Dynamic polymorphism in solids with multiple local minima on the atomic potential energy surface

Polymorphic solids of the same chemical composition can have different atomic structures. If transformations to other structures have sufficiently high enthalpy barriers, then each structure is either stable or metastable: it is stationary and does not spontaneously change with time. But what happens, if those barriers are low? As examples, we consider NiTi and FeRh alloys exhibiting large elasto-caloric and magneto-caloric effects. We suggest a model for dynamically polymorphic solids, consider an atomic structure stabilized by entropy, and compare theoretical predictions to experimental observations.