Vibrational modes and structure of vanadium(V) complexes in M2SO4-V2O5 (M = K or Cs) molten salt mixtures

Raman spectra of the M2SO4-V2O5 (M = K or Cs) binary molten salt systems ha ve been recorded at temperatures up to 570 degrees C under an oxygen atmosp here and at different compositions in the range 0.25 less than or equal to X(V2O5) less than or equal to 0.50. 33 mol% V2O5 mixtures contain V-v polym eric complexes consisted of VO3- and VO2(SO4)(2)(3-) units participating in chain-like or network-like configurations, while VO2SO4- units were also d etected in the K2SO4-V2O5 system. The spectral changes occurring upon addit ion of V2O5 up to X(V2O5) = 0.50 are interpreted to indicate: (i) a gradual transformation of VO2(SO4)(2)(3-) units, where vanadium is six-coordinated , to VO2SO4-, where vanadium is four-coordinated and (ii) extensive linking of polymeric chains giving rise to large and complex three-dimensional net works. The most characteristic bands observed for the various units compris ing the polymeric complexes in the K2SO4-V2O5 system are assigned as follow s: (i) for VO2(SO4)(2)(3-) at 1042 (terminal V=O stretches of six-coordinat ed vanadium), 940 (terminal S-O stretches of sulfate), 880 (bridging S-O), 668, 408 and 227 cm(-1); (ii) for VO3- units at 950 (terminal V=O stretches of four-coordinated vanadium), 486 and 365 cm(-1) and (iii) for VO2SO4- un its at 983 (terminal V=O stretches of four-coordinated vanadium) and 862 cm (-1) (bridging S-O). Similar values have been found for the band wavenumber s in the Cs2SO4-V2O5 system. The spectral data are discussed in terms of po ssible structural models. The spectral changes upon freezing the Cs2SO4-V2O 5 molten mixtures with 0.33 less than or equal to X(V2O5) less than or equa l to 0.50 indicate formation of the 1 :1 V2O5 . Cs2SO4 solid. This conclusi on is confirmed also by powder XRD spectroscopy. For the first time, high-t emperature vibrational spectroscopy has been used to establish the structur al and vibrational properties of M2SO4-V2O5 (M = K or Cs) molten salt mixtu res.