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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. [ABSTRACT FROM AUTHOR]

Third-order elastic coefficients can be determined from the velocity of small-amplitude sound waves in statically stressed media. For this purpose, exact expressions are derived for the sound velocity, for the natural velocity (natural unstressed length divided by the transit time), and for their stress derivatives, evaluated at zero stress, in terms of second- and third-order coefficients. Attention is given to the proper variation of propagation direction with static stress in order to maintain propagation normal to a given crystal face as in ultrasonic experiments, and to the proper identification of isothermal and isentropic coefficients. The most convenient form of the results employs the natural velocity, which is computed directly from experimental data with no need to correct the path length for the effect of stress. The formulas apply to arbitrary crystal symmetry and to arbitrary homogeneous stress systems depending on a single scalar variable. Special formulas for hydrostatic and uniaxial stress are given for several crystal classes and for isotropic materials. [ABSTRACT FROM AUTHOR]