Mechanism of formation of the equilibrium domain structure in crystals undergoing thermoelastic phase transitions

The mechanism of formation of the equilibrium domain structure during a thermoelastic phase transition is proposed. This mechanism is related to long-range elastic strain fields created by “elastic charges” at the free crystal surface. It is assumed that, during a phase transition, there appears not only the nonzero primary (antiferromagnetic, martensitic) order parameter in the crystal but also an internal (quasi-plastic) stress rigidly related to the order parameter. The orientation of this stress with respect to the crystallographic axes can be changed by external fields. Elastic charges arise due to those components of the internal stress tensor whose flux across the crystal surface is nonzero. The nonlocal destressing energy is found. It is shown that, for a certain shape of a sample, an inhomogeneous distribution of the primary order parameter (a domain structure) is energetically more favorable. The characteristic field at which a sample becomes a single domain is shown to be dependent on the shape of the crystal.