First-principles study of interstitial hydrogen in yttria-stabilized zirconia.*

Hydrogen is a common impurity in oxides and has been known to exhibit a dual behavior: either by being a dopant or alternatively an amphoteric impurity with the transition (pinning) level, E(+/-), lying inside the gap [1]. By means of calculations based on density-functional theory (DFT) and a hybrid-functional scheme (Heyd-Scuseria-Ernzerhof) we have studied the incorporation of hydrogen in yttria-stabilized zirconia. Equilibrium sites and formation energies were determined and the role of intrinsic oxygen vacancies needed to stabilize the cubic phase of the oxide was particularly examined. Whereas, in its positively-charged state, $H^{+}$, hydrogen was found exclusively to form a dative-type bond with O ions, the neutral paramagnetic $H^{0}$ displayed a coexistence with deep interstitial configurations with minimal lattice relaxation of the host lattice. A number of atomic-level mechanisms and migration paths were explored in order to understand this site interplay and the dynamics of neutral $H^{0}$ in a way that is consistent with the existing experimental data. [1]~C.G. Van de Walle and J. Neugebauer, Nature {\bf 423}, 626 (2003).