Molten AlF<sub>3</sub>-KF mixtures have been investigated in a wide composition range by Raman spectroscopy and with the use of a graphite windowless cell. The results of this study clearly show that the most intense spectrum components are made of three distinct bands whose intensities vary when the composition is changed. The most probable interpretation of our data is to consider the existence of an equilibrium between AlF<sub>6</sub><sup>3-</sup>, AlF<sub>5</sub><sup>2-</sup>, and AlF<sub>4</sub><sup>-</sup>, This explanation is a confirmation of the conclusions of recent papers suggesting the presence of these species in similar AlF<sub>3</sub>-NaF mixtures. The presence of only one major polarized band for AlF<sub>5</sub><sup>2-</sup> is explained by considering a fast exchange between the axial and equatorial fluoride at the elevated experimental temperature (1300 K). A quantitative analysis of the distribution of the fluoroaluminate species has been performed by decomposition of the Raman-band envelope. The calculated equilibrium constants (expressed in mole fraction units) for AlF<sub>5</sub><sup>2-</sup> ⇌ AlF<sub>4</sub><sup>-</sup> + F<sup>-</sup> and AlF<sub>6</sub><sup>3-</sup> ⇌ AlF<sub>5</sub><sup>2-</sup> + F<sup>-</sup> are <i>K</i><sub>A</sub> = 0.11 ± 0.01 and <i>K</i><sub>B</sub> = 5 ± 1, respectively. Compared with the NaF-AlF<sub>3</sub> system, these constants correspond to a stabilization of less coordinated fluoroaluminates when the size of the counterion increases.
E. Tixhon, E. Robert, and B. Gilbert, "Molten KF-AlF3 System: A Study by Raman Spectroscopy," Appl. Spectrosc. 48, 1477-1482 (1994)