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

Raman spectra of ZrF4-KF molten mixtures have been measured at compositions up to 66 mol% ZrF4 and at temperatures up to 1000 degreesC. The data indicate that in mixtures rich in alkali fluoride two kinds of chemical species predominate the melt structure; octahedral ZrF62- (nu (1)(A(1g)) 570 cm(-1) and nu (5) (F-2g) 248 cm(-1)) and pentagonal bipyramidal ZrF73- (nu (1)(A(1)') 535 cm(-1) and nu (9)(E-2') 340 cm(-1)). An equilibrium between the two species is established which depends on temperature and composition. Spectral changes upon melting A(2)ZrF(6) (A=Li or K) and A(3)ZrF(7) (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 nu (1)(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 approximate to 500 cm(-1), weak) and two weak depolarized bands (245 and approximate to 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.