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. Citation: Sakai, T., E. Ohtani, N. Hirao, and Y. Ohishi (2011), Stability field of the hcp-structure for Fe, Fe-Ni, and Fe-Ni-Si alloys up to 3 Mbar, Geophys. Res. Lett., 38, L09302, doi: 10.1029/2011GL047178.