Effects of self-interstitials and close pairs on the elastic constants of copper

Measurements of changes in the elastic constants of copper and their temperature dependences resulting from stage-IC close pairs and stage-ID isolated interstitials are used to investigate defect configurations. The relative elastic-constant changes below 4 K per unit concentration γ of isolated interstitials, dlnCdγ, are found to be 0±{}1, -30, and -15 for the bulk modulus, C₄₄, and C^'=(C₁₁-C₁₂)2, respectively. These results show that the bulk effect arising from the change in number and strength of interatomic bonds is small, but that there exist polarization effects expected from 100 > split interstitials. The temperature dependence of the elastic constants provides evidence of thermally excited resonance modes of frequency ω=5×10¹² Hz, which are also evidence for 100 > split interstitials. An elastic relaxation due to thermally activated stress-induced ordering of IC defects is observed at 18 K in C₄₄, but not in C^', providing symmetry information which leads to a model for the IC close-pair configuration.