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 have been recorded at temperatures up to 570°C under an oxygen atmosphere and at different compositions in the range 0.25⩽X(V2O5)⩽0.50. 33 mol% V2O5 mixtures contain VV polymeric complexes consisted of VO3- and VO2(SO4)23- units participating in chain-like or network-like configurations, while VO2SO4- units were also detected in the K2SO4–V2O5 system. The spectral changes occurring upon addition of V2O5 up to X(V2O5)=0.50 are interpreted to indicate: (i) a gradual transformation of VO2(SO4)23- 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 networks. The most characteristic bands observed for the various units comprising the polymeric complexes in the K2SO4–V2O5 system are assigned as follows: (i) for VO2(SO4)23- at 1042 (terminal VO stretches of six-coordinated 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 VO stretches of four-coordinated vanadium), 486 and 365 cm-1 and (iii) for VO2SO4- units at 983 (terminal VO stretches of four-coordinated vanadium) and 862 cm-1 (bridging S–O). Similar values have been found for the band wavenumbers in the Cs2SO4–V2O5 system. The spectral data are discussed in terms of possible structural models. The spectral changes upon freezing the Cs2SO4–V2O5 molten mixtures with 0.33⩽X(V2O5)⩽0.50 indicate formation of the 1:1 V2O5·Cs2SO4 solid. This conclusion is confirmed also by powder XRD spectroscopy. For the first time, high-temperature vibrational spectroscopy has been used to establish the structural and vibrational properties of M2SO4–V2O5 (M=K or Cs) molten salt mixtures.