On the high pressure high temperature orthorhombic phase of iron

In recent experiments, a dhcp and an orthorhombic distortion of the hcp structure have been proposed for iron. We have carried out first principles electronic structure total energy calculations in the high pressure region to evaluate the phase stability of the proposed orthorhombic structure. Our calculations reveal that the total energy of the orthorhombic phase is 35-50 mRy/atom higher than that of hcp. Besides, a monoclinic structure which could also be in the transit path of hcp-fcc transformation has been considered, and is found to be of higher energy than hcp. The entropy energy difference between the orthorhombic and hcp structures is estimated to be ≅ 2 mRy/atom at 2200 K. Thus the experimental observation of the orthorhombic structure is not supported by our calculations. (author)