Solution of the heavily stacking faulted crystal structure of the honeycomb iridate H3LiIr2O6

A powder sample of pure H3LiIr2O6 was synthesized from alpha-Li2IrO3 powder by a soft chemical replacement of Li+ with H+. The crystal structure of H3LiIr2O6 consists of sheets of edge sharing LiO6- and IrO6- octahedra forming a honeycomb network with layers stacked in a monoclinic distorted HCrO2 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 LiIr2O63--layers of alpha-Li2IrO3 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 H3LiIr2O6 is governed by interlayer O-H center dot center dot center dot O contacts. From the constitution of the layers, different stacking patterns with similar amounts of strong O-H center dot center dot center dot O contacts are considered. Random transitions among these stacking patterns can occur as faults in the crystal structure of H3LiIr2O6, which quantitatively describe the observed XRPD.