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

Abstract

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 (Δν_1/2) 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 Δν_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 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.