The pressure-stabilized polymorph of indium triiodide

A layered rhombohedral polymorph of indium(iii) triiodide is synthesized at high pressure and temperature. The unit cell symmetry and approximate dimensions are determined by single crystal X-ray diffraction. Its R3̄ crystal structure, with a = 7.214 Å, and c = 20.47 Å, is refined by the Rietveld method on powder X-ray diffraction data. The crystal structure is based on InI₆ octahedra sharing edges to form honeycomb lattice layers, though with considerable stacking defects. Different from ambient pressure InI₃, which has a monoclinic molecular structure and a light-yellow color, high pressure InI₃ is layered and has an orange color. The band gaps of both the monoclinic and rhombohedral variants of InI₃ are estimated from diffuse reflectance measurements.