Solution of the heavily stacking faulted crystal structure of the honeycomb iridate H 3 LiIr 2 O 6 .

A powder sample of pure H ₃ LiIr ₂ O ₆ was synthesized from α-Li ₂ IrO ₃ powder by a soft chemical replacement of Li ⁺ with H ⁺ . The crystal structure of H ₃ LiIr ₂ O ₆ consists of sheets of edge sharing LiO ₆ - and IrO ₆ -octahedra forming a honeycomb network with layers stacked in a monoclinic distorted HCrO ₂ type pattern. Heavy stacking faulting of the sheets is indicated by anisotropic peak broadening in the X-ray powder diffraction (XRPD) pattern. The ideal, faultless crystal structure was obtained by a Rietveld refinement of the laboratory XRPD pattern while using the LiIr ₂ O ₆ ^3- -layers of α-Li ₂ IrO ₃ as a starting model. The low radial distances of the PDF function, derived from synchrotron XRPD data, as constraints to stabilize the structural refinement. DIFFaX-simulations, structural considerations, high radial distances of the PDF function and a Rietveld compatible global optimization of a supercell were employed to derive a suitable faulting model and to refine the microstructure using the experimental data. We assumed that the overall stacking pattern of the layers in the structure of H ₃ LiIr ₂ O ₆ is governed by interlayer O-HO contacts. From the constitution of the layers, different stacking patterns with similar amounts of strong O-HO contacts are considered. Random transitions among these stacking patterns can occur as faults in the crystal structure of H ₃ LiIr ₂ O ₆ , which quantitatively describe the observed XRPD.