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

摘要

Raman spectra of molten ThCl_4-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 deg C. The spectral changes upon melting the binary compounds A_2ThCl_6, A_3ThCl_7 and the pure crystalline ThCl_4 were also measured. The data indicate that in molten mixtures rich in alkali-metal chloride the predominant species are the ThCl_6~(2-) octahedra [#nu#_1(A_(1g)297) and #nu#_5(F_(2g))125 cm~(-1)] in equilibrium with the ThCl_7~(3-) pentagonal bipyramid. With increasing ThCl_4 mole fraction above 0.3 the frequency of the #nu#_1(A_(1g)) band increases continuously reaching its maximum (approx= 340 cm~(-1)) in pure molten ThCl_4. It appears that in mixtures rich in ThCl_4 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. Th_2Cl_(10)~(2-), Th_3Cl_(14)~(2-), Th_3Cl_(10)~(2+), ...). Glassy ThCl_4 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.