Ab initio computational study of hydration thermodynamics in cubic yttria-stabilized zirconia

Abstract Hydration is a major process that controls defect equilibria in oxides through the exchange of oxygen and hydrogen species between the solid and its gaseous environment. For yttria-stabilized zirconia (YSZ), the presence of intrinsic oxygen vacancies that provide charge compensation to the acceptor dopants and the inherent structural disorder pose significant problems towards an understanding of how hydration operates at the atomistic level. First-principles calculations and ab - initio thermodynamics are employed in order to study the hydration reaction in cubic YSZ and the two types of defects appearing therein as reactants and products, the oxygen vacancies and protons, respectively, yielding the defect-formation energies, defect-induced deformation tensors and chemical expansion coefficients. The calculations are based on density-functional theory using a semilocal density functional and a screened-exchange functional approach and take into account the intrinsic structural disorder of the YSZ lattice. The various terms to the free energy of the hydration reaction are determined as a function of temperature and water-vapor partial pressure. The calculations provide estimates of the enthalpy and entropy of hydration in cubic YSZ examining how the solid-state and gas-phase contributions affect the free-energy balance. The final results are discussed in connection with experimental observations of hydration effects in YSZ and other oxides.