Cation self-diffusion in disordered VO_x

The diffusion of ⁴⁸V in disordered VO_x, crystals has been measured by a serial-sectioning technique as a function of temperature (1100-1500°C) over the homogeneity range 0.78 < x < 1.28. The temperature dependence of the cation tracer diffusivity at each fixed composition is characterized by an Arrhenius behavior in the temperature range 1100-1500°C. The Arrhenius parameters decrease rather sharply near the stoichiometric composition as the composition increases from the metal-rich to the metal-deficient regime; the activation energy for diffusion decreases from ~71 kcal mole to ~48 kcal mole, and the frequency factor decreases by nearly two orders of magnitude (from ~5 cm² s to ~0.05 cm² s). It is concluded that the significant difference in the cation self-diffusion behavior between the metal-rich and the metal-deficient VO_x may be attributed to the significant differences in the defect structures of the two regimes. The various possible diffusion mechanisms are explored, and comparisons of the cation diffusion behavior in VO_x, with that of the related transition-metal monoxides TiO_x and Fe_1-δO are made. It is concluded that the experimental results for the entire composition range are consistent with the process of diffusion occurring by the migration of monovacancies in equilibrium with defect clusters, the nature of the clusters being different for x < 1 and x #62;; 1.