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

Raman scattering spectra and thermodynamic measurements on the molten cryol ite mineral Na3AlF6 have been interpreted as giving evidence for the presen ce 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 o n NaAlF4 and (AlF4)(-). We show that within this model the uncommon fivefol d coordination of the Al ion in Na2AlF5 and Na3AlF6 is stabilized by the al kali counterions. (C) 1999 Elsevier Science B.V. All rights reserved.