Thermodynamic modeling and first-principles calculations of the Mo-O system

Oxidation resistance is the bottleneck to the development of high-performance Mo-based alloys for ultra-high temperature applications. In the present study, thermodynamic assessment of the Mo-O system was carried out using the CALPHAD method on the basis of experimental data in the literature. The derived set of self-consistent parameters gives good representation of the phase-equilibrium and thermodynamic properties of phases in the Mo-O system. Using the developed thermodynamic description, the chemical potential at T=1000 K was predicted, and a stability diagram was constructed. The lattice stability of oxide compounds in the Mo-O system was further studied using the first-principles density functional theory method. The impact on the phase stability of various exchange-correlation functional and van der Waals correction was also examined. The agreement in lattice parameters and enthalpy of formation between calculations and experimental data is acceptable except for using the local density functional. The present study also suggests that the value of the so-called correction term used to compensate for the error from calculating the energy of O₂ molecule and self-interacting error in oxides is not universal and may vary significantly depending on the individual transition metal-oxygen system. In addition, the present study also reveals that the phase stability among three reported MoO₃ polymorphs at low temperatures requires further experimental and theoretical investigation.