Third Order Elastic Constants of Ammonium Chloride

Abstract

Sound velocities along several directions in ammonium chloride crystals have been measured under hydrostatic and uniaxial pressures by means of an ultrasonic method. Second order elastic constants and their pressure derivatives were evaluated in the vicinity of the phase transition point in disordered phase, and third order elastic constants were calculated from the data. It was found that the absolute values of the third order elastic constants C₁₁₁, C₁₁₂ and C₁₂₃ considerably increased with decreasing temperature near the phase transition point.