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Recently, hybrid organic‐inorganic lead halide materials have attracted great attention for their applications in the fields of light‐emitting diodes, photovoltaics, photodetector. However, these hybrid materials usually suffer from environmental and human health issues because of the toxic lead moieties. Herein, we report a novel lead‐free bismuth iodide phase transition material, [NH3(CH2)7NH3][BiI5] (1), which undergoes a reversible phase transition at high temperature 351.7 K and exhibits a step fashion dielectric anomaly. The structural phase transition of 1 is mainly originated from the order‐disorder transition of the 1,7‐heptanediamine cations. Besides, the solid‐state UV‐vis spectrum of 1 displays an intense adsorption at around 658 nm; which gives an indirect bandgap of 1.89 eV. Such a small bandgap value is comparable to most of the organic‐inorganic hybrid lead halide type semiconductors. We believe this study will shed light on the exploration of lead‐free organic‐inorganic hybrid metal halide semiconductors toward future device applications. [ABSTRACT FROM AUTHOR]

Narrow band gaps and excellent ferroelectricity are intrinsically paradoxical in ferroelectrics as the leakage current caused by an increase in the number of thermally excited carriers will lead to a deterioration of ferroelectricity. A new molecular ferroelectric, hexane-1,6-diammonium pentaiodobismuth (HDA-BiI5), was now developed through band gap engineering of organic-inorganic hybrid materials. It features an intrinsic band gap of 1.89 eV, and thus represents the first molecular ferroelectric with a band gap of less than 2.0 eV. Simultaneously, low-temperature solution processing was successfully applied to fabricate high-quality ferroelectric thin films based on HDA-BiI5, for which high-precision controllable domain flips were realized. Owing to its narrow band gap and excellent ferroelectricity, HDA-BiI5 can be considered as a milestone in the exploitation of molecular ferroelectrics, with promising applications in high-density data storage and photovoltaic conversion. [ABSTRACT FROM AUTHOR]

• Synthesis two lead-free halides [3-FPEA] 3 (SbCl 4)Cl 2 and [3-FPEA] 4 (BiI 6)I. • Both hybrid perovskites have reversible phase transitions. • Both materials are lead-free semiconductor materials. Metal halide organic-inorganic hybrid perovskite is a kind of rapid development of semiconductors but its major development has been concentrated in lead-based perovskites. Here we report two zero-dimensional non-lead halide perovskites [3-FPEA] 3 (SbCl 4)Cl 2 (1) and [3-FPEA] 4 (BiI 6)I (2) (3-FPEA = 3-fluorophenethylamine) with the reversible phase transitions. The differential scanning calorimetry (DSC) and UV-vis absorption measurements of 1 and 2 revealed that the phase transition temperatures are 350 K (1) and 335 K (2), the bandgaps are 3.53 eV (1) and 2.14 eV (2), respectively. The room temperature X-ray single crystal diffraction showed the cation [3-FPEA]+ in 1 and 2 proceeds a twofold disorder, and the anions in 1 are a dimer [Sb 2 Cl 10 ] constructed with [SbCl 6 ] octahedra by sharing the Cl···Cl edge, and the anions in 2 are the discrete [BiI 6 ] octahedron. This study can provide an effective molecular design strategy for exploring and investigating non-lead-based halide perovskite semiconductive phase change materials. [Display omitted] [ABSTRACT FROM AUTHOR]