Locking of Methylammonium by pressure-enhanced H-bonding in (CH3NH3)PbBr3 Hybrid Perovskite

Organo-lead halide perovskites are nowadays considered to be an emerging photovoltaic material. It is clear that the peculiar hybrid nature of this class of materials is central for their outstanding optical and transport properties. However, the role of the organic cation and its interplay with the inorganic framework remains elusive. To get insight into the interactions at play, highpressure Raman, infrared, and X-ray absorption spectroscopy measurements were performed on MAPbBr3 (MA = CH3NH3+). Since lattice compression allows for a fine-tuning of the organic/inorganic interaction, we were able to follow the pressure evolution of the MA dynamics within the PbBr6 cage and identify different phases. From a MA dynamical disordered configuration, the system enters at first a cation ordered phase and, at higher pressure, a static disordered MA phase. Data analysis points at H-bonding as the driving force for molecular reorientation. Since the MA dynamics directly influence the formation of polarons in hybrid perovskites and their ferroelectric properties, the present results provide the basis for the understanding of the transport mechanisms at the core of the outstanding properties of this class of materials.