Suppressing Phase Transitions and High-Pressure Amorphization through Tethered Organic Cations in Organochalcogenide-Halide Perovskites

Polymorphism, where the same composition adopts different structures, is abundant in perovskites, with numerous phase transitions occurring as a function of temperature and pressure. The APbX₃ perovskites (A = monovalent cation; X = Cl^-, Br^-, I^-) show such phase transitions near ambient conditions, significantly impacting their optoelectronic device performance and stability. Herein, we show that the recently reported organochalcogenide-halide perovskites (RCh)PbX₂ (RCh = ⁺NH₃(CH₂)₂S^{- +}NH₃(CH₂)₂Se^-; X = Cl^-, Br^-) featuring an organic A-site cation that is covalently linked to the inorganic framework, show no phase transitions with temperature from 4 to 423 K and with pressure from 0 to 40 GPa. Furthermore, the RCh-perovskites remain crystalline even at 40 GPa, in striking contrast to AMX₃ (M = Pb, Sn) perovskites that rapidly become amorphous at pressures above ca. 5 GPa. By alloying RCh or the similar-sized ethylammonium as impurities into a (CH₃NH₃)PbBr₃ host, we find that the enhanced phase integrity of the RCh-perovskites may be attributed mostly to the covalent attachment of the A-site cation, which impedes octahedral tilting, a primary avenue for phase transitions. We also track the rotational isomerization of the RCh ligands with pressure, finding that the trans-to-gauche isomerization enables a shrinking A-site cavity volume, without drastic changes to the inorganic framework. Unlike the dynamic disorder seen in hybrid perovskite A-site cations, this static rotational isomerism appears to be unaffected by temperature from 93 to 373 K. The exceptional structural integrity of the RCh-perovskites motivates the design of similar strategies to impede phase transitions in technologically important perovskite compositions.