Isotope Effect for Diffusion of Iron in Vanadium

The diffusion coefficients of iron, using Fe⁵⁹ as the radioactive tracer, and the isotope-effect parameter, using Fe⁵⁵ and Fe⁵⁹ as tracers, have been measured in vanadium over a temperature range of more than 800^∘{}C by standard sectioning and counting techniques. The diffusion of Fe⁵⁹ in vanadium exhibits the anomalous behavior of having not one but two straight lines in a plot of lnD versus 1/T, with a change of slope occurring near 1350^∘{}C. The data have been analyzed into the sum of two exponentials: $D=2280$ exp(-101500/RT) at high temperatures, and $D=0.0936$ exp(-67600/RT) at low temperatures. The values of the isotope-effect parameter for the Fe⁵⁹ and Fe⁵⁵ isotopes are approximately constant near 0.7 at low temperatures, but decrease rapidly with increasing temperature above 1350^∘{}C. The measurements are interpreted to indicate that two mechanisms of diffusion operate in vanadium. The mechanism predominant at low temperatures is identified as motion of a single vacancy; that at high temperatures is probably motion of divacancies.