Raman spectroscopic studies of molten ZrF4–KF mixtures and of A2ZrF6, A3ZrF7 (A = Li, K or Cs) compounds

Abstract

Raman spectra of ZrF₄–KF molten mixtures have been measured at compositions up to 66 mol% ZrF₄ and at temperatures up to 1000 °C. The data indicate that in mixtures rich in alkali fluoride two kinds of chemical species predominate the melt structure; octahedral ZrF₆²⁻ (ν₁(A_1g) 570 cm⁻¹ and ν₅ (F_2g) 248 cm⁻¹) and pentagonal bipyramidal ZrF₇³⁻ (ν₁(A₁′) 535 cm⁻¹ and ν₉(E₂′) 340 cm⁻¹). An equilibrium between the two species is established which depends on temperature and composition. Spectral changes upon melting A₂ZrF₆ (A = Li or K) and A₃ZrF₇ (A = K or Cs) polycrystalline compounds support the proposed two species equilibrium scheme. At 33 mol% ZrF₄ the predominant species present are ZrF₆²⁻ octahedra. With increasing ZrF₄ mole fraction the strong ν₁(A_1g) band at 570 cm⁻¹ shifts continuously to higher wavenumbers and new bands appear in the spectra. At the maximum composition studied of 66 mol% ZrF₄ the spectra are characterized by two polarized (630 cm⁻¹, strong and ≈500 cm⁻¹, weak) and two weak depolarized bands (245 and ≈180 cm⁻¹). The observed spectral trends with variation of composition are similar to those found for ThCl₄–CsCl molten mixtures and are interpreted with an analogous model where the structure of the rich in ZrF₄ melts is dominated by small size chains formed by “ZrF₆” octahedra bound by corners and/or edges.