Stability field of the hcp‐structure for Fe, Fe‐Ni, and Fe‐Ni‐Si alloys up to 3 Mbar

The phase relations of Fe, Fe90Ni10 and Fe87.9Ni4.4Si7.7 were investigated up to 273(6) GPa and 4490(560) K, 250(19) GPa and 2730(110) K and 304(3) GPa and 2780(210) K, respectively, and the hexagonal close‐packed structure was found to be stable in all of these compounds under the pressure and temperature conditions studied. We did not observe other phases such as a body‐centered cubic structure. The axial ratio (c/a) of Fe87.9Ni4.4Si7.7 at 300 GPa showed an almost constant value of 1.593–1.596 against temperature. The weak temperature dependency of the axial ratio of hcp‐Fe‐Ni‐Si alloy indicates that the c‐axis is still harder than the a‐axis at high temperature condition. Therefore the alignment of hcp‐Fe–Ni–Si alloy with the c‐axis parallel to the Earth's rotation axis could account for the observation of the seismic wave anisotropy of the inner core.