Iron’s structure at Earth’s core

A team from Japan has measured the crystal structure of iron under conditions that prevail in Earth’s solid inner core—that is, at temperatures and pressures higher than 5000 K and 300 GPa. To reach those extreme values, Shigehiko Tateno and Kei Hirose of the Tokyo Institute of Technology and their collaborators placed Fe powder inside the 20-µm-wide cell of a diamond anvil. Tightening the anvil’s screw squeezed the sample to pressures up to 377 GPa, while two 100-W ytterbium fiber lasers raised the sample’s temperature as high as 5700 K. Placing the cell in a beamline at the SPring-8 synchrotron in Sayo, Japan, yielded the structural information and enabled the team to fill in the uncharted top corner of Fe’s phase diagram. Under ambient conditions, Fe adopts a body-centered cubic (bcc) structure (the red region in the bottom left corner of the phase diagram). As the temperature increases, the pressure needed to forestall melting increases too. Previous measurements (solid diamonds) had shown that Fe switches from a bcc to a hexagonal close-packed (hcp) structure (blue region) at modest temperatures and pressures. That the hcp structure survives at inner core conditions has now been established by the SPring-8 measurements (open symbols). If Fe in Earth’s inner core really is hcp, then the lengthening of the crystal’s c-axis parallel to Earth’s rotation would naturally account for a certain anomaly in seismic signals that pass through the core. (S. Tateno, K. Hirose, Y. Ohishi, Y. Tatsumi, Science 330 , 350, 2010 .)