Cesium Substitution Disrupts Concerted Cation Dynamics in Formamidinium 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, ¹H and ¹⁴N nuclear magnetic resonance spectroscopy, and ⁷⁹Br 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 sublattice and local or long-range distortions of the octahedral framework. We propose that cesium substitution suppresses the phase transitions through the relief of geometric frustration associated with the orientations of CH(NH₂)₂ ⁺ molecules, which retain their local dynamical degrees of freedom.