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 con sistent with a solid-solid rather than solid-liquid transition. Thus, anoth er solid phase of iron (besides the hexagonal close-packed epsilon -phase) may be stable at temperatures above 4000 K and pressures above 200 GPa. Whi le the Hugoniot data provide no structural constraint, the new phase is abo ut 0.7% less dense than c-iron. Since the slope of the phase boundary betwe en epsilon -iron and the new phase is not known, the stable phase of iron u nder Earth inner core conditions remains uncertain. Efforts to understand t he origin of inner core seismic anisotropy will require determination of th e new phase structure, its elastic anisotropy, and its regime of stability.