Stability and compressibility of a new iron-nitride -Fe7N3 to core pressures

We have examined the phase relations in the iron-rich portion of the Fe-N binary system up to 150GPa and 2720K in a laser-heated diamond anvil cell. Synchrotron X-ray diffraction measurements revealed the formation of a new phase (-Fe7N3) above 41GPa and similar to 1000K. The phase is structurally identical or very close to Fe7C3 and the most iron-rich Fe-N intermediate compound at its stability field. We also measured the volume of -Fe7N3 to 132GPa and found its elastic property very similar to that of Fe7C3. Recently, it has been proposed that the density and sound velocities, in particular, shear wave velocity, in the Earth's inner core are reconciled with Fe7C3, but such properties could be explained by -Fe7N3 equally well. It is therefore possible that the inner core consists of Fe-7(C,N)(3), which is found as diamond inclusions and likely stable under inner core conditions.