Our findings on the microscopic mechanisms of water-assisted B→Y phase transitions in CsPbI 3 may also be relevant to other perovskite materials such as CsPbBr x I 3-x 14 , CsSnI 3 , and FAPbI 3 . In the case of the MAPbI 3 system, there is no stable low-temperature non-perovskite structure; however, it degrades into PbI 2 and other by-products under humid conditions . Cryoelectron microscopy (cryo-EM) experiments have confirmed that this degradation process also follows the C-A-C mechanism, where an amorphous MAPbI 3 hydrate layer initially forms on the surface of MAPbI 3 before decomposing into PbI 2 and gaseous products . Additionally, Hidalgo et al. have noted that atmospheric oxygen synergistically accelerates the B→Y phase transition in FAPbI 3 , a process that may be accompanied by the formation of Pb(IO 3 ) 2 . The complex mechanism underlying the cooperative action of O 2 and H 2 O in this context requires further in-depth investigation.