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Energetics of bcc-fcc lattice deformation in iron.

The lattice deformation of the BCC-FCC martensitic transformation in iron can be described as a continuous change of the c/a parameter of the body-centered tetragonal (BCT) lattice from c/a=1 (BCC) to c/a=√2 (FCC). Along this deformation path, the total energy (as a function of volume), the enthalpy (as a function of pressure), the pressure-volume relations--both for nonmagnetic (NM) and ferromagnetic (FM) states--were calculated using the ab initio LMTO method. The ground-state magnetic properties: ferromagnetic contributions to the total energy and magnetic moments, were found by making use of the Stoner theory of itinerant ferromagnetism, rather than spin-polarized calculations. This circumvents the difficulties of using the traditional local spin-density approximation which failes to describe correctly the energetics of iron phases. The Stoner exchange parameter, I, was calculated from the linear response theory for each c/a as a function of volume. Then a constant enhancement factor, β, was introduced and the new Stoner parameter, βl, as used in all the calculations. The factor β was found by fitting the equilibrium atomic volume of the FM BCC phase to its experimental value. No other adjustments of any quantities were performed. The calculations revealed a somewhat unusual behavior of enthalpy along the deformation path. Instead of a double-well curve with a barrier maximum somewhere at 1