A DFT study of defects in paramagnetic Cr₂O₃

Cr₂O₃ is not only a promising functional material, but also an essential barrier to protect chromia-forming alloys against high temperature corrosion. The Cr₂O₃ protecting layer grows slowly via defect-mediated diffusion. Several types of point defects could be responsible for the diffusion process depending on the oxidation environment, resulting in different semiconductor characters of chromia. According to the literature, the defect chemistry of Cr₂O₃ in the antiferromagnetic (AFM) state has been well studied using density functional theory (DFT) calculations but not in the paramagnetic (PM) state, which is the fundamental state of Cr₂O₃ above 318 K. PM Cr₂O₃ is simulated in this study using special quasi-random structures (SQS). The formation energies of intrinsic point defects in AFM and PM Cr₂O₃ are calculated to study the defect chemistry and the semiconductor properties in different oxidation environments (temperature and oxygen partial pressure P_O) using a thermodynamic model. It is found that O vacancies and insulating-type Cr₂O₃, in which commensurate electrons and holes are dominant before atomic defects are more favorable at high temperatures and at low P_O, while Cr vacancies and p-type Cr₂O₃ are more favorable at low temperatures and at high P_O, according to the calculations both in AFM and PM Cr₂O₃. However, the limits of dominant zones for defects and for semiconductor characters shift to higher temperatures or lower P_O in PM state calculations.