Unusual Dynamical Properties of Self-Interstitials Trapped at Co Impurities in Al

Mössbauer spectroscopy was used to study the dynamics of Al self-interstitials trapped at ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ impurities. When a single interstitial atom is trapped at the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ atom, the Debye-Waller factor decreases rapidly by more than a factor 4 in the temperature interval between 13 and 20 K. We try to explain this anomaly by the following model: The ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ atom jumps between six different positions, thereby forming mixed Co-Al dumbbells with changing Al partners.