Neutron scattering near the order-disorder transition in Cu3Au: Evidence for a lower spinodal temperature

The binary alloy Cu 3 Au 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 , upper and lower spinodal temperatures, T su and T s 1 . These mark the first temperatures, upon approaching the phase transition from above and below, respectively, at which metastable droplets of the second phase can fluctuate out of the first phase. Until recently, there has, however, been little physical evidence supporting the existence of the spinodal temperatures. Elastic and inelastic neutron-scattering measurements have been carried out on Cu 3 Au over an extended temperature range with particular emphasis on temperatures near T c . The lattice constant data, order-parameter data, and phonon data provided by these measurements 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 spinodal temperature of Cu 3 Au.