FIRST-PRINCIPLES PREDICTION OF THE HIGH-PRESSURE PHASE-TRANSITION ANDELECTRONIC-STRUCTURE OF FEO - IMPLICATIONS FOR THE CHEMISTRY OF THE LOWER MANTLE AND CORE

Under shock-wave compression, Fe1-xO undergoes a transition to a dense metallic phase at pressures near 70 GPa. The geochemical significance of this transition has been unclear. Here, first-principles electroni c structure calculations (using the FLAPW method and GGA exchange-corr elation) show that the shock-wave discontinuity of FeO results from a RB1 (rhombohedrally distorted NaCl structure) to B8 (NiAs structure) t ransition. The metallic nature of the FeO (B8) phase is argued to resu lt from a breakdown of the Mott insulating condition, rather than an F e(3d)-O(2p) gap closure. As such, the metallization of FeO is probably not a basis for invoking oxygen in the Earth's core. The stability of FeO(Bs) over FeO (RB1) at high pressure is comparable to the ideal -T Delta S of mixing of FeO in (Mg,Fe)O at mantle temperatures. Conseque ntly, it is uncertain if FeO(B8) is present as a separate phase in the Earth's interior.