First-order versus second-order phase transformation in AuZn

The first-order versus second-order nature of the phase transition in AuZn has been examined by first-principles calculations. The calculated elastic constants of the high-temperature B2 phase have a large anisotropy, which suggests a possible instability in this phase. The first-principles calculations were extended to finite temperature by including vibrational and electronic contributions to the free energy. A small free-energy barrier was found between the high- (B2) and low-temperature (R) phases, which indicates that this is a weak first-order phase transition. Finally, we find that the calculated theoretical transformation temperature and entropy change (small latent heat) are in excellent agreement with the experimental observations for a first-order transition. Based on the entropy calculations for both phases, the high-temperature phase is found to be stabilized by the contribution of low-energy phonon modes to the lattice entropy.