Abstract
The elastic stiffness constants of single crystal potassium and their pressure derivatives have been measured at room temperature by the ultrasonic pulse echo method. The values found are: C = 1.88 C′ = 0.281 B 8 = 3.34 ( units of 10 10 dyn-cm −2) dC dP = 1.62 dC′ dP = 0.251 dB 8 dP = 3.97. The notation C = C 44, C′ = (C 11−C 12) 2 , B 8 = (C 11 + 2C 12) 3 has been used. The results have been interpreted in terms of Fuchs' theory of the electrostatic contribution to the shear stiffnesses of the alkali metals. The experimental observation that the elastic anisotropy, C/C′, does not depend on pressure indicates that, as Daniels found for sodium, it is not necessary to include an ion core interaction or a Fermi contribution to the shear stiffnesses in order to account for the results. The latter conclusion is consistent with a spherical Fermi surface. The bulk modulus and its pressure dependence fit very well the simplest model of the binding energy.
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