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The oxygen fugacity in the interior of the Earth is largely controlled by iron-bearing minerals. Recent studies have reported various iron oxides with chemical compositions between FeO and Fe 3 O 4  above ~10 GPa. However, the stabilities of these high-pressure iron oxides remain mostly uninvestigated. In this study, we performed in situ X-ray diffraction (XRD) measurements in a laser-heated diamond-anvil cell (DAC) to determine the phase relations in both Fe 5 O 6  and Fe 4 O 5  bulk compositions to 61 GPa and to 2720 K. The results show that Fe 5 O 6  is a high-temperature phase stable above 1600 K and ~10 GPa, while FeO + Fe 4 O 5  are formed at relatively low temperatures. We observed the decomposition of Fe 5 O 6  into 2FeO + Fe 3 O 4  above 38 GPa and the decomposition of Fe 4 O 5  into FeO + h-Fe 3 O 4  at a similar pressure range. The coexistence of FeO and Fe 3 O 4  indicates that none of the recently discovered compounds between FeO and Fe 3 O 4  (i.e., Fe 5 O 6 , Fe 9 O 11 , Fe 4 O 5 , and Fe 7 O 9 ) are formed beyond ~40 GPa at 1800 K, corresponding to conditions in the shallow lower mantle. Additionally, as some superdeep diamonds have genetic links with these high-pressure iron oxides, our results give constraints on pressure and temperature conditions of their formation.

The stability of Fe 5 O 6 and Fe 4 O 5 at high pressure and temperature