Stoichiometry, Vibrational Modes, and Structure of Niobium(V) Oxosulfato Complexes in the Molten Nb2O5−K2S2O7−K2SO4 System Studied by Raman Spectroscopy

The structural and vibrational properties of Nb v oxosulfato complexes formed in Nb 2 O 5 ―K 2 SO 7 and Nb 2 O 5 ―K 2 S 2 O 7 ―K 2 SO 4 molten mixtures with 0 ≤ X 0 Nb2O5 ≤ 0.25 have been studied by high-temperature Raman spectroscopy under static equilibrium at temperatures up to 700 °C. The spectral features for the binary Nb 2 O 5 ―K 2 S 2 O 7 molten system indicate that the dissolution of Nb 2 O 5 proceeds with consumption of S 2 O 7 2― leading to the formation of a Nb V oxosulfato complex according to Nb 2 O 5 + nS 2 O 7 2― → C 2n― ; a simple formalism exploiting the relative Raman band intensities is used for determining the stoichiometric coefficient, n, pointing to n = 3 and to the following reaction: Nb 2 O 5 + 3S 2 0 7 2― → 2NbO(SO 4 ) 3 3― , which is consistent with the Raman spectra of the molten mixtures. Nb 2 O 5 could be dissolved much easier when K 2 SO 4 was present in an equimolar (1:1) SO 4 2 /Nb ratio; the incremental presence of K 2 SO 4 in Nb 2 O 5 ―K 2 S 2 O 7 melts induces composition effects in the Raman spectra that terminate when n(SO 4 2 )/n(Nb) = 1. The composition effects and the temperature-dependent features of the Raman spectra obtained for Nb 2 O 5 ―K 2 S 2 O 7 ―K 2 SO 4 molten mixtures together with the spectral changes occurring upon freezing are accounted for by a Nb 2 O 5 ―3K 2 S 2 O 7 ·2K 2 SO 4 stoichiometry for the complete reaction taking place: Nb 2 O 5 + 3S 2 O 7 2― + 2SO 4 2 → NbO(SO 4 ) 4 S 2 O 7 7 + NbO 2 (SO 4 ) 2 3 . The spectral data are discussed in terms of the most plausible structural models, for which consistent band assignments are made. The most characteristic Raman bands for the Nb V oxosulfato complexes pertain to Nb=O modes: (i) at 937 cm ' for the mono-oxo Nb=O mode of NbO(SO 4 ) 3 3- ; (ii) at 958 cm ' for the mono-oxo Nb=O mode of NbO(SO 4 ) 4 S 2 O 7 7― ; and (iii) at 926 cm ―1 for the symmetric dioxo Nb(=O) 2 mode of NbO 2 (SO 4 ) 2 3― .