Crystal structure of iron at the center of the Earth

The inner core, most remote part of our planet, is composed of solid iron. Because the relevant ultrahigh pressure and temperature conditions were only accessible by dynamical shock-wave compression experiments, the crystal structure of iron at the inner core has long been under debate. Our first static experiments show that the hexagonal close-packed (hcp) structure is a stable form of iron up to 377 GPa and 5700 K, corresponding to inner core conditions. The observed weak temperature-dependence of the c/a axial ratio suggests that hcp-Fe is elastically anisotropic at core temperatures. Preferred orientation of the hcp phase may cause inner core seismic anisotropy.