Theory of the Phase and Twin Boundaries in Solid Helium and Reversibility of the bcc-hcp Phase Transition

Abstract

The phase transition between bcc and hcp in solid 4He has martensitic-like features. The phase transition was investigated theoretically in terms of coherent lattice transformations. A theory with three order parameters is developed to take into account the changes in volume and pressure. The order parameters are the relative displacement, the stretching strain, and the compressive strain of the atomic planes. Also we introduce a thermodynamic parameter which characterizes the deviation from phase equilibrium. The parameter describes the fine structure of a 180° boundary and it’s split into two phase boundaries. The twin boundaries are nuclei of the original crystal phase. On this basis we explain the bcc-hcp phase transition asymmetry and pressure change, found in solid 4He experiments, and propose a new experimental scheme to inverse this asymmetry.