Structural stability of FeO2 in the pressure range of lower mantle

Recently the finding of a new iron oxide pyrite-type FeO2 is reported to be involved in the oxygen cycles occurred in Earth's mantle. Understanding of the stability of FeO2 at high pressures will be critical to model the cyclical process of oxygen in the lower mantle. Here we report the possible pressure-driven phase transition of FeO2 in the pressure range of lower mantle by combining particle swarm optimization method and different density functional theory (DFT) techniques. We find a possible pressure-driven phase transition from PNNM symmetry phase to PA-3 symmetry (Pyrite-type) phase in the pressure range of lower mantle. Nevertheless, the stability of the PNNM symmetry phase is highly affected by temperature change. The possible pressure-driven phase transition is disappeared when the temperature exceeded 800 K. It is also found that the PNNM symmetry phase is mechanically unstable when temperature is beyond 800 K. Finally, the reactions Gibbs free energies of two possible decomposition reactions of FeO2 are calculated from different DFT techniques. It is found that the pyrite-type FeO2 decomposes to Fe2O3 and O2 is the more likely scenario in the relative low-pressure and high-temperature region. However, more detailed calculations, where the phase transitions and the different spin state of iron oxides are also taken into account, are needed to clarify this decomposition reaction.