Pure liquid niobium chloride and solid and liquid NbCl 5 -alkali chloride solutions were for the first time studied by 93 Nb NMR. Chemical shift (d) and line width (Dn 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 NbCl 5 -(LiCl/KCl) eut , where the salt mixture was not opaque, also Raman spectra could be recorded. For pure liquid NbCl 5 the monomerdimer equilibrium could be investigated. In the temperature range 200 C to ca. 500 C from the temperature dependence of both d and Dn 1/2 the equilibrium constant and the standard entropy and enthalpy for this reaction have been obtained. Binary NbCl 5 -CsCl and NbCl 5 -NaCl salt melts and solutions of NbCl 5 in solid and liquid eutectic mixtures were investigated by 93 Nb 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 93 Nb 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 Nb 5+ ! Nb 4+ . 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 93 Nb chemical shift (ca. 0.1 ppm K À1 ) in different salt mixtures is found to be in qualitative agreement with a theoretical estimate, which considers the Nb-Cl distance variation with temperature.

At elevated temperatures (about 10 3 K) liquid alkali metal-alkali halide solutions transform continuously from the nonmetallic to the metallic state (NM-M transition) as a function of the metal mole fraction x M . In this study we present results of new experiments on spectroscopic ellipsometry and on absorption spectroscopy across the transition regime. The data indicate that on both sides of the NM-M transition localized and mobile electronic states may coexist

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