  Metal halide perovskites are multifunctional semiconductors with tunable
structures and properties. They are highly dynamic crystals with complex
octahedral tilting patterns and strongly anharmonic atomic behaviour. In the
higher temperature, higher symmetry phases of these materials, several complex
structural features have been observed. The local structure can differ greatly
from the average structure and there is evidence that dynamic two-dimensional
structures of correlated octahedral motion form. An understanding of the
underlying complex atomistic dynamics is, however, still lacking. In this work,
the local structure of the inorganic perovskite CsPbI$_3$ is investigated using
a new machine learning force field based on the atomic cluster expansion
framework. Through analysis of the temporal and spatial correlation observed
during large-scale simulations, we reveal that the low frequency motion of
octahedral tilts implies a double-well effective potential landscape, even well
into the cubic phase. Moreover, dynamic local regions of lower symmetry are
present within both higher symmetry phases. These regions are planar and we
report the length and timescales of the motion. Finally, we investigate and
visualise the spatial arrangement of these features and their interactions,
providing a comprehensive picture of local structure in the higher symmetry
phases.
