Free-Energy Density of the Shape-Memory Alloy AuZn

The shape-memory alloy AuZn transforms martensitically (TM=65 K) from a cubic B2 (Pm 3m) to a rhombohedral R-phase (P3) through softening of the TA2[110] phonon branch. We report elastic constants, specific heat, and thermal expansivity measurements through the transition. A large elastic anisotropy, A=9 at TM, associated with softening of the TA2[110] phonon branch accompanied with a volume change (delta) V/V of 0.25% characterize the transition. We find that specific heat and thermal expansion are well represented by adding low-energy Einstein modes to the harmonic lattice. On the basis of these measurements, combined with established group-theoretical symmetry criteria, the free-energy density is presented with atomic shuffle displacements as the primary order parameter, Q. We use this free-energy model to explain the atomic displacements in the R-phase.