Mossbauer spectrum for diffusing atoms including fluctuating hyperfine interactions

Abstract

We present model calculations of Mossbauer emission spectra for mobile substitutional Mossbauer atoms in cubic lattices. The fluctuating hyperfine interactions (isomer shift and quadrupole interaction) due to approaching vacancies are considered. To describe the motion of the Mossbauer atom we use the five-frequency model appropriate for impurity diffusion including binding of Mossbauer atoms and vacancies. For suitable choice of the jump frequencies our model shows the combined effect of quadrupole splitting, motional narrowing, and diffusional broadening of the emitted line. These features have previously been discussed only separately. We present analytical results for diffusion on sc lattices and numerical results for fcc lattices. In particular we find the following. The diffusional broadening of the spectrum is governed by the effective jump frequency of the substitutional Mossbauer atoms which is essentially given by the product of the vacancy concentration and the vacancy-Mossbauer atom exchange jump rate whereas the fluctuation rates of the hyperfine interaction is directly given by the vacancy jump rates (without the factor vacancy concentration). Thus the hyperfine interactions are completely averaged out at temperatures where in normal metals diffusion broadening can be detected. The dynamics of the hyperfine relaxation can only be detected at low temperatures in alloys in which the vacancy concentration is enhanced. Promising systems are nonstoichiometric ordered alloys such as Co-Ga, Pd-Si, Fe-Al.