Pretransition phenomena in first-order order-disorder transitions

Experimental evidence for regions of heterogeneous and homogeneous nucleation and continuous ordering is reviewed; a number of effects have been detected just above T _c in Cu₃Au which appear to be due to heterophase fluctuations: 1) an increase in the Debye Waller factor and the diffuse intensity due to the variance in atomic displacements, 2) the presence of optical phonon modes, 3) phonon mode broadening, and 4) diffuse intensity due to anharmonic interactions. The analysis of these results indicates that there are large quasi static mean square displacements of Au atoms near T_c for Cu₃Au. A model for the heterophase regions consistent with this result is that the regions contain a high density of intersecting anti-phase domain boundaries.