  Lead halide hybrid perovskites consist of an inorganic framework hosting a
molecular cation located in the interstitial space. These compounds have been
extensively studied as they have been identified as promising materials for
photovoltaic applications with the interaction between the molecular cation and
the inorganic framework implicated as influential for the electronic
properties. CH3NH3PbCl3 undergoes two structural transitions from a high
temperature cubic unit cell to a tetragonal phase at 177 K and an orthorhombic
transition at 170 K. We have measured the low-frequency lattice dynamics using
neutron spectroscopy and observe an energy broadening in the acoustic phonon
linewidth towards the symmetry point QX =(2,1/2,0) when approaching the
transitions. Concomitant with these zone boundary anomalies is a hardening of
the entire acoustic phonon branch measured near the (2, 0, 0) Bragg position
with decreasing temperature. Measurements of the elastic scattering at the
Brillouin zone edges QX = (2,1/2,0), QM = (3/2,1/2,0), and QR = (3/2,3/2,5/2)
show Bragg peaks appearing below these structural transitions. Based on
selection rules of neutron scattering, we suggest that the higher 177 K
transition is displacive with a distortion of the local octahedral environment
and the lower transition is a rigid tilt transition of the octahedra. We do not
observe any critical broadening in energy or momentum, beyond resolution, of
these peaks near the transitions. We compare these results to the critical
properties reported near the structural transitions in other perovskites. We
suggest that the simultaneous onset of static resolution-limited Bragg peaks at
the zone boundaries and the changes in acoustic phonon energies near the zone
center is evidence of a coupling between the inorganic framework and the
molecular cation.
