Kinetically Stable Single Crystals of Perovskite-Phase CsPbI.

We use a solid-state method to synthesize single crystals of perovskite-phase cesium lead iodide (γ-CsPbI) that are kinetically stable at room temperature. Single crystal X-ray diffraction characterization shows the compound is orthorhombic with the GdFeO structure at room temperature. Unlike conventional semiconductors, the optical absorption and joint density-of-states of bulk γ-CsPbI is greatest near the band edge and decreases beyond the for at least 1.9 eV. Bulk γ-CsPbI 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 shows signatures of quantum confinement. By flowing gases during powder X-ray diffraction measurements, we confirm that γ-CsPbI is stable to oxygen but rapidly and catalytically converts to non-perovskite δ-CsPbI in the presence of moisture. Our results provide vital parameters for theoretical and experimental investigations into perovskite-phase CsPbI that will the guide the design and synthesis of atmospherically stable inorganic halide perovskites.