Cubic Invariants in the Landau Theory Applied to Elastic Phase Transitions

Abstract
The third-order invariants in the Landau free-energy expansion for an elastic phase transition reached by group theory are determined from third-order elastic constants measured from the effects of hydrostatic and uniaxial pressure on ultrasonic wave velocities in In-Tl alloys near the fcc-to-fct transition. Results extend application of the Landau theory to a first-order transition in which the atomic displacements are small.