Kinetics of nucleation under bcc-hcp structural transition in solid helium

An experimental study of the kinetics of formation of nuclei hcp phase in supercooled bcc phase of solid 4He and 3He-4He solution containing 1% 3He is realised. The method precision measurement of pressure crystal cooled along isochore near the line of phase equilibrium bcc-hcp is used. It is found that in 4He lifetime metastable bcc phase has a complex dependence on the magnitude of hypothermia, which can be described with the involvement of two mechanisms of nucleation - homogeneous and heterogeneous. It is shown that at large overcooling bcc phase of carefully annealed crystals homogeneous nucleation mechanism realized, and at low overcooling a heterogeneous mechanism realized, which is characterized by the values of activation coefficients several orders of magnitude less homogeneous. The effect of defects and impurities on the nucleation process is investigated. Revealed that in the unannealed (defective) 4He crystal nucleation rate shows huge variation in the values, indicating poor reproducibility from crystal to crystal. It is s found that for the annealed samples 3He-4He data obtained on the nucleation kinetics can be described only by heterogeneous nucleation mechanism.