Perovskite lead–halides (PLHs) are approximately an order of magnitude softer than their oxide counterparts. While this property (among others) makes them promising candidates for applications in e.g. flexible (opto-)electronic devices, there is a strong correlation between mechanical strain and the device stability, which has been attributed to changes in the activation energies for ion transport in the PLH layer due to residual strain, and for anion vacancy transport in particular [1,2]. On the other hand, the interplay between strain and ion transport could be exploited for e.g. memristive applications [3]. While there exists computational evidence that activation energies for anion-vacancy hopping are lowered in the presence of positive (tensile) strain and increased in the presence of negative (compressive) strain [4,5,6], a detailed atomistic analysis of this interplay is currently lacking, and cation transport has not been considered at all.

Therefore, we carried out a DFT-based study on the effects of imposing biaxial strain on Br- and Cs-vacancy diffusion in a model perovskite halide, orthorhombic (Pnma) CsPbBr₃. We calculate the activation energies for the hopping of vacancies between all pairs of nearest-neighbour lattice sites, and use the resulting values to parametrise a kinetic scheme, and thereby to calculate vacancy diffusivity tensors.

Our results indicate that the relationship between strain and vacancy diffusion is significantly more complex than previously thought – activation energies for vacancy hopping may increase or decrease for both negative (compressive) or positive (tensile) imposed biaxial strain, depending both on the plane in which strain is imposed and the particular pair of sites between which the vacancy hops, and the relationships are non-linear in general and often non-monotonic. Furthermore, we find that the influence of imposed biaxial strain on the diffusivity is significantly greater for Cs vacancies than for Br vacancies, and in particular, that values of Cs-vacancy diffusivity approach those of Br-vacancy diffusivity under certain conditions. This suggests that phenomena attributed solely to the transport of anion vacancies in PLHs might owe a significant contribution to cation-vacancy transport, particularly if a strong strain dependence is observed, such as in the stability of PLH based devices.