Ionic clusters in aluminium sodium fluoride melts

Abstract

From Raman scattering experiments three types of charged AlFn cluster (with n = 4, 5 and 6) have been proposed to coexist in liquid alkali fluoroaluminates such as cryolite (Na3AlF6). We characterize these isolated clusters by a phenomenological ionic model and employ it to examine how local screening of the excess charge on a cluster may be effected by sodium ions at various compositions in the liquid. We provide the first theoretical explanation for the stability of the AlF5 cluster at the compositions Na3AlF6 and Na2AlF5.