Diffusivity Tensors of Br and Cs Vacancies in Biaxially Strained Perovskite CsPbBr₃

Using density functional theory and kinetic modeling, we investigate the relationship between imposed biaxial strain and Br- and Cs-vacancy diffusion in orthorhombic (Pnma) CsPbBr₃, in the dilute limit. We calculate the activation energies for the hopping of vacancies between all pairs of nearest-neighbor lattice sites and use the resulting values to parametrize 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 nonlinear in general and often nonmonotonic. 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.