Stoichiometry, vibrational modes, and structure of niobium(V) oxosulfato complexes in the molten Nb(2)O(5)-K(2)S(2)O(7)-K(2)SO(4) system studied by Raman spectroscopy.

The structural and vibrational properties of NbV oxosulfato complexes formed in Nb2O5-K2S2O7 and Nb2O5-K2S2O7-K2SO4 molten mixtures with 0 C2n-; 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: Nb2O5 + 3S2O72- --> 2NbO(SO4)33-, which is consistent with the Raman spectra of the molten mixtures. Nb2O5 could be dissolved much easier when K2SO4 was present in an equimolar (1:1) SO42-/Nb ratio; the incremental presence of K2SO4 in Nb2O5-K2S2O7 melts induces composition effects in the Raman spectra that terminate when n(SO42-)/n(Nb) = 1. The composition effects and the temperature-dependent features of the Raman spectra obtained for Nb2O5-K2S2O7-K2SO4 molten mixtures together with the spectral changes occurring upon freezing are accounted for by a Nb2O5.3K2S2O7.2K2SO4 stoichiometry for the complete reaction taking place: Nb2O5 + 3S2O72- + 2SO42- --> NbO(SO4)4S2O77- + NbO2(SO4)23-. 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 NbV oxosulfato complexes pertain to Nb=O modes: (i) at 937 cm-1 for the mono-oxo Nb=O mode of NbO(SO4)33-; (ii) at 958 cm-1 for the mono-oxo Nb=O mode of NbO(SO4)4S2O77-; and (iii) at 926 cm-1 for the symmetric dioxo Nb(=O)2 mode of NbO2(SO4)23-.