THE DEFECT STRUCTURE OF 2S NB1+XS2 AND THE HIGH-TEMPERATURE SULFIDATION OF NIOBIUM

An approximate model for the defect structure of the niobium sulfide 2s Nb1+xS2 is developed on the basis of experimental data concerning its deviation from stoichiometry as a function of sulfur pressure. The model involves the presence of doubly-charged metal interstitials and metal vacancies as well as of free electrons and electron holes. The possible effects of hydrogen dissolution on the concentration of the intrinsic defects in this compound are also evaluated for various hydrogen species with an effective charge different from zero, but no final conclusion concerning the existence of an actual doping effect of this compound is reached The rate constant for the sulfidation of niobium at 950-degrees-C is calculated on the basis of this defect structure as a function of the sulfur pressure and is compared with the experimental results concerning the sulfidation of niobium in H-2-H2S mixtures. It is concluded that both metal vacancies and metal interstitials contribute significantly to the growth of the sulfide 2s Nb1+xS2.