  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 R-3 crystal
structure, with a = 7.217 {\AA}, and c = 20.476 {\AA}, 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 variations. 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.
