A DFT study of defects in paramagnetic Cr₂O₃.

Area of Science:

• Materials Science
• Solid State Chemistry
• Surface Science

Background:

• Chromium oxide (Cr2O3) is vital for protecting alloys from high-temperature corrosion.
• Cr2O3 protective layer growth relies on defect-mediated diffusion.
• Defect chemistry in antiferromagnetic (AFM) Cr2O3 is well-studied, but not in the paramagnetic (PM) state (above 318 K).

Purpose of the Study:

• To investigate the defect chemistry and semiconductor properties of paramagnetic Cr2O3.
• To compare defect behavior in PM Cr2O3 with the established AFM state.
• To understand how oxidation environment (temperature, oxygen partial pressure) influences defect types and semiconductor character.

Main Methods:

• Simulated PM Cr2O3 using special quasi-random structures (SQS).
• Calculated formation energies of intrinsic point defects in both AFM and PM Cr2O3 using density functional theory (DFT).
• Employed a thermodynamic model to analyze defect chemistry and semiconductor properties under varying temperature and oxygen partial pressure (PO2).

Main Results:

• Oxygen vacancies and insulating Cr2O3 (with dominant electrons/holes) are favored at high temperatures and low PO2.
• Chromium vacancies and p-type Cr2O3 are favored at low temperatures and high PO2.
• The transition boundaries for dominant defects and semiconductor types shift to higher temperatures or lower PO2 in the PM state compared to the AFM state.

Conclusions:

• The defect chemistry and semiconductor properties of Cr2O3 are significantly influenced by its magnetic state (AFM vs. PM).
• Paramagnetic Cr2O3 exhibits distinct defect behavior, particularly under varying temperature and oxygen partial pressure conditions.
• Understanding these PM state characteristics is crucial for accurately modeling Cr2O3 protective layer growth and high-temperature corrosion resistance.