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

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 a t temperatures up to 960 degrees C. The spectral changes upon melting the b inary compounds A(2)ThCl(6), A(3)ThCl(7) and the pure crystalline ThCl4 wer e also measured. The data indicate that in molten mixtures rich in alkali-m etal chloride the predominant species are the ThCl62- octahedra [nu(1)(A(1g )) 297 and nu(5)(F-2g) 125 cm(-1)] in equilibrium with the ThCl73- pentagon al bipyramid. With increasing ThCl4 mole fraction above 0.3 the frequency o f the nu(1)(A(1g)) band increases continuously reaching its maximum (approx imate to 340 cm(-1)) in pure molten ThCl4. It appears that in mixtures rich in ThCl4 edge bridging of thorium(IV) octahedra occurs yielding chain spec ies of the type [ThnCl4n + 2](2-) and [ThnCl4n - 2](2+) where the end Th at oms of the chain are six- and four-fold co-ordinated for the anion and cati on 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 forme d by slow cooling of the melt. The vibrational modes and the inferred struc ture of the glass are similar to those of the melt.