Orientational Glass Formation in Substituted Hybrid Perovskites

Hybrid organic-inorganic perovskites have gained notoriety in the photovoltaic community for their composition-tunable band gaps and long-lived electronic excited states, which are known to be related to the crystalline phase. While it is known that the inorganic and organic components are coupled through structural phase transitions, it remains unclear as to what role each plays in directing the structure of hybrid perovskites such as methylammonium lead halides (CH₃NH₃PbX₃). Here, we present crystallographic and spectroscopic data for the series (CH₃NH₃)_1-xCs_xPbBr₃. CH₃NH₃PbBr₃ behaves as a plastic crystal in the high temperature cubic phase, and substitution of CH₃NH₃⁺ with Cs⁺ leads to the formation of an orientational glass. While the organic molecule exhibits slow, glassy reorientational dynamics, the inorganic framework continues to undergo crystallographic phase transitions. These crystallographic transitions occur in the absence of thermodynamic signatures in the specific heat from molecular orientation transitions, which suggests that the phase transitions result from underlying instabilities intrinsic to the inorganic lattice. However, these transitions are not decoupled from the reorientations of the organic molecule, as indicated by inelastic and quasielastic neutron scattering. Observation of a reentrant phase transition in (CH₃NH₃)_0.8Cs_0.2PbBr₃ permits the resolution of these complex behaviors within the context of strain mediated interactions. Together, these results provide critical insight into the coupled phase behavior and dynamics in hybrid perovskites.