  Using muon spin relaxation ($\mu$SR) measurements on formamidinium lead
iodide [FAPbI$_3$, where FA denotes HC(NH$_2)_2$] we show that, among the five
structurally distinct phases of FAPbI$_3$ exhibited through two different
temperature hysteresis, the reorientation motion of FA molecules is
quasi-static below $\approx50$ K over the time scale of 10$^{-6}$ s in the
low-temperature (LT) hexagonal (Hex-LT, $<160$ K) phase which has relatively
longer photo-excited charge carrier lifetime ($\tau_{\rm c}\sim$10$^{-6}$ s).
In contrast, a sharp increase in the FA molecular motion was found above
$\approx50$ K in the Hex-LT phase, LT-tetragonal phase (Tet-LT, $<140$ K), the
high-temperature (HT) hexagonal phase (Hex-HT, 160-380 K), and the
HT-tetragonal phase (Tet-HT, 140-280 K) where $\tau_{\rm c}$ decreases with
increasing temperature. More interestingly, the reorientation motion is further
promoted in the cubic phase at higher temperatures ($>380/280$ K), while
$\tau_{\rm c}$ is recovered to comparable or larger than that of the LT phases.
These results indicate that there are two factors that determine $\tau_{\rm
c}$, one related to the local reorientation of cationic molecules that is not
unencumbered by phonons, and the other to the high symmetry of the bulk crystal
structure.
