Spectroscopic behavior and coordination of nickel(II) in liquid mixtures of zinc and cesium chlorides

Abstract

Optical absorption spectra of dilute solutions of NiCl2 in molten ZnCl2-CsCl mixtures were measured over the complete range of solvent compositions. The temperature extremes investigated were 260°C for melts containing 38 mole % CsCl and 1000°C for 75 mole % CsCl. At temperatures within 100-300°C of the liquidus (depending on melt composition) the spectra were highly composition and temperature dependent and consisted of relatively narrow absorption bands. Analysis of these spectra showed the presence under different conditions of six kinds of nickel(II) centers with well-defined coordination geometries. An equilibrium between octahedral and tetrahedral centers occurred in 0-8 mole % CsCl such that the octahedral form was favored with increasing CsCl content. A different octahedral-tetrahedral pair occurred in 20-50 mole % CsCl with the tetrahedral form favored by increasing CsCl content and temperature. In CsCl-rich melts tetrahedrally coordinated Cl3NiClZnCl33- and NiCl42- were found. In ZnCl2-rich melts at high temperatures the absorptions became broad and only mildly dependent on solvent composition. Suggestions are made regarding the relationships between these coordination effects and structure in the solvent.