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

Abstract

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-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 H 3 LiIr 2 O 6 , which quantitatively describe the observed XRPD.

Crystal structure and stacking faults in the layered honeycomb, delafossite-type materials Ag3LiIr2O6and Ag3LiRu2O6

“…44 For the optimization of the microstructures of Ag 3 LiIr 2 O 6 and Ag 3 LiRu 2 O 6 the monoclinic unit cells of the initial faultless structure models were transformed into P1-pseudo-orthorhombic cells as described for H 3 LiIr 2 O 6 . 9 During each simulation 100 supercells with 200 layers each were simulated and averaged. The simulated pattern was refined against the measured pattern while keeping all parameters fixed except the scaling factor.…”

“…The microstructural properties of Ag 3 LiIr 2 O 6 and Ag 3 LiRu 2 O 6 are similar to H 3 LiIr 2 O 6 . 9 This may point to the fact that the presence of planar defects is an intrinsic property of this kind of layered honeycomb structures.…”

“…In fact, the crystal structure of H 3 LiIr 2 O 6 was found to be heavily faulted. 9 The presence of stacking faults is in general a common phenomenon in layered honeycomb iridates that seriously impedes their structural characterization. 4,10,11 The occurrence of defects causes diffuse scattering that among others can lead to strong peak broadening.…”

“…8 By considering planar defects and developing a suitable microstructural model, the crystal structure of H 3 LiIr 2 O 6 was further sharpened to a layered honeycomb structure in space group C2/m with a completely ordered cation sublattice. 9 A detailed understanding of the crystal structures of honeycomb lattices is crucial, as H 3 LiIr 2 O 6 was found to be the first materialization of the quantum spin liquid state on a honeycomb lattice, 13 that may be relevant for the Kitaev quantum spin liquid. 7 The interlayer protons in H 3 LiIr 2 O 6 which exhibit a linear coordination can be replaced by silver cations.…”

Thermodynamic Evidence of Proximity to a Kitaev Spin Liquid in Ag3LiIr2O6

“…LiIr 2 O 6where the region of stacking faults must be excluded to obtain a reasonable refinement[20]. Thus, the in-plane bond randomness in H 3 LiIr 2 O 6[22] is negligible in Ag 3 LiIr 2 O 6 .…”