Mössbauer Absorption inFe57in Metallic Iron from the Curie Point to theγ−δTransition

Mössbauer absorption in ${\mathrm{Fe}}^{57}$ in metallic iron has been observed for absorber temperatures from 22 to 1413°C, with particular emphasis on the region from the Curie point through the $\gamma -\delta$ phase transition. Values of recoilless fraction $f_A$ and Debye temperature $\Theta$ for iron were determined as a function of temperature from Mössbauer absorption line areas. The recoilless fraction takes on values from 0.84±0.04 at room temperature to 0.012±0.006 at a point just above the $\gamma -\delta$ transition temperature $T_{\gamma \delta }$. At the $\alpha -\gamma$ and $\gamma -\delta$ phase transition points there are discontinuities in $f_A$ of $\delta f_A=+0.030\mathrm{}\pm 0.008 \mathrm{and} -0.06\mathrm{}\pm 0.01$, respectively. The values of $\Theta$ behave correspondingly, decreasing from (510±80)°K at room temperature to (226±14)°K at the point just above $T_{\gamma \delta }$, with $\delta \Theta =(8\mathrm{}\pm 3)°$K at $T_{\alpha \gamma }$ and - (70±15)°K at $T_{\gamma \delta }$. The determinations of $f_A$ and $\Theta$ in the region from the Curie point up to the $\gamma -\delta$ transition were the most accurate, with statistical errors of about ±0.004 in $f_A$ and ±2°K in $\Theta$. The energy shift in the iron absorber was determined as a function of temperature from the centroid of the Mössbauer absorption spectrum. At the $\alpha -\gamma$ transition, a discontinuity occurs in the energy shift of - (0.041±0.004) mm/sec, three times too large to be explained solely by an isomer shift discontinuity due to an assumed scaling of the $4s$ atomic electron density with the volume. At the $\gamma -\delta$ transition, a discontinuity in the energy shift of - (0.10±0.02) mm/sec is observed, too large and in the wrong direction to be accounted for by scaling the $4s$ density with volume. The discrepancies in the shift discontinuities are of the same sign at $T_{\alpha \gamma }$ and $T_{\gamma \delta }$, and indicate the existence of a mechanism for increasing the electron density at the nucleus at both phase transitions on going into the higher-temperature phase.