  Microscopic pathways of structural phase transitions are difficult to probe
because they occur over multiple, disparate time and length scales. Using $in$
$situ$ nanoscale cathodoluminescence microscopy, we visualize the
thermally-driven transition to the perovskite phase in hundreds of
non-perovskite phase nanowires, resolving the initial nanoscale nucleation and
subsequent mesoscale growth and quantifying the activation energy for phase
propagation. In combination with molecular dynamics computer simulations, we
reveal that the transformation does not follow a simple martensitic mechanism,
and proceeds via ion diffusion through a liquid-like interface between the two
structures. While cations are disordered in this liquid-like region, the halide
ions retain substantial spatial correlations. We find that the anisotropic
crystal structure translates to faster nucleation of the perovskite phase at
nanowire ends and faster growth along the long nanowire axis. These results
represent a significant step towards manipulating structural phases at the
nanoscale for designer materials properties.
