Nucleation of crystals at the bcc-hcp transition line in solid 4He

The nucleation phenomenon in the structural transition between the bcc and hcp phase of solid 4 He on the melting line was investigated. At the lower transition point T c 1 (1.457 K), the transition was triggered by a single new phase seed nucleated on the wall in contact with the superfluid and the seed grew up into the superfluid region. The nucleation of the new phase has nothing to do with the original solid. The situation is almost the same for the two cases of bcc to hcp (cooling) and hcp to bcc (warming) transitions, whereas at the higher transition temperature T c 2 (1.778 K), where the crystal coexisted with normal liquid, the new phase seeds were nucleated at many sites inside the original crystal and the crystal was cracked into many grains. This is strongly reminiscent of martensitic transition. The supercooling and superheating phenomena observed in those transitions were analyzed based on the standard homogeneous nucleation theory, though it is clear that those transitions are nucleated heterogeneously. The temperature width of supercooling is larger than that of superheating at T c 1 , whereas it is larger for superheating at T c 2 . That is, supercooling (and superheating) is more likely in the transition from bcc to hep regardless of warming or cooling. The transition event rate, Σ = (N 0 -N)/N 0 , is well reproduced by the formula Σ=(N 0 -N)/N 0 =1-exp [-(δT/c)Γ 0 exp(-ΔE/kT)]. The transition barrier ΔE is estimated by the fitting.