Theoretical evidence for the stability of the (AlF5)2- complex anion

Raman scattering spectra and thermodynamic measurements on the molten cryolite mineral Na3AlF6 have been interpreted as giving evidence for the presence of the (AlF5)(2-) complex anion as a majority cluster species. We study various NanAlFn+3 clusters in vacuo with model ionic interactions which are adjusted and tested against experimental data and ab initio calculations on NaAlF4 and (AlF4)(-). We show that within this model the uncommon fivefold coordination of the Al ion in Na2AlF5 and Na3AlF6 is stabilized by the alkali counterions. (C) 1999 Elsevier Science B.V. All rights reserved.