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The honeycomb and hyperhoneycomb polymorphs of IrI$_3$

The synthesis of IrI$_3$ 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 IrI$_3$ ($β$-IrI$_3$), is also described. Its structure in space group Fddd is determined by single crystal XRD. Both materials are highly-resistive diamagnetic semiconductors, consistent with a low spin d$^6$ configuration for Ir(III). The two- and three-dimensional Ir arrays in these polymorphs of IrI$_3$ are analogous to those found in the $α$- and $β$- polymorphs of Li$_2$IrO$_3$, although the Ir electron configurations are different.