  Hybrid organic-inorganic perovskites (HOIPs) have recently emerged as highly
promising solution-processable materials for photovoltaic (PV) and other
optoelectronic devices. HOIPs represent a broad family of materials with
properties highly tuneable by the ions that make up the perovskite structure as
well as their multiple combinations. Interestingly, recent high-efficiency PV
devices using HOIPs with substantially improved long-term stability have used
combinations of different ionic compositions. The structural dynamics of these
systems are unique for semiconducting materials and are currently argued to be
central to HOIPs stability and charge-transport properties. Here, we studied
the impact of ionic composition on phonon speeds of HOIPs from Brillouin
spectroscopy experiments and density functional theory calculations for
FAPbBr$_3$, MAPbBr$_3$, MAPbCl$_3$, and the mixed halide
MAPbBr$_{1.25}$Cl$_{1.75}$. Our results show that the acoustic phonon speeds
can be strongly modified by ionic composition, which we explain by analysing
the lead-halide sublattice in detail. The vibrational properties of HOIPs are
therefore tuneable by using targeted ionic compositions in the perovskite
structure. This tuning can be rationalized with non-trivial effects, for
example, considering the influence of the shape and dipole moment of organic
cations. This has an important implication to further improvements in the
stability and charge-transport properties of these systems.
