Co-ordination of thorium(IV) in molten alkali-metal chlorides and the structure of liquid and glassy thorium(IV) chloride

Abstract

Raman spectra of molten ThCl₄–ACl (A = Li, Na, K or Cs) mixtures have been measured. The complete composition range has been studied for all systems at temperatures up to 960 °C. The spectral changes upon melting the binary compounds A₂ThCl₆, A₃ThCl₇ and the pure crystalline ThCl₄ were also measured. The data indicate that in molten mixtures rich in alkali-metal chloride the predominant species are the ThCl₆^2– octahedra [ν₁(A_1g) 297 and ν₅(F_2g) 125 cm^–1] in equilibrium with the ThCl₇^3– pentagonal bipyramid. With increasing ThCl₄ mole fraction above 0.3 the frequency of the ν₁(A_1g) band increases continuously reaching its maximum (≈340 cm^–1) in pure molten ThCl₄. It appears that in mixtures rich in ThCl₄ edge bridging of thorium(IV) octahedra occurs yielding chain species of the type [Th_nCl_4n + 2]^2– and [Th_nCl_4n – 2]²⁺ where the end Th atoms of the chain are six- and four-fold co-ordinated for the anion and cation respectively. The known ionic character of these melts suggests that the chain length is rather small and that most probably the species have low n values (e.g. Th₂Cl₁₀^2–, Th₃Cl₁₄^2–, Th₃Cl₁₀²⁺, . . .). Glassy ThCl₄ was formed by slow cooling of the melt. The vibrational modes and the inferred structure of the glass are similar to those of the melt.