Boundary between bcc and hcp lattices in a 4He quantum crystal

The mechanism of transition between bcc and hcp phases, which is applicable to both classical and quantum crystals, has been considered at the microscopic level. The relative orientations of the lattices that provide coherent phase matching are specified. It is suggested that phase boundaries are coherent and atomic displacements do not exceed the unit-cell size. A new order parameter—relative displacement of atomic layers—is introduced. The phase boundary is described within the double-and triple-well potential models. The relationship of the lattice constant and barrier height with the parameters of these potentials has been found, as well as the shape and energy of the phase boundary. A mechanism leading to a decrease in the barrier height for the phase boundary in a quantum crystal is proposed. This mechanism makes it possible to explain the results of recent experimental investigations of the nucleation occurring during the bcc-hcp transition in solid ⁴He.