Optical signatures of the Jeff=1/2 state in Ir4+halides

We study the vibrational and electronic excitations in cubic iridium halides (NH4)2IrCl6, K2IrCl6, and K2IrBr6 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 jeff = 32 to jeff = 12 transitions (spin-orbit excitons), and t2g-to-eg excitations above & 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 eg states lie at lower energies in the bromide compared to the chlorides, in agreement with density-functional band-structure calculations