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

Abstract

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 V^V polymeric complexes consisted of VO₃^- and VO₂(SO₄)₂^3- 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)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 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)₂^3- 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.