Relative Occurrence of Oxygen-Vacancy Pairs in Yttrium-Containing Environments of Y2O3-Doped ZrO2 Can Be Crucial to Ionic Conductivity

Yttria-stabilized zirconia (YSZ) is widely used as an electrolyte in solid oxide fuel cells. Much of current research on ionic conductivity in YSZ has focused on understanding migration barriers for O2- ion movement and vacancy trapping behavior in an attempt to understand why dopant cation edges result in fewer vacancy hopping transitions. We show that the free energy of finding O2--vacancy (O2--vac) pairs in a local environment can also be crucial to the hopping transitions. Higher probability of O2--vac pairs can result in a greater number of transitions. O2- ion movement in bulk YSZ is studied here using multiple independent short molecular dynamics (MD) trajectories. Analysis of the MD trajectories yields free energy of O2--vac pairs in 42 different local cation (Y3+/Zr4+) environments, to our knowledge calculated for the first time, as well as coarse-grained O2- hopping rates and Arrhenius parameters. On the basis of the free energies we conclude that it is possible that ionic movement is hindered in some environments not only because of high migration barriers or vacancy trapping as believed earlier but also because O2--vac pairs are destabilized by these environments. Increasing the temperature and/or decreasing the dopant composition stabilizes O2--vac pairs in these environments, which in turn affects the YSZ conductivity. Another newly found aspect is the connection between the kinetic rate constants for different environments; namely, the rates of interconversion between two environments depend on their O2--vac free energy differences.