Supercool and Superheat in bcc-hcp Transition of Solid 4He

The nucleation phenomenon in the structural transition between bcc and hcp phase of solid 4 He is investigated by an optical method together with pressure and temperature, At T c1 (1.46 K), the single new phase seed was nucleated at some spot on the wall in contact with the superfluid and grown into the superfluid region. The nucleation is nothing to do with the original solid. The situation is almost the same for both cases of bcc to hcp (cooling) and hcp to bcc (warming). Whereas at T c2 (1.77 K), the new phase seeds were nucleated at many sites inside the original crystal, which is the typical Martensitic transition. Since the transition is of first order, supercool and superheat were observed. Though the transition event occurred at random, accumulated events were pretty well represented by a standard nucleation probability model. The temperature width of supercool is larger than that of superheat at T c1 . Whereas it is larger for superheat at T c2 . In other words, supercool (and superheat) is more likely in the transition from bcc to hcp regardless of warming or cooling. It is qualitatively understood by the fact that the surface tension of hcp crystal against liquid is much larger than that of bcc crystal.