  Atomic dynamics takes a fundamental part in numbers of physical properties of
solids like high-Tc superconductivity, semiconducting transports, and
thermoelectricity. Perovskite CH3NH3PbI3 exhibits outstanding photovoltaic
performances, but the exact physical scenario has not been established yet, due
to the inadequate understanding of the atomic dynamics and exceptional
transport properties. We present a complete atomic dynamic picture consisting
of phonons, rotational modes of protons and molecular vibrational modes, which
is constructed by carrying out high-resolution time-of-flight inelastic neutron
scattering measurements in a wide energy window ranging from 0.0036 to 54 meV
on a large single crystal sample. A three-fold rotational mode of protons
activated around 80 K reduces the lifetimes of acoustic and optical phonons
down to about 4.5 ps and below 1 ps at 150 K, respectively. The orthorhombic to
tetragonal phase transition takes place with a slower four-fold rotational mode
of the C-N axis concomitantly setting in at ~ 165 K, above which the optical
phonons are too broadened to be distinguished whereas the acoustic ones are
still robust. The significantly reduced lifetimes of optical phonons are linked
to the smaller mobility of charge carriers while the ultralow lattice thermal
conductivity is attributed to nanoscale mean free paths of acoustic phonons.
These microscopic insights provide a solid standing point, on which perovskite
solar cells can be understood more accurately and their performances are
perhaps further optimized. The revealed rotor-phonon coupling opens up an
emergent opportunity to create unprecedented functionalities of materials.
