Stable phases of iron at terapascal pressures

Abstract

We have used density-functional-theory methods to study phases of iron at terapascal (TPa)pressures. Hexagonal-close-packed (hcp) iron is stable from below 0.1 TPa to multi-TPapressures, where we find a window of 7–21 TPa in which the face-centred-cubic (fcc) phase isslightly more stable. At 34 TPa we find a transition to a body-centred-tetragonal (bct)phase, which is a small distortion of the body-centred-cubic (bcc) structure. The bcc phasealso becomes more stable than the hcp above 35 TPa. The bct and bcc phases becomeconsiderably more stable than hcp and fcc at even higher pressures, and we expect thisresult to hold at high temperatures.