High temperature 93Nb NMR and Raman spectroscopic investigation of the structure and dynamics of solid and liquid NbCl5-alkali chloride solutions

Pure liquid niobium chloride and solid and liquid NbCl5-alkali chloride solutions were for the first time studied by 93Nb NMR. Chemical shift (δ) and line width (Δν1/2) measurements were made by using a laboratory made probe head for experiments at elevated temperatures up to 650 °C. In the case of NbCl5-(LiCl/KCl)eut, where the salt mixture was not opaque, also Raman spectra could be recorded. For pure liquid NbCl5 the monomer–dimer equilibrium could be investigated. In the temperature range 200 °C to ca. 500 °C from the temperature dependence of both δ and Δν1/2 the equilibrium constant and the standard entropy and enthalpy for this reaction have been obtained. Binary NbCl5–CsCl and NbCl5–NaCl salt melts and solutions of NbCl5 in solid and liquid eutectic mixtures were investigated by 93Nb NMR. For some mixtures where the phase behaviour was only partially known, additional differential thermoanalysis (DTA) measurements of the phase diagrams have been performed. By 93Nb NMR a distinctly different behaviour of CsCl and NaCl containing melts is observed, where the the CsCl melts show relative broad resonance lines and a large negative chemical shift, whereas the NaCl melts reveal typically narrow lines and positive shifts. The broad lines are discussed with respect to the self-reduction reaction Nb5+ → Nb4+. In the high temperature solid phases of both NaCl and CsCl containing mixtures unusually narrow lines are explained by a fast ionic hopping process in these materials. The stability of oxy complexes in different salt matrices is also discussed. Finally, the observed temperature dependence of the 93Nb chemical shift (ca. 0.1 ppm K−1) in different salt mixtures is found to be in qualitative agreement with a theoretical estimate, which considers the Nb–Cl distance variation with temperature.