Incorporation and migration of hydrogen in yttria-stabilized cubic zirconia: Insights from semilocal and hybrid-functional calculations

Hydrogen is a common impurity in oxides and is known to exhibit dual behavior: It can act either as a dopant or alternatively as a compensating impurity, depending on whether its transition (pinning) level, E(+/-), intersects the conduction band or lies deep in the energy gap. In the present work the incorporation of isolated hydrogen in 10.3 mol% yttria-stabilized zirconia was studied by ab initio calculations employing a semilocal exchange-correlation density functional and a hybrid-functional approach. Equilibrium sites and formation energies were determined for the different charged states of hydrogen and the role of intrinsic oxygen vacancies needed to stabilize the cubic phase of the oxide was particularly examined. Hydrogen was found to be an amphoteric impurity with the equilibrium charge-transition levels, E(q,q'), lying deep inside the gap. Whereas, in its positively charged state, H+, hydrogen was found exclusively to form a dative-type bond with the lattice oxygens, the negatively charged and neutral states also adopt interstitial configurations provided by the empty cubes and the intrinsic structural vacancies of the anion sublattice. Two distinct paramagnetic configurations of hydrogen, H-0, are predicted and both of them induce deep localized levels in the band gap. The first configuration is higher-energy compact atomlikewith the hydrogen at the interstitial sites, whereas the second one is a bond-type deep donor configuration with the unpaired 4d electron localized predominantly at an undercoordinated Zr cation in the vicinity of the impurity. Minimum-energy paths and corresponding classical barriers of migration were also determined for H-0 with the aid of the nudged elastic-band method providing insight on the feasibility of site interplay of H-0 and interconversion among its interstitial and donor configurations. Oxygen vacancies and lattice relaxation were found to have a major effect on the energy profiles of the paths, the position of the transition states, and the magnitude of the migration barriers.