Elastic Constants of Ammonium Dihydrogen Phosphate (ADP) and the Laval Theory of Crystal Elasticity

The acoustic shear wave velocities $v_{\mathrm{yz}}$ and $v_{\mathrm{zy}}$ have been measured in ADP by the direct method of piezo-electric resonances, and by the direct pulse-echo method. These velocities are found with each method to be equal within the experimental uncertainty. The resonance results have a probable error of 0.1% for the difference between the corresponding Laval elastic constants $c_{44}$ and $c_{77}$. This experimental result disagrees with those previously obtained with ADP by indirect methods.