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 $\sim 1$ 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.

• Received 31 August 2022
• Revised 26 April 2023
• Accepted 6 June 2023

DOI:https://doi.org/10.1103/PhysRevB.107.235142