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

We study within an ionic model the static and dynamic structure and the ene rgetics of neutral and charged molecular clusters which have been proposed to exist in liquid ZrCl4 and in molten CsCl-ZrCl4 mixtures from Raman scatt ering 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 polarizabilit ies, using three disposable parameters which are adjusted to the bond lengt h and to vibrational frequencies of the isolated ZrCl4 molecule. The Zr2Cl8 dimer, which is believed to be present in saturated vapours and in the pur e melt, is found in its ground state to consist of two double-corner-sharin g tetrahedra, yielding a distorted fivefold coordination for the Zr ions at low temperatures. This distortion is essentially averaged out by thermal f luctuations already at room temperature. Chlorination through mixing with C sCl at various compositions favours the ZrCl5 anion as well as sixfold, oct ahedral-like coordinations of the Zr ions in the ZrCl6 monomer and in dimer s 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 e nergetics of their equilibria. (C) 2000 Elsevier Science B.V. All rights re served.