The choice of organic cation in hybrid perovskites has large implications for optoelectronic properties, material stability, and crystal structure. In particular, formamidinium ((CH(NH 2 ) 2 + ) perovskites exhibit unusual temperature-dependent trends in photoluminescence, dielectric constant, and phase behavior that are hypothesized to relate to CH(NH 2 ) 2 + reorientations. This contribution describes five distinct, temperaturedependent phase transitions in CH(NH 2 ) 2 PbBr 3 that produce changes in the steady-state photocurrent. Three of these phase transitions do not resolve crystallographically and relate to the orientation and dynamics of CH(NH 2 ) 2 + . These crystallographically unresolvable phase transitions resemble ferroelastic transitions with the formation of nanoscale domains, which we hypothesize are mediated by the strain exerted from geometric frustration of CH(NH 2 ) 2 + quadrupoles. This work demonstrates the importance of cation orientation and dynamics, domain behavior, and their interdependence in the steady-state optoelectronic properties of hybrid perovskites.

Although initial studies on hybrid perovskites for photovoltaic applications focused on simple compositions, the most technologically relevant perovskites are heavily substituted. The influence of chemical substitution on the general phase behavior and specific physical properties remains ambiguous. The hybrid perovskite formamidinium lead bromide, CH(NH2)2PbBr3, exhibits complex phase behavior manifesting in a series of crystallographically-unresolvable phase transitions associated with changes in the cation dynamics. Here, we characterize the molecular and lattice dynamics of CH(NH2)2PbBr3 as a function of temperature, and their evolution upon chemical substitution of CH(NH2)2 + for cesium (Cs + ) with crystallography, neutron scattering, 1 H and 14 N nuclear magnetic resonance spectroscopy, and 79 Br nuclear quadrupolar spectroscopy. Cs + substitution suppresses the four low-temperature phase transitions of CH(NH2)2PbBr3, which propagate through concerted changes in the dynamic degrees of freedom of the organic sub-lattice and local or long-range distortions of the octahedral framework. We propose that cesium substitution suppress the phase transitions through the relief of geometric frustration associated ** Both authors contributed equally.with the orientations of CH(NH2)2 + molecules, which retain their local dynamical degrees of freedom.

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