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 Nb-93 NMR. Chemical shift (delta) and line width (Deltav(1/2)) measurements were made by using a laboratory made probe head for experiments at elevated temperatures up to 650degreesC. 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 200degreesC to ca. 500degreesC from the temperature dependence of both delta and Deltav(1/2) 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 Nb-93 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 Nb-93 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 Nb-93 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.