The Honeycomb and Hyperhoneycomb Polymorphs of IrI_3

The synthesis of IrI3 at high pressure in its layered honeycomb polymorph is reported. Its crystal structure is refined by single crystal X-ray diffraction. Faults in the honeycomb layer stacking are observed by single crystal diffraction, synchrotron powder diffraction, and transmission electron microscopy. A previously unreported “hyperhoneycomb” polymorph of IrI3 (β-IrI3), is also reported - Its structure in space group Fddd is determined by single crystal XRD. Both materials are highly-resistive diamagnetic semiconductors, consistent with a low spin d6 configuration for Ir(III). The two- and three-dimensional Ir arrays in these polymorphs of IrI3 are analogous to those found in the α- and β-polymorphs of Li2IrO3, although the Ir electron configurations are different. IrI3 may be of future interest in the fabrication of new quantum materials.