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.