Influence of Force-Constant Changes and Localized Modes on the V:Fe57Mössbauer System

An experimental and theoretical study is made of the Mössbauer effect of ${\mathrm{Fe}}^{57}$ in a vanadium matrix. We have measured the recoil-free fraction and second-order Doppler shift over the temperature range 100 to 700°K. The experimental results are interpreted using recently derived expressions for the mean-square velocity and displacement of the defect which allow for changes in the force constants as well as the mass at the impurity site. A good fit to the data is obtained for an increase in force constant of approximately 2.5. The data are also analyzed in the high-temperature limit to confirm this increase. Evidence for the presence of localized modes is found from the velocity shift, and the positions of the modes are determined.