**Abstract**

Strain-rate cycling tests associated with the ultrasonic oscillation were conducted for the purpose of investigation on the interaction between dislocation and dopant ions during plastic deformation of seven kinds of single crystals: NaCl doped with Li+ , K+ , Rb+ , Cs+ , F<sup>−</sup>, Br<sup>−</sup> or I<sup>−</sup> ions separately. Relative curves between the stress decrement (Δ*τ*) due to ultrasonic oscillatory stress and strain-rate sensitivity (*λ*) of flow stress under superposition of the oscillation are obtained by the original method (combination method of strain-rate cycling tests and application of ultrasonic oscillations) at 77 K to room temperature and have stair-like shapes for the specimens at low temperatures. The Gibbs free energy for overcoming of the dopant ion by dislocation at absolute zero is calculated from the data analyzed in terms of Δ*τ* vs. *λ*. As a result, the obtained energies are found to be varied linearly with the isotropic defect around it in the each specimen.

**Keywords:** dislocation, ultrasonic oscillatory stress, activation energy, monovalent ion, isotropic strain
