  The exceptional optoelectronic properties of metal halide perovskites (MHPs)
are presumed to arise, at least in part, from the peculiar interplay between
the inorganic metal-halide sublattice and the atomic or molecular cations
enclosed in the cage voids. The latter can exhibit a roto-translative dynamics,
which is shown here to be at the origin of the structural behavior of MHPs as a
function of temperature, pressure and composition. The application of high
hydrostatic pressure allows for unraveling the nature of the interaction
between both sublattices, characterized by the simultaneous action of hydrogen
bonding and steric hindrance. In particular, we find that under the conditions
of unleashed cation dynamics, the key factor that determines the structural
stability of MHPs is the repulsive steric interaction rather than hydrogen
bonding. Taking as example the results from pressure and temperature-dependent
photoluminescence and Raman experiments on MAPbBr$_3$ but also considering the
pertinent MHP literature, we provide a general picture about the relationship
between the crystal structure and the presence or absence of cationic dynamic
disorder. The reason for the structural sequences observed in MHPs with
increasing temperature, pressure, A-site cation size or decreasing halide ionic
radius is found principally in the strengthening of the dynamic steric
interaction with the increase of the dynamic disorder. In this way, we have
deepened our fundamental understanding of MHPs; knowledge that could be coined
to improve performance in future optoelectronic devices based on this promising
class of semiconductors.
