Cubic or Orthorhombic? Revealing the Crystal Structure of Metastable Black-Phase CsPbI₃ by Theory and Experiment

Room-temperature films of black-phase cesium lead iodide (CsPbI₃) are widely thought to be trapped in a cubic perovskite polymorph. Here, we challenge this assumption. We present structural refinement of room-temperature black-phase CsPbI₃ in an orthorhombic polymorph. We demonstrate that this polymorph is adopted by both powders and thin films of black-phase CsPbI₃, fabricated either by high- or low-temperature processes. We perform electronic band structure calculations for the orthorhombic polymorph and find agreement with experimental data and close similarities with orthorhombic methylammonium lead iodide. We investigate the structural transitions and thermodynamic stability of the various polymorphs of CsPbI₃ and show that the orthorhombic polymorph is the most stable among its other perovskite polymorphs, but it remains less stable than the yellow nonperovskite polymorph.