Diffusion Broadening of the Mössbauer Line in Wüstite

The temperature dependence of the diffusion broadening for ${\mathrm{Fe}}^{57}$ in wüstite, $\mathrm{Fe}{\mathrm{O}}_x$, from 800 to 1050 °C is reported. Measurements were done at various compositions $x$ by adjusting the ratio of partial pressures of carbon dioxide to carbon monoxide in which the samples were annealed. Our data indicate that the Mössbauer line broadening is only about half of that predicted by the Singwi-Sjölander theory. Our results support the recent tracer-diffusion measurements of Chen, Peterson, and Robinson, which contradict several earlier studies on the diffusion of iron in wüstite. The temperature dependence of the recoil-free fraction is reported in the region where diffusion broadening is observed.