A theoretical study of the stabilization of the (AlF5)2- complex anion by alkali counterions

We evaluate the relative stability of various states of coordination for the aluminium ion by fluorines in MnAlFn+3 microclusters, with M = Li, Na or K and n = 2 or 3. The calculations use ionic model interactions which have been adjusted and tested against experimental data and ab initio calculations on the (AlF4)(-) anion and on MAlF4 clusters. We confirm earlier results showing that the fivefold (AlF5)(2-) anion is stabilized by the counterions and assess the sensitivity of this result to the details of the model as well as the effect of alkali substitution. We evaluate the variation of the breathing mode frequency of the complex anions in these clusters for comparison with Raman scattering data from liquid mixtures of AlF3 and MF in the basic range of composition.