Electronic Spectra and Coordination of Nickel Centers in Liquid Lithium Chloride—Potassium Chloride Mixtures

Abstract

The electronic absorption spectra (4–28 kK) of dilute solutions of NiCl2 in liquid LiCl–KCl mixtures were measured at representative solvent compositions and temperatures from pure LiCl to pure KCl and and from 363° to 1070°C. Phenomenologically the spectra respond to changes in temperature at high temperature and changes in solvent composition at low temperatures in ways that are quantitatively very regular. At intermediate temperatures the behavior is more intricate. The data agree well with the following model. Nickel is partitioned between two types of centers, labeled T and O, which are in equilibrium. The T/O concentration ratio increases with increasing temperature and KCl content in the solvent. At low temperatures the fraction of each kind of center varies linearly with the mole fraction of KCl. The O-center concentration falls below the detection limit at 900°C in all solvents and in KCl at all temperatures. The coordination geometries of these centers are best described as being distributions about particular averages rather than as sharply defined arrangements. At low temperatures T centers have a distribution of coordination geometries that is narrow, composition invariant, and are clustered about a tetrahedral average. Their outer-shell cations are mostly K+. At high temperatures the distribution broadens and in LiCl-rich melts, Li+ ions invade their outer shells. The O centers are accurately studied only at rather low temperatures where they have a relatively broad coordination-geometry distribution with an average distinctively different from tetrahedral. Their outer shells consist largely of Li+ ions.