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Using muon spin relaxation ($\mu$SR) measurements on formamidinium leadiodide [FAPbI$_3$, where FA denotes HC(NH$_2)_2$] we show that, among the fivestructurally distinct phases of FAPbI$_3$ exhibited through two differenttemperature hysteresis, the reorientation motion of FA molecules isquasi-static below $\approx50$ K over the time scale of 10$^{-6}$ s in thelow-temperature (LT) hexagonal (Hex-LT, $<160$ K) phase which has relativelylonger 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), thehigh-temperature (HT) hexagonal phase (Hex-HT, 160-380 K), and theHT-tetragonal phase (Tet-HT, 140-280 K) where $\tau_{\rm c}$ decreases withincreasing temperature. More interestingly, the reorientation motion is furtherpromoted 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 $\tau_{\rmc}$, one related to the local reorientation of cationic molecules that is notunencumbered by phonons, and the other to the high symmetry of the bulk crystalstructure.

Photo-excited charge carrier lifetime enhanced by slow cation molecular dynamics in lead iodide perovskite FAPbI$_3$