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The interaction of phase fronts and twin boundaries are observed in NaNbO3 and PbHfO3 single crystals. It is manifest by the stopping of a phase front by a twin boundary, as well as by their joint movement. A phase front stopped by an immovable defect of an arbitrary nature is described and expressions for the force of interaction between the defect and phase front and for the velocity of their joint movement are obtained. It is shown that (independently of the concrete defect and phase front structure) these values are determined by the difference between the free energies of the phases. The exact solution describing a phase front stopped on a narrow coherent twin boundary is obtained. It is shown that this solution exists in the whole region of the crystal phase diagram.

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