Oxygen ion diffusivity in strained yttria stabilized zirconia: where is the fastest strain?

Abstract

We present the mechanism and the extent of increase in the oxygen anion diffusivity in Y2O3 stabilized ZrO2 (YSZ) under biaxial lattice strain. The oxygen vacancy migration paths and barriers in YSZ as a function of lattice strain was assessed computationally using density functional theory (DFT) and nudged elastic band (NEB) method. Two competing and non-linear processes acting in parallel were identified to alter the migration barrier upon applied strain: (1) the change in the space, or electronic density, along the migration path, and (2) the change in the strength of the interatomic bonds between the migrating oxygen and the nearest neighbor cations that keep the oxygen from migrating. The increase of the migration space and the weakening of the local oxygen–cation bonds correspond to a decrease of the migration barrier, and vice versa. The contribution of the bond strength to the changes in the migration barrier is more significant than that of the opening of migration space in strained YSZ. A database of migration barrier energies as a function of lattice strain for a set of representative defect distributions in the vicinity of the migration path in YSZ was constructed. This database was used in kinetic Monte Carlo (KMC) simulations to estimate the effective oxygen diffusivity in strained YSZ. The oxygen diffusivity exhibits an exponential increase up to a critical value of tensile strain, or the fastest strain. This increase is more significant at the lower temperatures. At the strain states higher than the critical strain, the diffusivity decreases. This is attributed to the local relaxations at large strain states beyond a limit of elastic bond strain, resulting in the strengthening of the local oxygen–cation bonds that increases the migration barrier. The highest enhancement of diffusivity in 9%-YSZ compared to its unstrained state is 6.8 × 103 times at 4% strain and at 400 K. The results indicate that inducing an optimal strain state by direct mechanical load or by creating a coherent hetero-interface with lattice mismatch can enable desirably high ionic conductivity in YSZ at reduced temperatures. The insights gained here particularly on the nonlinear and competing consequences of lattice strain on the local bonding structure and charge transport process are of importance for tuning the ionic transport properties in a variety of solid-state conducting material applications, including but not limited to fuel cells.