Effects of biaxial strain on bulk 8\% yttria-stabilised zirconia ion conduction through molecular dynamics

Tensile strain is thought to give rise to enhanced conduction properties in ion conducting compounds. However, most experimental studies in the field involve simultaneous presence of interface structures and strain, thus complicating separation of the individual effects. Here, we present molecular dynamics calculations that clarify the influence of biaxial strain in bulk yttria-stabilised zirconia. Such a study mimics what may be experimentally observed in epitaxially deposited films. We show that, as expected, tensile strain leads to enhanced ion conduction properties. The maximum enhancement is observed for a 2-3\% tensile strain. We show that the increase of bulk diffusion is in part due to an opening of the Zr-Zr and Zr-Y distances induced by tensile strain, leading to a smaller oxygen migration energy. Above a 3\% tensile strain, the diffusion coefficient of oxygen is strongly reduced, reaching values even lower than without strain. This decrease is associated with important structural changes of the cation and oxygen network. Also, we show that the diffusion coefficient increases by less than a factor 2 at 833 K for the optimal strain value. This confirms that the great increase of conductivity observed in zirconia/strontium titanate multilayers was due either to an electron contribution from strontium titanate or to the presence of interfaces, but not to the direct influence of strain on the oxygen diffusion coefficient in zirconia. Copyright \textcopyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.