Ab initio study of the martensitic bcc-hcp transformation in iron

We present an extensive study of the pressure-induced bcc to hcp martensitic transformation in iron, using a spin-polarized full-potential total energy technique. The calculated pressure where the phases have equal enthalpies, 10.3 GPa, agrees well with the experimental value. Total energy surfaces as a function of the atomic displacements, which in the bcc phase correspond to the T₁ N-point phonon mode and a long-wavelength shear, are calculated for six different volumes. We observe that the bcc phase is thermodynamically unstable with respect to the hcp phase, long before it becomes dynamically unstable. The transition pressure at room temperature is estimated to approximately 50 GPa. We find that magnetism is the primary stabilizing mechanism of the bcc structure. Furthermore, we observe a sudden drop in the magnetic moment at a certain point in the transition path, which results in a discontinuous derivative in the energy surface. This is a clear signature of a first order ferromagnetic to nonmagnetic transition, responsible for the main part of the latent heat developed in this martensitic transformation. We also observe low-spin states at certain structures and pressures. Finally we employ Stoner theory to explain the behavior of the magnetism along the transition path.