In solid state oxygen ion conductors such as CeO₂ and ZrO₂, aliovalent cations are used to introduce oxygen vacancies which allow ionic conduction, but which also limit the maximum achievable conductivity due to defect-defect interactions. Materials are optimised based on the size and concentration of the substituted cations, in order to minimise defect association and maximise ionic transport. To push the performance of such materials beyond their compositional limit, significant attention has turned to the application of mechanical strain.

Computational studies have shown that the energy barrier for migration in CeO₂ and ZrO₂ can be manipulated using lattice strain. A number of studies are in broad agreement, showing that significant enhancements in the conductivity under tensile strain can be achieved.[1],[2] However, experimentally, the results on strained thin films and multilayers have been both controversial and inconstant, with vastly different findings in nominally similar systems.[3] Typically strained layers due to interfacial mismatch are grown and the thickness varied to control the volume fraction of the strained material. However this leads to difficulties controlling the density of grain boundaries, and isolating changes in the conductivity in the layers.[4] These complications mean that comparisons between experimental and computational work are challenging.

Furthermore, much of the work so far on the effects of strain on the transport properties have focused on the so-called ‘optimised’ ionic conductors, such as commercial electrolytes Y-stabilised ZrO₂ or Gd- or Sm-doped CeO₂. Very little attention has been payed to the ‘non-optimised’ electrolytes where the defect-defect interactions are much more significant. However, as the defect association limits the conductivity for these materials a much larger enhancement in the transport properties (or a less significant reduction) due to strain may be possible.

We have fabricated both ‘optimised’ and ‘non-optimised’ doped-CeO₂ films by pulsed laser deposition on MgO substrates. The dopants used were La, Gd, and Yb, which represent an ionic radii mismatch with the host Ce lattice of 19.6%, 8.6%, and 1.5% respectively. Epitaxy was maintained using a double BaZrO₃ and SrTiO₃ buffer layer resulting in films free from grain boundaries. The thickness of the films was kept constant and the strain varied by thermal annealing (from 600°C to 1000°C) to relax the intrinsic strain occurring during growth. Crucially, this means that the strain in these materials can be tailored and the conductivity measured without complications due to grain boundary effects or changes in the volume fraction of the interface and surface.

The out-of-plane and in-plane strain in the films was characterised in detail using X-ray diffraction and Raman spectroscopy, and the microstructure evaluated by high-resolution transmission electron microscopy. Impedance spectroscopy measurements showed that the conductivity of the relaxed films was in excellent agreement with bulk rare earth substituted CeO₂. With increasing in-plane compressive strain, the conductivity of the films was reduced and the activation energy increased. The observed change in the activation energy with strain is in excellent agreement with computational work, and consistent with other experimental findings. In addition we show that the effect is more pronounced for Yb-substituted CeO₂ than La- or Gd-substituted CeO₂, suggesting that larger changes in the conductivity with strain can be obtained with ‘non-optimised’ dopant cations.

[1] A. Kushima and B. Yildiz, “Oxygen ion diffusivity in strained yttria stabilized zirconia: where is the fastest strain?,” J. Mater. Chem., vol. 20, no. 23, p. 4809, 2010.

[2] R. A. De Souza, A. Ramadan, and S. Hörner, “Modifying the barriers for oxygen-vacancy migration in fluorite-structured CeO2 electrolytes through strain: a computer simulation study,” Energy Environ. Sci., vol. 5, no. 1, p. 5445, 2012.

[3] K. Wen, W. Lv, and W. He, “Interfacial lattice-strain effects on improving the overall performance of micro-solid oxide fuel cells,” J. Mater. Chem. A, vol. 3, no. 40, pp. 20031–20050, 2015.

[4] G. F. Harrington, A. Cavallaro, D. W. McComb, S. J. Skinner, and J. A. Kilner, “The effects of lattice strain, dislocations, and microstructure on the transport properties of YSZ films,” Phys. Chem. Chem. Phys., vol. 19, pp. 14319–14336, 2017.