Noncubic local distortions and spin-orbit excitons in K2IrCl6

The cubic antifluorite ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$ has recently garnered renewed attention due to its relevance to Kitaev magnetism. Combining Raman spectroscopy with numerical calculations, we investigate its electronic structure and ensuing low-lying excitations as well as lattice instabilities. For temperatures below ${T}^{*}\ensuremath{\approx}180$ K, we observe several lattice anomalies: (i) a gradual appearance of the symmetry-forbidden phonons; (ii) a central-mode-like excitation; and (iii) a soft-mode-like behavior of the ${\mathrm{\ensuremath{\Gamma}}}^{5+}$ mode involving vibrations of the ${\mathrm{K}}^{+}$ ion relative to the ${\mathrm{Cl}}^{\ensuremath{-}}$ ion. All these features indicate the occurrence of local noncubic distortions. At high energies, we observe spin-orbit (SO) excitons made of five peaks at $\ensuremath{\omega}=0.62--0.79$ eV as well as a weak electronic excitation at $\ensuremath{\omega}=0.12--0.86$ eV. Our numerical calculations reproduce their spectral energy and shape with the electronic parameters: SO coupling $\ensuremath{\lambda}=465$ meV; Hund's coupling ${J}_{H}=300$ meV; tetragonal distortion strength ${\mathrm{\ensuremath{\Delta}}}_{t}=30$ meV and on-site Coulomb interaction $U=2.2$ eV. The multiple SO excitons are interpreted in terms of bounded SO excitons of the $|{j}_{\mathrm{eff}}=\frac{3}{2},\ifmmode\pm\else\textpm\fi{}\frac{1}{2}\ensuremath{\rangle}$ states arising from the coupling between the SO excitons and electron-hole excitations. Our results showcase that ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$ is on the brink of a structural phase transition.