Towards Cubic Symmetry for Ir4+: Structure and Magnetism of the Antifluorite K2IrBr6

Abstract

Crystal structure, electronic state of Ir4+, and magnetic properties of the antifluorite compound K2IrBr6 are studied using high-resolution synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS), thermodynamic and transport measurements, and ab initio calculations. The crystal symmetry is reduced from cubic at room temperature to tetragonal below 170 K and eventually to monoclinic below 122 K. These changes are tracked by the evolution of the noncubic crystal-field splitting Δ measured by RIXS. Nonmonotonic changes in Δ are ascribed to the competing effects of the tilt, rotation, and deformation of the IrBr6 octahedra as well as tetragonal strain on the electronic levels of Ir4+. The Néel temperature of TN=11.9 K exceeds that of the isostructural K2IrCl6, and the magnitude of frustration on the fcc spin lattice decreases. We argue that the replacement of Cl by Br weakens electronic correlations and enhances magnetic couplings. © 2021 American Physical Society.N.K. thanks Somnath Ghara for his help with resistivity measurements. A.A.T. thanks Adam Aczel and Anna Efimenko for fruitful discussions on the antifluorites, and Yurii Skourski for performing the high-field magnetization measurements. The work in Augsburg was supported by the Federal Ministry for Education and Research through the Sofja Kovalevskaya Award of Alexander von Humboldt Foundation (A.A.T.). The work was partially supported by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, Project No. FEUZ-2020-0060). We acknowledge ESRF and APS for providing synchrotron beamtime for this project, and thank Andy Fitch for his technical support during the experiment at ID22, ESRF. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Extraordinary facility operations were supported in part by the DOE Office of Science through the National Virtual Biotechnology Laboratory, a consortium of DOE national laboratories focused on the response to COVID-19, with funding provided by the Coronavirus CARES Act. We also acknowledge the support of the HLD at HZDR, member of European Magnetic Field Laboratory (EMFL)