Systematic study of electron localization in saltrich Mx(MX)1−x melts (M=Na, K, Cs; X=Cl, I) by optical absorption spectroscopy

Abstract

We report optical absorption spectra of Kx(KI)1−x, Csx(CsI)1−x, and Csx(CsCl)1−x solutions at temperatures around 800 °C and in the saltrich concentration range up to metal mole fractions of xM⩽0.04 for energies 0.5⩽ℏω⩽5 eV. Employing a high-temperature electrochemical cell, we were able to change and determine the alkalimetal activity and the alkalimetal mole fraction in situ simultaneously with the optical absorption spectra. The high quality of the absorption spectra allows to distinguish spectral contributions to localized electronic states and mobile electrons. Together with previously measured spectra of Nax(NaI)1−x melts a systematic investigation of the alkali iodide melts reveals differences in the nature of the strongly localized electronic states with varying cation. Interpretation of the spectroscopic results with the aid of a chemical defect model shows that the formation of localized dimeric electron states (bipolarons) is pronounced in melts of smaller cations. The analysis of the optical contribution due to mobile electrons has been performed with a simple Drude model for nearly free electrons and the results are in good agreement with independent measurements of electronic transport properties.