Elastic softening in Fe 7 C 3 with implications for Earth's deep carbon reservoirs

Iron carbide Fe 7 C 3 has recently emerged as a potential host of reduced carbon in Earth's mantle and a candidate component of the inner core, but the equation of state of Fe 7 C 3 is still uncertain, partly because the nature of pressure‐induced magnetic transitions in Fe 7 C 3 and their elastic effects remain controversial. Here we report the compression curve of hexagonal Fe 7 C 3 in neon medium with dense pressure sampling and in comparison with pure iron in the same loading. The results revealed elastic softening between 7 GPa and 20 GPa, which can be attributed to noncollinear alignment of spin moments in a state between the ferromagnetic and paramagnetic phases, as expected for Invar‐type alloys. The volume reduction associated with the softening would enhance the stability of Fe 7 C 3 in the deeper part of the upper mantle and transition zone. As a result of subsequent spin crossover at higher pressures, Fe 7 C 3 at inner core conditions likely occurs as the nonmagnetic phase, which remains a candidate for the major component of the Earth's central sphere.