Hydrogen and Carbon Insertion in the Cr₂O₃ System: A First-Principles Study

This study provides a comprehensive analysis of hydrogen and carbon insertion into the Cr2O3 system using first-principles calculations with the PBEsol + U and meta-GGA SCAN functionals. The general properties of the Cr2O3 system were also re-examined with both functionals, yielding consistent results, with a slight preference for PBEsol + U. To investigate the insertion of interstitial species, several configurations were proposed for corundum-type structures. We explored different charge states (neutral, positive, and negative) and various insertion sites for both hydrogen and carbon. The most stable configurations for both species depend on the Fermi level position within the oxide, and the requirement of electroneutrality leads to distinct behaviors for hydrogen and carbon. For hydrogen, stable interstitial configurations compatible with electroneutrality include the neutral (Hx), positive (H· or H+), and negative (H′ or H–) charge states. The presence of the H2x molecule is found to be minimal compared to the atomic hydrogen states. Similarly, for carbon, a variety of charge states (C··, Cx, and C′) are observed, with the specific configuration determined by the electronic state. We examine how the position of the Fermi level, which can be adjusted by doping, can influence the nature of the hydrogen and carbon charge in the oxide. The insertion of these elements is also analyzed in terms of their equilibrium with the gases (H2, O2, H2O, CO, and CO2).