Optical signatures of the Jeff = 12 state in Ir4+ halides

We study the vibrational and electronic excitations in cubic iridium halides (${\mathrm{NH}}₄{)}₂{\mathrm{IrCl}}₆, {\mathrm{K}}₂{\mathrm{IrCl}}₆$, and ${\mathrm{K}}₂{\mathrm{IrBr}}₆$ by infrared reflectivity and transmission measurements of single crystals. All three compounds show optical excitations in the energy range of 0.5--0.8 eV, which can be ascribed to ${j}_{\mathrm{eff}}=\frac{3}{2}$ to ${j}_{\mathrm{eff}}=\frac{1}{2}$ transitions (spin-orbit excitons), and ${t}_2g$-to-${e}_g$ excitations above $\ensuremath{\sim}1$ eV. We observe at least two peaks due to spin-orbit excitons in the bromide and four peaks in both chlorides, suggesting local deviations from the cubic symmetry already at room temperature. We further show that the ${e}_g$ states lie at lower energies in the bromide compared to the chlorides, in agreement with density-functional band-structure calculations.