Intervalence charge transfer in mixed valence neodymium iodide melts: Electronic conductivity and optical absorption spectra

Abstract

We have studied the optical absorption spectra and the electrical conductivity of highly corrosive liquid NdI₃–NdI₂ mixtures at various temperatures up to 957 °C. The conductivity as a function of composition shows a clear maximum near the 1∶1 composition, indicative of a considerable partial electronic conductivity of ∼1 Ω⁻¹ cm⁻¹ and an activation energy E_a = 0.4 eV. Optical spectra have been obtained for the first time in NdI₂-rich solutions in the range 0.5 ⩽ ħω/eV ⩽ 3. They are characterized by the typical f–f transitions in the visible range, by an absorption edge in the ultraviolet range that exhibits Urbach behaviour and, in particular, by a strong and broad absorption band with a peak near 1.5 eV. The latter shows the main characteristic features of an intervalence charge transfer band which is shown by fitting the spectra with a two-site polaron model.