Strain-Modified Oxygen Transport in Rare-Earth Substituted Ceria: Migration Direction and Defect Association
Authors: Harrington George Frederick , Kim Sunho , Yildiz Bilge , Sasaki Kazunari , Tuller Harry L. 
Published: December 2020

Traditionally, the search for new electrolytes and electrodes for ionic devices with improved properties has been focused on compositional tuning and the search for new functional oxides. Recently, there has been increasing interest on the interplay of the chemical, electrical, and mechanical properties in electroceramic materials, referred to as electro-chemo-mechanics. This approach may offer a route to better than state-of-the-art materials making use of composites, nanoscale design, and engineered interfaces.

In particular, perturbing the crystal lattice away from the equilibrium structure via an applied lattice strain has been investigated from some time as a method to realise considerable improvements in oxygen-ion conductivity. Interest in this approach has, however, waned over recent years for two primary reasons: (i) a lack of consistency and reproducibility in the reported experimental findings and (ii) typically only modest changes in ionic transport are observed. In this presentation, we will address both reasons and make the case for lattice strain still being a promising route to enhanced oxygen transport.

By using an unconventional method of thermally annealing out strain which occurs during deposition of epitaxial rare earth-substituted films grown by pulsed laser deposition (PLD), we were able to tailor the strain with no influence from grain boundaries or interfaces (Figure 1a). The full strain states of the films, both out-of-plane and in-plane, as characterised by high-resolution X-ray diffraction, could be correlated with changes in the magnitude and activation energies of ionic conduction (Figure 1b). Through careful analysis of the literature, we managed, for the first time, to develop a quantitative consensus on the variation of the transport properties of ceria as a function of lattice strain, across both experimental and computational reports. Furthermore, by comparing the transport properties both in-plane and out-of-plane, we were able to experimentally demonstrate the effects of migration direction with respect to the biaxially strained plane (Figure 1c).

By investigating the substituent rare-earth cation, we were also able to vary the influence of defect-association on the ionic conductivity. Comparing La-, Gd-, and Yb-substituted ceria, we find that the change in activation energy with strain is dependent on the size of the dopant cation (Figure 1d), indicating that strain-modified conductivity is dependent both on the migration edge barrier and the defect-association. These findings give unique insights into the atomistic interaction of strain on the ionic transport of oxygen in substituted CeO2, and suggest that substantially improved conductivity in strained oxides may yet still be achieved if migration direction and defect association are optimised.

Figure 1 - (a) XRD peaks from the (004) out-of-plane reflection and (b) variation in the ionic conductivity of Gd-substituted films as a function of annealing temperature. (c) Schematic of a biaxially-strained CeO2 unit cell with out-of-plane and in-plane migration directions labelled. (d) Effect of strain on the out-of-plane and in-plane activation energies of conduction for Gd and Yb substituents.

Figure 1