Towards cubic symmetry for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi mathvariant="normal">Ir</mml:mi><mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>: Structure and magnetism of the antifluorite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">K</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>IrBr</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>
N. Khan, Danil Prishchenko, M. H. Upton, V. G. Mazurenko, Alexander A. Tsirlin
Abstract
Crystal structure, electronic state of ${\mathrm{Ir}}^{4+}$, and magnetic properties of the antifluorite compound ${\mathrm{K}}_{2}{\mathrm{IrBr}}_{6}$ 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 $\mathrm{\ensuremath{\Delta}}$ measured by RIXS. Nonmonotonic changes in $\mathrm{\ensuremath{\Delta}}$ are ascribed to the competing effects of the tilt, rotation, and deformation of the ${\mathrm{IrBr}}_{6}$ octahedra as well as tetragonal strain on the electronic levels of ${\mathrm{Ir}}^{4+}$. The N\'eel temperature of ${T}_{N}=11.9$ K exceeds that of the isostructural ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$, 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.