Mössbauer Absorption in Fe 57 in Metallic Iron from the Curie Point to the γ − δ Transition

M\"ossbauer absorption in ${\mathrm{Fe}}^{57}$ in metallic iron has been observed for absorber temperatures from 22 to 1413\ifmmode^\circ\else\textdegree\fi{}C, with particular emphasis on the region from the Curie point through the $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\delta}$ phase transition. Values of recoilless fraction ${f}_{A}$ and Debye temperature $\ensuremath{\Theta}$ for iron were determined as a function of temperature from M\"ossbauer absorption line areas. The recoilless fraction takes on values from 0.84\ifmmode\pm\else\textpm\fi{}0.04 at room temperature to 0.012\ifmmode\pm\else\textpm\fi{}0.006 at a point just above the $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\delta}$ transition temperature ${T}_{\ensuremath{\gamma}\ensuremath{\delta}}$. At the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\gamma}$ and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\delta}$ phase transition points there are discontinuities in ${f}_{A}$ of $\ensuremath{\delta}{f}_{A}=+0.030\ifmmode\pm\else\textpm\fi{}0.008 \mathrm{and} \ensuremath{-}0.06\ifmmode\pm\else\textpm\fi{}0.01$, respectively. The values of $\ensuremath{\Theta}$ behave correspondingly, decreasing from (510\ifmmode\pm\else\textpm\fi{}80)\ifmmode^\circ\else\textdegree\fi{}K at room temperature to (226\ifmmode\pm\else\textpm\fi{}14)\ifmmode^\circ\else\textdegree\fi{}K at the point just above ${T}_{\ensuremath{\gamma}\ensuremath{\delta}}$, with $\ensuremath{\delta}\ensuremath{\Theta}=(8\ifmmode\pm\else\textpm\fi{}3)\ifmmode^\circ\else\textdegree\fi{}$K at ${T}_{\ensuremath{\alpha}\ensuremath{\gamma}}$ and - (70\ifmmode\pm\else\textpm\fi{}15)\ifmmode^\circ\else\textdegree\fi{}K at ${T}_{\ensuremath{\gamma}\ensuremath{\delta}}$. The determinations of ${f}_{A}$ and $\ensuremath{\Theta}$ in the region from the Curie point up to the $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\delta}$ transition were the most accurate, with statistical errors of about \ifmmode\pm\else\textpm\fi{}0.004 in ${f}_{A}$ and \ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}K in $\ensuremath{\Theta}$. The energy shift in the iron absorber was determined as a function of temperature from the centroid of the M\"ossbauer absorption spectrum. At the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\gamma}$ transition, a discontinuity occurs in the energy shift of - (0.041\ifmmode\pm\else\textpm\fi{}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 $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\delta}$ transition, a discontinuity in the energy shift of - (0.10\ifmmode\pm\else\textpm\fi{}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 ${T}_{\ensuremath{\alpha}\ensuremath{\gamma}}$ and ${T}_{\ensuremath{\gamma}\ensuremath{\delta}}$, 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.