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

Abstract

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.