Size dependence of the Mossbauer recoilless fraction in beta-Sn nanocrystals

We study the size dependence of the Mossbauer recoilless fraction in beta-Sn nanocrystals by performing conversion electron Mossbauer spectroscopy at different temperatures. The Mossbauer recoilless fraction is intimately related to dynamical processes involving the nuclei during the gamma-ray emission. The beta-Sn nanocrystals are embedded in a SiO2 matrix, and they have a mean diameter ranging from 7 to 17 nm. A lowering of the recoilless fraction with decreasing cluster size is observed. The smallest nanocrystals reveal a 60% reduction of the recoilless fraction compared to the bulk value, while for the largest clusters we observe the same Mossbauer recoilless fraction as in the bulk. This suggests that the dynamical properties of the beta-Sn nanocrystals in SiO2 approach those of the bulk for a critical mean diameter above 16 nm. The experimental results are compared with theoretical values obtained by a continuum model in which the relevant parameters are determined ab initio.