Ionic interactions in alkali aluminium tetrafluoride clusters

Complex anion structures ((AlF4)(-), (AlF5)(2-) and (AlF6)(3-)) coexist in liquid mixtures of aluminium trifluoride and alkali fluorides in compositio n-dependent relative concentrations and are known to interact with the alka li counterions. We present a comparative study of the static and vibrationa l structures of MAlF4 molecules (with M = any alkali), with the aim of deve loping and testing a refined model of the ionic interactions for applicatio ns to the M-Al fluoride mixtures. We find that, whereas an edge-bridged coo rdination is strongly favoured for Li in LiAlF4, edge-bridging and face-bri dging of the alkali ion become energetically equivalent as one moves from N a to the heavier alkalis. This result is sensitive to the inclusion of alka li polarizability and may be interpreted as implying (for M = K, Rb or Cs) almost free relative rotations of the M+ and (AlF4)(-) partners at temperat ures of relevance to experiment. The consistency of such a viewpoint with e lectron diffraction data on vapours and with Raman spectra on melts is disc ussed.