The honeycomb and hyperhoneycomb polymorphs of IrI₃

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.