Stability of the pressure-induced orthorhombic phase of iron

Abstract

Structure of iron in the pressure range of 40--100 GPa at temperatures of 1500--2400 K is controversial. A double hcp (dhcp) and an orthorhombic distortion of the hcp structure have been proposed in recent experiments. We have carried out first-principles electronic structure total-energy calculations in this high-pressure region to evaluate the phase stability of these new structures. Besides, a monoclinic structure with two atoms per unit cell, which could be in the transit path of \ensuremath{\varepsilon}-Fe (hcp)--\ensuremath{\gamma}-Fe (fcc) transformation has also been considered. Our calculations reveal that the total energies of the fcc and the dhcp structures are close to each other (within 1 mRy/atom), and 6--8 mRy above that of hcp. The orthorhombic phase is 40--55 mRy/atom higher in energy. The monoclinic structure is also of higher energy with respect to the hcp phase by about 50 mRy/atom. The entropy energy difference between the orthorhombic and hcp structures is estimated to be \ensuremath{\cong}2 mRy/atom at 2200 K. Thus the experimental observation of the orthorhombic structure by Andrault et al. is not supported by our calculations. However, these calculations are in agreement with the recent double-sided laser-heated (for avoiding thermal gradients) diamond-anvil cell experiments of Shen et al., in which only hcp and fcc phases were observed in situ at high pressures and temperatures.