Nick E. B. Cowern, Chihak Ahn, Nick S. Bennett, Sergei Simdyankin, Jonathan P. Goss, Jean-Michel Hartmann, Ardechir Pakfar, Silke Hamm, Jérôme Valentin, Enrico Napolitani, Davide De Salvador, Elena Bruno, Salvatore Mirabella
A vast array of crystalline properties arise from the behaviour of atomic-scale 'point' defects. Theory of simple point defects - single atoms added interstitially or missing from the lattice - is well established but experiments under conditions relevant to industrial processing hint at more complex entities. Here we provide powerful input from experiment to theory: we show the self-interstitial in Ge has a simple low-temperature and complex high-temperature form. The complex form has a Gibbs free energy at the diffusion saddle point G = (6.1 \pm 0.2)eV - 30kT, and dominates at temperatures above 0.65 x the melting point. Its extreme 30k entropy and low energy/entropy ratio suggests a mobile complex with structural elements common to the amorphous phase, a 'morph'. A simple model based on this construct explains previously unaccountable features of diffusion in Ge and Si and is predictive for other materials with low amorphous-crystalline energy gap. Atomistic modelling of morph structures has potential to improve understanding of defects, diffusion, precipitation, irradiation damage, electrical properties, and the dynamics of transitions from the crystalline to the amorphous or liquid phase.
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http://arxiv.org/abs/1210.2902
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