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 ThCl62– octahedra [ν1(A1g) 297 and ν5(F2g) 125 cm–1] in equilibrium with the ThCl73– pentagonal bipyramid. With increasing ThCl4 mole fraction above 0.3 the frequency of the ν1(A1g) 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 [ThnCl4n + 2]2– and [ThnCl4n – 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. Th2Cl102–, Th3Cl142–, 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.