Configuration-space approach to the fcc to hcp structural transition

The Shoji-Nishiyama mechanism prescribes a way of continuously distorting an fcc into an hcp structure. We show that the atomic displacements of this deformation mechanism can be considered as the sum of a modulation and a strain. A transition path between these two structures can therefore be characterized by a parametrization of the modulation and strain amplitudes. The free energy as a function of these two amplitudes is shown to be highly constrained and, in particular, is invariant under a two-dimensional space group.