Crystal structure and stacking faults in the layered honeycomb, delafossite-type materials Ag3LiIr2O6 and Ag3LiRu2O6.

Sebastian Bette,Robert E Dinnebier,Agnieszka Poulain,Hidenori Takagi,Tomohiro Takayama,Viola Duppel

doi:10.1039/c9dt01789e

Abstract

Powder samples of Ag3LiIr2O6 and Ag3LiRu2O6 were synthesized from α-Li2IrO3 and Li2RuO3 respectively by ion exchange in an AgNO3 melt. The crystal structures of the title compounds were solved from high resolution laboratory X-ray powder diffraction (XRPD) patterns and from pair distribution function (PDF) analysis using synchrotron X-ray powder diffraction data. In both crystal structures edge sharing LiO6/3- and (Ir/Ru)O6/3-octahedra form honeycomb like layers that are stacked in a staggered fashion. Silver cations, situated in-between the layers mediate the interlayer interactions by linear O-Ag-O bonds. Anisotropic peak broadening in the XRPD patterns and diffuse scattering occurring as streaks in the precession electron diffraction (PED) patterns indicate the presence of stacking faults, which could be also visualized by high resolution transmission electron microscopy (HRTEM). Possible alternative stacking sequences were derived from the ideal crystal and incorporated into a microstructure model. By applying a supercell approach that randomly generates and averages stacking sequences based on transition probabilities and combining it with a grid search algorithm, the microstructures, i.e. the degrees of faulting in the structures of the title compounds were refined to the measured XRPD data. In result the crystal structures of Ag3LiIr2O6 and Ag3LiRu2O6 were found to be vastly faulted with almost no coherence of the stacked layers.

Highlights

Iridium oxides as α-1 and β-Li2IrO3 2,3 with a honeycomb like motif in their layered structure and related iridates, like Na2IrO3 4,5 are currently attracting broad interest in the field of quantum magnetism and frustrated magnetism[6] because of their possible relevance for the Kitaev quantum spin liquid state.[7]
If the occurrence of stacking fault disorder and the microstructure of the sample is neglected during the process of the crystal structure solution, an occupational disorder in the intralayer cation sublattice is introduced, which is connected with a higher lattice symmetry
By considering planar defects and developing a suitable microstructural model, the crystal structure of H3LiIr2O6 was further sharpened to a layered honeycomb structure in space group C2/m with a completely ordered cation sublattice.[9]

Summary

Introduction

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 9250 | Dalton Trans., 2019, 48, 9250–9259. In this study we describe the solution of the real crystal structure of the layered honeycomb delafossite-type phases Ag3LiIr2O6 and Ag3LiRu2O6, which includes the development of suitable microstructure models and their refinement to the measured data. For comparison the XRPD-patterns of the solid phases were collected at room temperature on a laboratory powder diffractometer in Debye– Scherrer geometry (Stadi P-Diffractometer (Stoe), Ag-Kα1 radiation from primary Ge(111)-Johann-type monochromator, Mythen 1 K detector (Dectris)). Due to the high fluorescence of ruthenium when using silver radiation, the diffraction pattern of Ag3LiRu2O6 was collected using the same type of diffractometer equipped with a Mo-cathode. The collected Debye–Scherrer rings were subsequently azimuthally integrated with the pyFAI-software[26] to one-dimensional powder diffraction patterns in Q [nm−1] and 2θ [°] versus intensity. Nickel was previously measured as a standard material to determine the Q-damp and Q-broad parameters which are the parameters that correct the PDF envelope function for instrumental resolution effects.[33,34]

Results and discussion

Conflicts of interest

Conclusions