Abstract
The optical and transport properties in the paramagnetic phase of α‐RuCl~3~ are studied here for the first time. Values are reported for the dc conductivity of single crystals over the temperature range 110 to 500 °K in the two crystallographic directions, along and perpendicular to the optic c‐axis: at 300 °K, ϱ∥c = 8 × 10^6^ Ωcm and ϱ⟂c = 1.7 × 10^3^ Ωcm. The activation energy for both directions is ≈ 0.1 eV below 300 °K and ≈ 0.5 eV for temperatures above 400 °K. Detailed absorption spectra in the energy range 0.1 to 4 eV, and in the temperature interval 80 to 300 °K, reveal three weak bands (α ≈ 10^3^ cm^−1^) at ≈ 0.28, 0.53, and 0.75 eV, two stronger peaks (α ≈ 10^4^ to 10^5^ cm^−1^) at ≈ 1.18, 2.08 and a step at 3.10 eV (values at 80 °K). Reflectivity measurements in the energy range 1 to 9 eV show maxima which, at 300°K, are centred at ≈ 1.1, 1.9, and 3.10 eV and a marked reflection peak, at ≈ 5.10 eV, probably related to p(Cl) → s(Ru) band‐to‐band transitions. Ac photoconductivity spectra in the 0.6 to 2.8 eV region show structures closely related to the absorption bands in the same energy range. The different temperature dependence of the absorption and photocurrent spectra over the 80 to 300 °K interval is discussed. After an analysis of the transport data, the absorption spectrum up to 3 eV is discussed in terms of the Tanabe and Sugano diagram for a d^5^ configuration.