Twin boundaries as nuclei of a new phase in body-centered cubic-hexagonal close-packed phase transitions in solid helium

The reversibility of the coherent phase transition from a body-centered cubic (bcc) phase to a hexagonal close-packed (hep) phase has been considered. Th e topological classification and qualitative description of arising grains (domains) and 60A degrees, 90A degrees, 120A degrees, and 180A degrees twin boundaries have been performed. The thermodynamic parameter of deviation from the phase equilibrium has been introduced. This parameter has made it possible to analytically describe the fine structure of the 180A degrees boundary and its splitting into two interphase boundaries during the reverse phase transition. It has been demonstrated that, in the vicinity of the phase transition, this description is applicable to the 60A degrees, 90A degrees, and 120A degrees boundaries. The twin boundaries have been shown to be nuclei of the initial (parent) phase, for which the structure can be completely restored. On this basis, an explanation has been offered for the origin of the asymmetry of the bcc-hcp phase transition observed in experiments carried out in solid (4)He. A scheme has been proposed for a new experiment performed with the aim of reversing the asymmetry of this phase transition.