Stable phases of iron at terapascal pressures

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-TPa pressures, where we find a window of 7-21 TPa in which the face-centred-cubic (fcc) phase is slightly 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 phase also becomes more stable than the hcp above 35 TPa. The bct and bcc phases become considerably more stable than hcp and fcc at even higher pressures, and we expect this result to hold at high temperatures. (fast track communication)