First-principles thermoelasticity and stability of pyrite-type FeO2 under high pressure and temperature

The pyrite-type FeO2 was proposed recently as a new structure that exists in the deep Earth. Taking into account the influence of temperature, we investigated the stability and elastic properties of pyrite-type FeO2 under high pressure. The obtained lattice constants and bulk modulus agree well with the available experimental and theoretical data. We found the stability of pyrite-type FeO2 is greatly affected by temperature and pressure. For example, pyrite-type FeO2 is predicted to be both mechanically and thermodynamically unstable when temperature is beyond 1700 K at 300 GPa. The stable boundary of pyrite-type FeO2 is obtained at high pressure and temperature. Furthermore, the pressure dependences of elastic properties of pyrite-type FeO2 are investigated for the first time. It is found that the values of C44 and shear modulus G are nearly invariable with the increasing pressure and temperature.