(178e) Step Formation On Yttria-Stabilized Zirconia: Combining DFT and Reactive Force Fields to Interpret Surface Characterization

Yttria-stabilized zirconia (YSZ) is a mixed oxide with high oxygen ion conductivity due to the presence of Y³⁺ induced oxygen defects within the cubic phase of zirconia. The high oxygen conductivity of YSZ facilitates its application in a variety of applications such as oxygen sensors, solid oxide fuel cell (SOFC) membranes, and heterogeneous catalyst supports. The surface structure of YSZ influences its performance in each of these applications, motivating characterization of stable YSZ surfaces at the atomistic scale. We report the results of density functional theory (DFT) calculations as well as reactive force field (ReaxFF) molecular dynamics and Monte Carlo simulations used to interpret and supplement scanning tunneling microscopy (STM) imaging of the YSZ(111) surface.

Imaging of the YSZ(111) surface reveals a high concentration of monatomic height step edges, formed upon Ar-ion sputtering and vacuum annealing. We calculate surface formation energies within the Vienna Ab-initio Simulation Program (VASP) for pure ZrO₂ and YSZ, for both flat and stepped surfaces. The surface formation energy of the stepped YSZ(111) surface is less than that of the pure ZrO2 stepped surface, showing that the addition of Y to cubic zirconia stabilizes monatomic steps on the (111) termination. To allow consideration of larger scale step structures, ReaxFF was used to perform molecular dynamics to simulate annealing. Surface energies calculated by DFT are used to parameterize the ReaxFF force field to facilitate accurate treatment of the YSZ surface. We apply Monte Carlo methods to sample the possible sites for yttrium and oxygen vacancies on the stepped YSZ(111) surface to find a minimum energy configuration. We perform simulated annealing on our minimum energy stepped surface model to probe surface reorganization and step stability. Our results show that Yttria-doping of cubic zirconia stabilizes the formation of monatomic steps on the (111) surface, in agreement with STM results.