Structural transition in liquid FeO

FeO is an important end-member in planetary physics. Its properties under high pressure are crucial for the Earth's mantle, as (Mg,Fe)O ferropericlase is a major mineral in the lower mantle, but also for the Earth's core, as O is potentially the major light element in the liquid outer core. Furthermore FeO is one of the archetypal minerals used to model the mantles of telluric planets of the solar system as well as of telluric exoplanets. However, up to now, no data were available on the liquid structure of pure FeO under high pressure.

Angle dispersive x-ray diffraction measurements have been carried out on liquid FeO in laser-heated diamond anvil cell, collecting data ~300 K above the melting curve from 20 to 100 GPa. The diffuse scattering signal was analyzed by Fourier transform and the local structure of the liquid was extracted, allowing to follow the evolution with pressure of the Fe-O and the Fe-Fe bonds. While the Fe-O bond distance is observed to remain relatively constant over this pressure range, the Fe-Fe bond distance reduces from 3.2 to 2.5 Angstroms from 0 to 40 GPa, and then stays relatively stable up to the maximum investigated pressure. This structural evolution in the liquid is tentatively related to the reported insulator to metal transition in solid FeO. Such an important structural change in the liquid state is likely to significantly affect major physical properties, such as compressibility, with potential implications for the magma ocean dynamics.