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Diffusion coefficients of Nb(V) and Nb(IV) fluoride complexes in chloride and fluoride melts were obtained by linear sweep voltammetry, chronopotentiometry and chronoamperometry methods. The values found by these methods are in good agreement with each other. Decreasing of diffusion coefficients was obtained when the cation in the second coordination sphere changes from Na + to Cs + . It was determined that diffusion coefficients decrease when chloride melts are converted to fluoride melts. Optimized geometrical structures and energies of such model particles as (M + ) n [Nb(V)F 7 ] 2− and (M + ) n [Nb(IV)F 7 ] 3− (M + = Na + , K + , Cs + ; n = 1-6) were obtained by quantum-chemical calculations. There most stable compositions of these particles have been determined. Energies of both the [NbF 7 ] complexes for all outersphere compositions, of the [NbF 7 ] complex bond with its outersphere shell, and the (Nb-F) and (Nb-M) bond lengths have been calculated. Variations of specified quantities in the Na + -K + -Cs + and Nb(IV)-Nb(V) series have been compared with changes in diffusion coefficients. This has allowed to make some guesses concerning the mechanism of the [NbF 7 ] complex diffusion in melts.Studying of transport properties in molten electrolytes, including of diffusion coefficients (D), is interesting both for the kinetics electrode processes of and melt structure, and for selecting the optimum conditions of refractory metal electrodeposition. For the most part, the experimental values of D in molten salts are obtained by chronopotentiometry (CP), although linear sweep voltammetry (LSV), cyclic voltammetry (CV) and chronoamperometry (CA) are often used as well. 1 Despite extensive electrochemical studies of molten salts, there are not too much data on diffusion coefficients of refractory metals. 2,3 Diffusion coefficients of Nb(V) and Nb(IV) complexes in chloridefluoride and fluoride melts, obtained before this study are presented in Table I. As can be seen from Table I, there are mostly results concerning diffusion coefficients of Nb(V) and Nb(IV) complexes. Data on activation energies for the diffusion process are poor, there is no information on the influence of the composition of the first and second coordination spheres on the D Nb(V) and D Nb(IV) . This paper discusses the D of niobium obtained by the authors and the effect of the second coordination sphere on diffusion coefficients. ExperimentalChemicals; preparation of salts.-Alkali chlorides (NaCl, KCl, and CsCl) were purchased from Prolabo (99.5% min.). They were dehydrated by continuous and progressive heating just above the melting point under a gaseous HCl atmosphere in quartz ampoules. Excess HCl was removed from the melt by argon. The salts were handled in a glove box and stored in sealed glass ampoules. The alkali metal fluorides (Aldrich 99.5% min.) were purified by double melt recrystallization: NaF, KF and CsF were dried in a glassy carbon crucible (SU-2000) at 673-773 K under vacuum, then heated up to a temperature of 50 K above their...

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