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-$I_C$ close pairs and stage-$I_D$ isolated interstitials are used to investigate defect configurations. The relative elastic-constant changes below 4 K per unit concentration $\gamma$ of isolated interstitials, $\frac{d\ln C}{d\gamma }$, are found to be 0±1, -30, and -15 for the bulk modulus, $C_{44}$, and $C^{\prime }=\frac{(C_{11}-C_{12})}{2}$, 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 $\omega =5\mathrm{}\times {10}^{12}$ Hz, which are also evidence for < 100 > split interstitials. An elastic relaxation due to thermally activated stress-induced ordering of $I_C$ defects is observed at 18 K in $C_{44}$, but not in $C^{\prime }$, providing symmetry information which leads to a model for the $I_C$ close-pair configuration.