AB-INITIO STUDY OF THE MARTENSITIC BCC-HCP TRANSFORMATION IN IRON

We present an extensive study of the pressure-induced bce to hcp marte nsitic transformation in iron, using a spin-polarized full-potential t otal energy technique. The calculated pressure where the phases have e qual enthalpies, 10.3 GPa, agrees well with the experimental value. To tal energy surfaces as a function of the atomic displacements, which i n the bcc phase correspond to the T-1 N-point phonon mode and a long-w avelength shear, are calculated for six different volumes. We observe that the bcc phase is thermodynamically unstable with respect to the h cp 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 bce st ructure. Furthermore, we observe a sudden drop in the magnetic moment at a certain point in the transition path, which results in a disconti nuous 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 transfo rmation. We also observe low-spin states at certain structures and pre ssures. Finally we employ Stoner theory to explain the behavior of the magnetism along the transition path.