NEUTRON-SCATTERING NEAR THE ORDER-DISORDER TRANSITION IN CU3AU - EVIDENCE FOR A LOWER SPINODAL TEMPERATURE

The binary alloy Cu3Au undergoes a first-order phase transition at 667 +/- 3 K Within the context of the Landau theory of phase transitions, there exist, in addition to the order-disorder temperature, T-c, uppe r and lower-spinodal temperatures, T-su and T-sl. These mark the first temperatures, upon approaching the phase transition from above and be low, respectively, at which metastable droplets of the second phase ca n fluctuate out of the first phase. Until recently, there has, however , been little physical evidence supporting the existence of the spinod al temperatures. Elastic and inelastic neutron-scattering measurements have been carried out on Cu3Au over an extended temperature range wit h particular emphasis on temperatures near T-c. The lattice constant d ata, order-parameter data, and phonon data provided by these measureme nts all indicate that there are two temperature regimes just below T-c with a crossover between these regimes in the range of (T-c - 35) to (T-c - 25) K. This crossover temperature is interpreted as the lower s pinodal temperature of Cu3Au.