Fist-principles total-energy calculations with WIEN97 on ferromagnetic iron in body-cantered
tetragonal structure under hydrostatic pressure have shown that the body-centered cubic
(bee) phase exists up to 1500 kbar of pressure. At that pressure a shear constant vanishes and the
phase becomes unstable. A body-centered tetragonal (bet) phase is shown to come into existence
at 1300 kbar and becomes stable at 1825 kbar and above. This phase may have been observed in
the phase diagram of iron above 2000 kbar at high temperature. The pressure dependences of the
elastic constants of bee Fe and of the bet phase are calculated by using the epitaxial Bain path
(EBP) of tetragonal Fe generalized to finite pressure and used to define a free energy. The
minima at the free energy along the EBP locate the phases; second derivatives of the free energy
at the minima give the elastic constants of both the bee and bet phases as functions of pressure.
Minima of tetragonal energies calculated at constant volume are shown to be. unreliable for
determining stability of a phase.
By: Hong Ma, S. L. Qiu, P. M. Marcus
Published in: RC22235 in 2001
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