Structure and energetics of neutral and Cesium-compensated charged clusters in Zirconium chloride melts

Abstract

We study within an ionic model the static and dynamic structure and the energetics of neutral and charged molecular clusters which have been proposed to exist in liquid ZrCl4 and in molten CsCl-ZrCl4 mixtures from Raman scattering experiments by G. M. Photiadis and G. N. Papatheodorou (J. Chem. Soc. Dalton Trans. 1998, 981). The model accounts for ionic-shell deformability through (i) effective valences and (ii) electric and overlap polarizabilities, using three disposable parameters which are adjusted to the bond length and to vibrational frequencies of the isolated ZrCl4 molecule. The Zr2Cl8 dimer, which is believed to be present in saturated vapours and in the pure melt, is found in its ground state to consist of two double-corner-sharing tetrahedra, yielding a distorted fivefold coordination for the Zr ions at low temperatures. This distortion is essentially averaged out by thermal fluctuations already at room temperature. Chlorination through mixing with CsCl at various compositions favours the ZrCl5 anion as well as sixfold, octahedral-like coordinations of the Zr ions in the ZrCl6 monomer and in dimers built from face-sharing, edge-sharing or corner-sharing octahedra (Zr2Cl9, Zr2Cl10 and Zr2Cl11, respectively). We demonstrate a crucial role of the Cs counterions in stabilizing these complex anions and in determining the energetics of their equilibria. (C) 2000 Elsevier Science B.V. All rights reserved.