Cation reorientation and octahedral tilting in the metal-organic perovskites MAPI and FAPI

The metal-organic halide perovskites may have excellent photovoltaic properties, useful to create cheap and efficientsolar cells, photodetectors and LEDs with a wide range of colors. Great efforts are spent for improving theirscarce stability and clarifying the mechanisms that allow the photocarriers to have so long lifetimes. These phenomenaare closely connected to the structural transformations and to the dynamics of the reorienting organiccations, both of which can be studied by anelastic measurements, here combined with dielectric spectroscopy.The complex dynamic Young's modulus and permittivity of MAPI and FAPI have been measured on samples obtainedby pressing the powders into bars or disks. Both perovskites are cubic at and above room temperature(? phase), with freely rotating FA and MA cations, and undergo two tilt transitions of the PbI6 octahedra into atetragonal (?) and orthorhombic (?) phase, accompanied by losses of the orientational degrees of freedom of theMA and FA molecular cations. These ? -> ? and ? -> ? transitions are accompanied respectively by sharp softeningand stiffening of the Young's modulus, while the permittivity has a major drop from a paraelectric rise atthe ? -> ? transition of MAPI and is practically flat everywhere in FAPI. From these behaviors we argue that the?/? transition is mainly due to octahedral tilting while the latter to the blocking of MA/FA reorientational modeslinearly coupled with strain. Comparisons are made with elastic anomalies found in other hybrid 3D and layeredperovskites.