  Lattice vibrations in CH$_3$NH$_3$PbI$_3$ are strongly interacting, with
double well instabilities present at the Brillouin zone boundary. Analysis
within a first-principles lattice dynamics framework reveals anharmonic
potentials with short phonon quasi-particle lifetimes and mean-free paths. The
phonon behaviour is distinct from the inorganic semiconductors GaAs and CdTe
where three-phonon interaction strengths are three orders of magnitude smaller.
The implications for the applications of hybrid halide perovskites arising from
thermal conductivity, band-gap deformation, and charge-carrier scattering
through electron-phonon coupling, are presented.
