Third Order Elastic Constants of Ammonium Bromide near λ-Point

Abstract

Second order elastic constants of ammonium bromide were measured near λ-point in disordered phase under hydrostatic and uniaxial pressures by means of an ultrasonic method. Third order elastic constants were calculated from pressure derivatives of the second order elastic constants. Cubic and third order invariants in free energy expressed in terms of irreducible strains were derived near λ-point. It was found that the third order invariant played a dominant role in the vicinity of λ-point in comparison with the cubic invariant.