10.1021/jacs.9b06055.s001 Daniel B. Straus Daniel B. Straus Shu Guo Shu Guo Robert J. Cava Robert J. Cava Kinetically Stable Single Crystals of Perovskite-Phase CsPbI<sub>3</sub> American Chemical Society 2019 γ- CsPbI 3 film γ- CsPbI 3 Kinetically Stable Single Crystals GdFeO 3 structure Bulk γ- CsPbI 3 powder X-ray diffraction measurements Single crystal X-ray diffraction characterization perovskite-phase CsPbI 3 non-perovskite δ- CsPbI 3 Perovskite-Phase CsPbI 3 bulk γ- CsPbI 3 room temperature 2019-07-08 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Kinetically_Stable_Single_Crystals_of_Perovskite-Phase_CsPbI_sub_3_sub_/8859041 We use a solid-state method to synthesize single crystals of perovskite-phase cesium lead iodide (γ-CsPbI<sub>3</sub>) that are kinetically stable at room temperature. Single crystal X-ray diffraction characterization shows the compound is orthorhombic with the GdFeO<sub>3</sub> structure at room temperature. Unlike conventional semiconductors, the optical absorption and joint density-of-states of bulk γ-CsPbI<sub>3</sub> is greatest near the band edge and decreases beyond the <i>E</i><sub>g</sub> for at least 1.9 eV. Bulk γ-CsPbI<sub>3</sub> does not show an excitonic resonance and has an optical band gap of 1.63(3) eV, ∼90 meV smaller than has been reported in thin films; these and other differences indicate that previously measured thin film γ-CsPbI<sub>3</sub> shows signatures of quantum confinement. By flowing gases <i>in situ</i> during powder X-ray diffraction measurements, we confirm that γ-CsPbI<sub>3</sub> is stable to oxygen but rapidly and catalytically converts to non-perovskite δ-CsPbI<sub>3</sub> in the presence of moisture. Our results provide vital parameters for theoretical and experimental investigations into perovskite-phase CsPbI<sub>3</sub> that will the guide the design and synthesis of atmospherically stable inorganic halide perovskites.