  Metal halide perovskites (MHPs) are nowadays one of the most studied
semiconductors due to their exceptional performance as active layers in solar
cells. Although MHPs are excellent solid-state semiconductors, they are also
ionic compounds, where ion migration plays a decisive role in their formation,
their photovoltaic performance and their long-term stability. Given the
above-mentioned complexity, molecular dynamics simulations based on classical
force fields are especially suited to study MHP properties, such as lattice
dynamics and ion migration. In particular, the possibility to model mixed
compositions is important since they are the most relevant to optimize the
optical band gap and the stability. With this intention, we employ DFT
calculations and a genetic algorithm to develop a fully transferable classical
force field valid for the benchmark inorganic perovskite compositional set
CsPb(Br_xI_(1-x))_3 (x = 0,1/3,2/3,1). The resulting force field reproduces
correctly, with a common set of parameters valid for all compositions, the
experimental lattice parameter as a function of bromide/iodide ratio, the
ion-ion distances and the XRD spectra of the pure and mixed structures. The
simulated thermal conductivities and ion migration activation energies of the
pure compounds are also in good agreement with experimental trends. Our
molecular dynamics simulations make it possible to predict the compositional
dependence of the ionic diffusion coefficient on bromide/iodide ratio and
vacancy concentration. For vacancy concentrations of around 9 10^21 cm^-3, we
obtained ionic diffusion coefficients at ambient temperature of 10^-11 and
10^-13 cm2/s for CsPbBr3 and CsPbI3, respectively. Interestingly, in comparison
with the pure compounds, we predict a significantly lower activation energy for
vacancy migration and faster diffusion for the mixed perovskites.
