The advantageous performance of hybrid organic–inorganic perovskite halide semiconductors in optoelectronic applications motivates studies of their fundamental crystal chemistry. In particular, recent studies have sought to understand how dipolar, dynamic, and organic cations such as methylammonium (CH3NH3 +) and formamidinium (CH(NH2)2 +) affect physical properties such as light absorption and charge transport. To probe the influence of organic–inorganic coupling on charge transport, we prepared the series of vacancy-ordered double perovskite derivatives A 2SnI6, where A = Cs+, CH3NH3 +, and CH(NH2)2 +. Despite nearly identical cubic structures by powder X-ray diffraction, replacement of Cs+ with CH3NH3 + or CH(NH2)2 + reduces conductivity through a reduction in both carrier concentration and carrier mobility. We attribute the trends in electronic behavior to anharmonic lattice dynamics from the formation of hydrogen bonds that yield coupled organic–inorganic dynamics. This anharmonicity manifests as asymmetry of the interoctahedral I–I pair correlations in the X-ray pair distribution function of the hybrid compounds, which can be modeled by large atomistic ensembles with random rotations of rigid [SnI6] octahedral units. The presence of soft, anharmonic lattice dynamics holds implications for electron–phonon interactions, as supported by calculation of electron–phonon coupling strength that indicates the formation of more tightly bound polarons and reduced electron mobilities with increasing cation size. By exploiting the relatively decoupled nature of the octahedral units in these defect-ordered perovskite variants, we interrogated the impact of organic–inorganic coupling and lattice anharmonicity on the charge transport behavior of hybrid perovskite halide semiconductors.
Hybrid perovskites are a technologically relevant family of materials, with potential applications in photovoltaics, solid-state lighting, and radiation detection. Interactions between the inorganic octahedral framework and the organic sublattice have been implicated in the structure and optoelectronic properties, but characterization of these interactions has been challenging, because of competition between organic−inorganic coupling and intraoctahedral interactions. Owing to their decreased octahedral connectivity, vacancy-ordered double perovskites present an ideal case study to examine organic−inorganic coupling in hybrid perovskites and their derivatives. Here, we describe the lowtemperature, hysteretic phase transition of formamidinium tin(IV) iodide from the high-symmetry cubic phase to a lower-symmetry monoclinic phase. We propose that the hysteresis stems from organic−inorganic coupling mediated by local and spontaneous strain from the orientations of the formamidinium cations, which result in a ferroelastic phase transition.
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