Orientational Glass Formation in Substituted Hybrid Perovskites

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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–xCsxPbBr3. 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 orie...