Absolute values of the recoil-free fraction $f$ of the 14.4-keV $\gamma$ rays of ${\mathrm{Fe}}^{57}$ are reported at temperatures from 4 to 1050°K. ${\mathrm{Co}}^{57}$, the parent of ${\mathrm{Fe}}^{57}$, is a dilute impurity in Au, Cu, Ir, Pd, Pt, Rh, and Ti. For intermediate temperatures the measured $f$ is consistent with the calculations of Visscher, and Maradudin and Flinn when the impurity-host coupling constant is taken to be about the same as the host-host coupling constant. At the highest temperatures, the $f\text{'}\mathrm{s}$ of Au, Cu, and Pd fall off faster than simple theory would predict due to diffusion and anharmonicities. The results are compared with the anharmonic lattice theory of Maradudin and Flinn. Chemical and temperature-shift data are also reported.

• Received 18 December 1963

DOI:https://doi.org/10.1103/PhysRev.134.A716