Effect of the deviation from stoichiometry on cation self-diffusion and isotope effect in wüstite,Fe1?xO

Diffusion of ⁵⁹Fe in Fe_1xO crystals has been measured by a serial-sectioning technique as a function of temperature and deviation from stoichiometry. The results indicate that the diffusivity increases slightly at 1200°C, decreases at 802°C with an increase in x, and is insensitive to change in x at 1003°C. The temperature dependence of the cation diffusivity in Fe_0.94O is given by the expression, $\text{D=(0.6±0.5)×10}{}^{\mathrm{?3}}\text{exp(?29350 ±}\text{300}\text{RT}\text{)cm}{}^2\text{/se}$ the temperature range 700–1340°C. The isotope effect for cation self-diffusion was measured by simultaneous diffusion of ⁵²Fe and ⁵⁹Fe in Fe_1?xO at various temperatures and values of x. Although the measured values of fΔK are independent of temperature within the limits of experimental error, they decrease with an increase in the deviation from stoichiometry. The experimental results appeared to be consistent with the diffusion of Fe ions via “free mobile vacancies” that coexist with defect clusters. As a consequence of a “site-blocking” effect, the mobility of “free mobile vacancies” and the apparent correlation factor for cation tracer diffusion decrease with an increase in deviation from stoichiometry.