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Lead halide hybrid perovskites consist of an inorganic framework hosting amolecular cation located in the interstitial space. These compounds have beenextensively studied as they have been identified as promising materials forphotovoltaic applications with the interaction between the molecular cation andthe inorganic framework implicated as influential for the electronicproperties. CH3NH3PbCl3 undergoes two structural transitions from a hightemperature cubic unit cell to a tetragonal phase at 177 K and an orthorhombictransition at 170 K. We have measured the low-frequency lattice dynamics usingneutron spectroscopy and observe an energy broadening in the acoustic phononlinewidth towards the symmetry point QX =(2,1/2,0) when approaching thetransitions. Concomitant with these zone boundary anomalies is a hardening ofthe entire acoustic phonon branch measured near the (2, 0, 0) Bragg positionwith decreasing temperature. Measurements of the elastic scattering at theBrillouin 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 onselection rules of neutron scattering, we suggest that the higher 177 Ktransition is displacive with a distortion of the local octahedral environmentand the lower transition is a rigid tilt transition of the octahedra. We do notobserve any critical broadening in energy or momentum, beyond resolution, ofthese peaks near the transitions. We compare these results to the criticalproperties reported near the structural transitions in other perovskites. Wesuggest that the simultaneous onset of static resolution-limited Bragg peaks atthe zone boundaries and the changes in acoustic phonon energies near the zonecenter is evidence of a coupling between the inorganic framework and themolecular cation.

Lifetime-shortened acoustic phonons and static order at the Brillouin zone boundary in the organic-inorganic perovskite CH$_3$NH$_3$PbCl$_3$