5 - Tantalum and niobium complexes in fluoride melts

Abstract

For a long period, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. Molten salts are characterized by the formation of discrete complex ions that are subjected to coordination phenomenon. Such complex ions have specific compositions that are related to the rearrangement of their electronic configuration and to the formation of partially covalent bonds. The lifetime of the coordinated ions is longer than the contact period of the individual ions. Molten salt investigation methods are divided into two classes: thermodynamic and kinetic. In some cases, the analysis of melting diagrams and isotherms of physical-chemical properties, such as density, surface tension, viscosity, and electroconductivity enables the determination of the ionic composition of the melt. Direct investigation of the complex structure is performed using spectral methods.