Influence of the Second Coordination Sphere on the Diffusion Coefficients of Niobium Fluoride Complexes in Chloride and Fluoride Melts

摘要

The diffusion coefficients of Nb(Ⅴ) and Nb(Ⅳ) fluoride complexesin chloride and fluoride melts were obtained by linear sweepvoltammetry, chronopotentiometry and chronoamperometrymethods. The values found by these methods are in a goodagreement with each other. The decreasing of diffusion coefficientswas obtained when the cation in the second coordination spherechanges from Na to Cs. It was determined that the diffusioncoefficients decrease when going from chloride melts to fluoridemelts. Optimized geometrical structures and energies of such modelparticles 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. Themost stable compositions of these particles were determined;energies of the [NbF_7] complexes for all outersphere compositions,energy of the [NbF_7] complex bonds with its outersphere shell, andthe (Nb-F) and (Nb-M) bond lengths were calculated. Variations ofthe specified quantities in the Na-K-Cs and Nb(Ⅳ)-Nb(Ⅴ) seriesare compared with a changes of diffusion coefficients. It hasallowed making some guesses related mechanism of the [NbF_7]complex diffusion in melts.