Electron localization in metal–molten salt solutions: An optical study with <i>in situ</i> variation of composition

Abstract

Optical absorption spectra are reported for liquid KxKCl1−x (10−5≤x≤5×10−2) and NaxNaBr1−x (10−3≤x≤3×10−2) solutions at 800 °C in the energy range 0.5≤hν≤5 eV. The composition has been varied in situ by Coulometric titration of the metal. With this method of simultaneous measurement of optical spectra and metal activity relative changes in the spectra are detected with high precision. In this way it has been possible, for the first time, to separate distinct components in the optical spectra of metal solutions. A contribution which is quantitatively described by the Drude model is attributed to weakly localized electrons in conduction band tail states with low mobility. The determined electronic mobilities are of the order of 0.1 cm2 V−1 s−1. The concentration dependence of the corresponding electronic conductivity quantitatively agrees with independent measurements of the dc-electronic conductivity. The main component in the absorption spectra is due to excitations of localized electrons which are different in character in the KxKCl1−x and NaxNaBr1−x melts. This is concluded from the distinct concentration dependence and is discussed in the light of quantum molecular dynamics calculations and further results of the electronic properties.