Superconducting high-pressure phase of cesium iodide

The evolutionary methodology for crystal structure prediction is employed to resolve a long-held puzzle in the high-pressure crystallography of cesium iodide. Above 42 GPa, an orthorhombic $Pnma$ structure with a hexagonal-closed-packed (hcp) stacking which resembles hcp Xe is uncovered to be stable and successfully explains the observed diffraction patterns. At strong compression (>100 GPa), this structure becomes metallic via band-gap closure, and superconducting originated from the peculiar reverse electron donation from I^- to Cs⁺.