Pressure instability of bcc iron

Abstract

First-principles total-energy calculations with WIEN97 on ferromagnetic iron in body-centered tetragonal structure under hydrostatic pressure have shown that the body-centered cubic (bcc) phase exists up to 1500 kbar of pressure. At that pressure a shear constant vanishes and the phase becomes unstable. A body-centered tetragonal (bct) phase is shown to come into existence at 1300 kbar and becomes stable at 1825 kbar and above. The minima of a free energy evaluated along the epitaxial Bain path generalized to finite pressure give the tetragonal phases of iron under pressure. Second derivatives of the free energy at the minima define elastic constants of both the bcc and bct phases as functions of pressure, which are appropriate to determine stability. These elastic constants are the same functions of pressure as the usual elastic constants computed from stress-strain relations under pressure. Minima of tetragonal energies calculated at constant volume are shown to be unreliable for determining stability of a phase.