Mössbauer absorption in Fe57 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 - phase transition. Values of recoilless fraction fA and Debye temperature for iron were determined as a function of temperature from Mössbauer absorption line areas. The recoilless fraction takes on values from 0.840.04 at room temperature to 0.0120.006 at a point just above the - transition temperature T. At the - and - phase transition points there are discontinuities in fA of fA=+0.0300.008 and -0.060.01, respectively. The values of behave correspondingly, decreasing from (51080)°K at room temperature to (22614)°K at the point just above T, with =(83)°K at T and - (7015)°K at T. The determinations of fA and in the region from the Curie point up to the - transition were the most accurate, with statistical errors of about 0.004 in fA and 2°K in The energy shift in the iron absorber was determined as a function of temperature from the centroid of the Mössbauer absorption spectrum. At the - transition, a discontinuity occurs in the energy shift of - (0.0410.004) 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 - transition, a discontinuity in the energy shift of - (0.100.02) 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 and T, 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. © 1969 The American Physical Society.
