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

Abstract

A powder sample of pure H₃LiIr₂O₆ was synthesized from α-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 α-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⋯O contacts. From the constitution of the layers, different stacking patterns with similar amounts of strong O–H⋯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.