Reconstructive transitions between ordered phases: The martensitic fcc-hcp and the graphite-diamond transitions

A phenomenological model is proposed for the martensitic fcc-hcp and graphite-diamond transitions. The observed phases are interpreted as ordered phases, induced by independent mechanisms from a disordered latent parent structure of symmetry $D_{6h}^1$. The symmetries of the corresponding order parameters are worked out. The fcc-hcp model is shown to provide a good interpretation of the phonon spectrum of cobalt, and of the pressure-temperature phase diagram of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$.