Localized and mobile electrons in metal - molten-salt solutions

Abstract

At elevated temperatures (about 10 3 K) liquid alkali metal-alkali halide solutions transform continuously from the nonmetallic to the metallic state (NM-M transition) as a function of the metal mole fraction x M . In this study we present results of new experiments on spectroscopic ellipsometry and on absorption spectroscopy across the transition regime. The data indicate that on both sides of the NM-M transition localized and mobile electronic states may coexist

Presence of Li Clusters in Molten LiCl-Li

“…The conglomeration of Li atoms in the form of clusters may explain why the F − center model notably overestimates the electrical conductivity of the LiCl-Li system while accurately predicting such properties for alternative solutions of alkali metal–alkali halide salts 19 43 44 . The F − center model used to describe electron mobility in metal-salt solutions operates on the assumption that each metal atom acts as an electron donor to the electronic structure of the molten system 45 46 47 48 . Under this assumption, the melt should exhibit a rapid increase in electrical conductivity with the inclusion of a small concentration of electron donor atoms.…”

“…Alternatively, if Li atoms suspended in molten LiCl-Li form clusters their valance electrons would be confined to the clusters instead of extending into shared electron states of the melt as a whole. This effect can therefore account for the consumption of what would be “free” electrons under metal saturated conditions, resulting in a suppression of the electrical conductivity of the melt 43 45 46 47 48 49 . Similarly, the F − center model does not apply to polyvalent metal-salt solutions such as Bi-BiI 3 50 51 .…”

“…Furthermore, the alteration to the Li|Li + open circuit potential that occurs with varying concentrations of Li in the melt is not Nernstian; it behaves as if the activity of the reduced form, Li is not unity and changes based on concentration 13 . These facts suggest that molten solutions of LiCl and Li contain additional Li complexes other than LiCl and Li; a hypothesis that has been debated without confirmation for decades 15 20 21 46 48 50 51 52 . It was noted that Li clusters possess significantly different ionization potentials than metallic Li 36 37 ; therefore they are expected to exhibit thermodynamic activity that is different from the metallic phase.…”

Raman spectroscopic study of mixed valence neodymium and cerium chloride solutions in eutectic LiCl–KCl melts

“…This is in analogy to that what has been proposed previously for the alkali metal-alkali halide systems. [2][3][4] In the latter case the divalent oxidation state is stable. Thus, the metal excess electrons can be effectively trapped and localized.…”

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

“…2,3 The electronic structure of these non-metallic systems is characterized by strongly localized states like F-centers and spin paired dimers or bipolarons. [4][5][6] For the electronic transport a dynamic equilibrium between these localized states and mobile electrons has to be considered, whereby the latter have a mobility of $0.1 cm 2 V À1 s À1 in dilute salt rich solutions, 7 i.e. about two orders of magnitude larger than that of the ions.…”