Phonons inCu3Au near the order-disorder transition

Selected phonon modes in ${\mathrm{Cu}}_3$Au have been studied at temperatures in the vicinity of the order-disorder transition by neutron-scattering techniques. Marked changes occur predominantly at temperatures well below the region of the precipitous drop in the order parameter. Our results give considerable support to the concept of spinodal ordering near a discontinuous phase transition, and are consistent with a lower spinodal temperature of $T_c$-24 K as inferred from earlier x-ray scattering results.