Noncubic local distortions and spin-orbit excitons in K2IrCl6

The cubic antifluorite K2IrCl6 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* approximate to 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 Gamma(5+) mode involving vibrations of the K+ ion relative to the Cl- 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 omega = 0.62-0.79 eV as well as a weak electronic excitation at omega = 0.12-0.86 eV. Our numerical calculations reproduce their spectral energy and shape with the electronic parameters: SO coupling lambda = 465 meV; Hund's coupling J(H) = 300 meV; tetragonal distortion strength 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 vertical bar j(eff) = 3/2,+/- 1 /2 > states arising from the coupling between the SO excitons and electron-hole excitations. Our results showcase that K2IrCl6 is on the brink of a structural phase transition.