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.