Structural Transitions and Stability of FAPbI(3) and MAPbI(3): The Role of Interstitial Water

We studied the influence of water on the structural stability and transformations of MAPI and FAPI by anelastic and dielectric spectroscopies under various temperature and H [Formula: see text] O partial pressure protocols. Before discussing the new results in terms of interstitial water in MAPI and FAPI, the literature is briefly reviewed, in search of other studies and evidences on interstitial water in hybrid halide perovskites. In hydrated MAPI, the elastic anomaly between the cubic [Formula: see text] and tetragonal [Formula: see text] phases may be depressed by more than 50%, demonstrating that there are H [Formula: see text] O molecules dispersed in the perovskite lattice in interstitial form, that hinder the long range tilting of the PbI [Formula: see text] octahedra. Instead, in FAPI, interstitial water accelerates in both senses the reconstructive transformations between 3D [Formula: see text] and 1D [Formula: see text] phases, which is useful during the crystallization of the [Formula: see text] phase. On the other hand, the interstitial H [Formula: see text] O molecules increase the effective size of the MA and FA cations to which are bonded, shifting the thermodynamic equilibrium from the compact perovskite structure to the open [Formula: see text] and hydrated phases of loosely bonded chains of PbI [Formula: see text] octahedra. For this reason, when fabricating devices based on hybrid metal-organic halide perovskites, it is important to reduce the content of interstitial water as much as possible before encapsulation.