SELF-DIFFUSION IN SOLID HE-4 AND HE-3-HE-4 MIXTURES NEAR THE BCC-HCP PHASE-TRANSITION

We report experiments on the plastic flow of solid He-4 and He-3-He-4 mixtures of 1.4% and 2.8% near the bcc-hcp transition. Plastic flow wa s generated by moving a wire through a macroscopic single crystal. We found that the plastic flow rate both in pure He-4 and in mixture heli um crystals is enhanced in vicinity of the bcc-hcp phase transition. T he results are interpreted in terms of self diffusion in the solid. Va lues of the self diffusion coefficient D-s at the respective transitio n temperatures of pure He-4 and of the mixtures are very close, and re ach that found in normal liquids, The activation energy for self diffu sion in the mixtures is lower by up To 3 K than in pure He-4, We sugge st that similar to what is observed in bce metals, selfdiffusion in so lid He rakes place through phonon assisted atom-vacancy exchange. The enhancement of the diffusion near the bcc-hcp transition is a result o f the softening of a short wavelength transverse phonon, The temperatu re dependence of the energy of rite phonon calculated using our data i s in accord with the Landau theory of a phase transition driven by a s oft mode. Work hardening was observed in mixture crystals, Bur not in pure He-4, This implies that He-3 impurities pin dislocation lines.