Kinetically Stable Single Crystals of Perovskite-Phase CsPbI${_3}$

We use solid-state methods to synthesize single crystals of perovskite-phase cesium lead iodide (${\gamma}$-CsPbI3) that are kinetically stable at room temperature. Single crystal X-ray diffraction characterization shows that the compound is orthorhombic with the GdFeO3 structure at room temperature. Unlike conventional semiconductors, the optical absorption and the joint density-of-states of bulk ${\gamma}$-CsPbI3 is greatest near the band edge and decreases beyond Eg for at least 1.9 eV. Bulk ${\gamma}$-CsPbI3 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 ${\gamma}$-CsPbI3 shows signatures of quantum confinement. By flowing gases over ${\gamma}$-CsPbI3 during in situ powder X-ray diffraction measurements, we confirm that ${\gamma}$-CsPbI3 is stable to oxygen but rapidly and catalytically converts to non-perovskite ${\delta}$-CsPbI3 in the presence of moisture. Our results on bulk ${\gamma}$-CsPbI3 provide vital parameters for theoretical and experimental investigations into perovskite-phase CsPbI3 that will the guide the design and synthesis of atmospherically stable inorganic halide perovskites.