The equation of state of iron to 450 GPa: Another high pressure solid phase?

At the 75% confidence level, Hugoniot data for iron exhibit a small volume‐change discontinuity at 200 GPa, the same pressure as a previously observed discontinuity in sound velocity. The sharpness of the discontinuity is consistent with a solid‐solid rather than solid‐liquid transition. Thus, another solid phase of iron (besides the hexagonal close‐packed ϵ‐phase) may be stable at temperatures above 4000 K and pressures above 200 GPa. While the Hugoniot data provide no structural constraint, the new phase is about 0.7% less dense than ϵ‐iron. Since the slope of the phase boundary between ϵ‐iron and the new phase is not known, the stable phase of iron under Earth inner core conditions remains uncertain. Efforts to understand the origin of inner core seismic anisotropy will require determination of the new phase structure, its elastic anisotropy, and its regime of stability.