The Crystal Structure of Arsenic Triiodide, AsI3

The crystal structure of arsenic triiodide has been investigated by means of Laue, powder, and oscillation photographs. It is shown to be a layer lattice. The underlying lattice is ${\Gamma }_{\mathrm{Th}}$ and the space group ${C_{3i}}^2$. The unit rhombohedron has dimensions $r=8.25\mathrm{}$ A and $\alpha ={51}^{\circ }{20}^{\prime }$; there are two molecules to the unit cell. The coordinates of the atoms are arsenic $\pm \mathrm{uuu}$ where $u={60}^{\circ }$; iodine $\pm \mathrm{xyz}$, $\pm \mathrm{zxy}$, $\pm \mathrm{yzx}$ where $x={152}^{\circ }$, $y={28}^{\circ }$, $z=-{90}^{\circ }$. Distances of closest approach of iodine are about 4.28A and 4.13A and of arsenic and iodine about 2.97A. The iodine ions are in hexagonal close packing; each arsenic ion is surrounded by six iodine ions.