Does the fcc phase exist in the Fe bcc–hcp transition? A conclusion from first-principles studies

Despite considerable experimental and theoretical efforts, the underlying atomistic process of the body-centered cubic (bcc) to hexagonal close-packed (hcp) phase transition in Fe is still not clear. In particular, whether an intermediate fcc phase exists in the transition has long been controversial. In this work, we carry out a study on this problem from the viewpoint of energy based on accurate first-principles calculations. The results indicate that the occurrence of the metastable fcc state in the transition is energetically unfavorable, which can explain why no fcc phase was observed in recent in situ x-ray diffraction experiments. A transition mechanism that mainly consists of simultaneous shear and shuffle of the (1 1 0)_bcc planes together with reduction of the (1 1 0)_bcc interplane distance is proposed. It can explain both the anomalous structures and the temperature dependence of the c/a ratio of the hcp phase observed at the beginning of the transition in diamond anvil cell (DAC) experiments. Therefore, a metastable fcc state, which was proposed by Wang and Ingalls, is not needed to account for those observations in the DAC experiments. Moreover, we find that the fcc phase appearing in molecular dymanics simulations may be just an artifact of the semiempirical potential being employed, by which the differences between fcc and hcp are not well described. (paper)