Diffusion of iron in aluminum studied by Mössbauer spectroscopy

The broadening of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer line due to diffusion jumps of the Fe atoms in an Al single crystal has been measured as a function of crystal orientation. The broadening is strongly anisotropic. Theoretical calculations for the broadening using the five-frequency model for the motion of an Fe atom via vacancies show that the following information about the geometry and perturbed jump frequencies of the Fe-vacancy complex can be obtained: (i) The anisotropy of the broadening yields information about the jump geometry and jump-frequency ratios, (ii) the absolute value and the temperature dependence of the broadening yield the diffusion coefficient and the activation energy for Fe diffusion, and (iii) the anomalous decrease of the Mössbauer intensity in the vicinity of the melting point yields information about the binding energy of the Fe-vacancy complex. Our results are consistent with the following interpretation: Fe in Al moves by exchanging sites with nearest-neighbor vacancies. The binding energy of an Fe-vacancy complex is $E_b\le 0.29$ eV. The diffusion coefficient of Fe in Al is $D_{\mathrm{Fe}}=1.1\mathrm{}\times {10}^{4\pm 1}\exp [-\frac{(2.3\mathrm{}\pm 0.2 \mathrm{eV})}{k_{B}T}]$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{-1\mathrm{}}$ in reasonable agreement with results from tracer experiments. Values are obtained for different jump frequencies of a vacancy in the vicinity of an Fe atom.