Exploring phase transformations with neutron scattering

Phase transformations have for many years intrigued physicists as dramatic and yet subtle phenomena for study. In the early 1960's came the belated realization that there exists a particularly simple mechanism responsible for a certain class of structural transformations in solids. This mechanism involves an instability in a mode of vibration of the solid; that is, a “soft phonon” (one of low energy) is said to “condense” into the lattice to cause the phase transformation. The process is susceptible to experimental study even in complex structures and has given us a handle on the dynamics of this cooperative phenonenon. Here we will discuss such phase transformations, which have come to be called “displacive transformations,” and what can be learned about them by inelastic neutron-scattering experiments.