Abstract
Two anelastic relaxation effects were found and studied ultrasonically in commercial alkali halide crystals. They are caused by (H2O)− and (OH)− impurities substituting for the halogen ions and orienting themselves along the six 〈100〉 directions. This degeneracy is lifted by elastic waves producing unequal strains along the cube axes. The resulting relaxation of the water ions is thermally activated with activation energies of E0 = 1.6 × 10−15 erg in NaCl and E0 = 3.7 × 10−15 erg in KCl. The high-temperature limits of the relaxation times are τ∞ = 2.5 × 10−11 and τ∞ = 2.2 × 10−11 sec, respectively. The relaxation of the hydroxyl ions does not depend on an activation energy. Relaxation times of the order of 10−8 sec are found in KCl crystals, in agreement with results from dielectric measurements. A simple model exhibiting all experimental features is presented.
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