Insulating Nature of Iridium Oxide Ca₅Ir₃O₁₂ Probed by Synchrotron-Radiation-Based Infrared Spectroscopy

Ca5Ir3O12 shows semiconducting behavior, although it is expected to have metallic conduction because of its 1/6-filled band. To investigate the insulating nature of Ca5Ir3O12, synchrotron-radiation-based infrared spectroscopy and electrical resistivity measurements were performed. Optical conductivity obtained using Kramers–Krönig transformation is compared with ab initio calculations based on density-functional and random-phase-approximation calculations, and their differences are carefully discussed. A detailed analysis of the temperature dependence of electrical resistivity along the c-axis reveals that ln ρ ∝ T−2/3. This temperature dependence indicates that the electrical conduction of Ca5Ir3O12 along the c-axis follows Efros–Shklovskii variable-range hopping. The optical conductivity with polarization parallel to the c-axis shows no clear onset at 1,300 cm−1 (0.16 eV) of the energy gap obtained by assuming the Arrhenius law. This feature of the optical conductivity is consistent with the fact that Ca5Ir3O12 follows Efros–Shklovskii variable-range hopping.