Mixing I and Br in Inorganic Perovskites: Atomistic Insights from Reactive Molecular Dynamics Simulations

All-inorganic halide
perovskites have received a great deal of
attention as attractive alternatives to overcome the stability issues
of hybrid halide perovskites that are commonly associated with organic
cations. To find a compromise between the optoelectronic properties
of CsPbI3 and CsPbBr3, perovskites with CsPb(BrxI1–x)3 mixed compositions are commonly used. An additional benefit
is that without sacrificing the optoelectronic properties for applications
such as solar cells or light-emitting diodes, small amounts of Br
in CsPbI3 can prevent the inorganic perovskite from degrading
to a photo-inactive non-perovskite yellow phase. Despite indications
that strain in the perovskite lattice plays a role in the stabilization
of the material, a full understanding of such strain is lacking. Here,
we develop a reactive force field (ReaxFF) for perovskites starting
from our previous work for CsPbI3, and we extend this force
field to CsPbBr3 and mixed CsPb(BrxI1–x)3 compounds.
This force field is used in large-scale molecular dynamics simulations
to study perovskite phase transitions and the internal ion dynamics
associated with the phase transitions. We find that an increase of
the Br content lowers the temperature at which the perovskite reaches
a cubic structure. Specifically, by substituting Br for I, the smaller
ionic radius of Br induces a strain in the lattice that changes the
internal dynamics of the octahedra. Importantly, this effect propagates
through the perovskite lattice ranging up to distances of 2 nm, explaining
why small concentrations of Br in CsPb(BrxI1–x)3 (x ≤ 1/4) have a significant impact on the phase stability of
mixed halide perovskites.