable decrease of the lifetime. The study was supplemented with PAS measurements on samples of monoclinic zirconia. From the PAS data two distinct, high-intensity components were resolved originating from different defect states. The corresponding lifetimes of 187 and 225 ps were very close to the theoretical predictions.