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...