Elusive Co₂O₃: A Combined Experimental and Theoretical Study

Despite several oxides with trivalent cobalt ions are known, the sesquioxide M2O3 with Co3+ ions remains elusive. Our attempts to prepare Co2O3 have failed. However, 50% of Co3+ ions could be substituted for Ln3+ ions in Ln2O3 (Ln = Y and Lu) with a cubic bixbyite structure where the Co3+ ions are in the intermediate-spin state. We have therefore examined the structural stability of Co2O3 and the special features of solid solutions (Ln0.5Co0.5)2O3 (Ln = Y and Lu). The experimental results are interpreted in the context of ab initio-based density functional theory, molecular dynamics (AIMD), and crystal orbital Hamiltonian population (COHP) analysis. Our AIMD study signifies that Co2O3 in a corundum structure is not stable. COHP analysis shows that there is instability in Co2O3 structures, whereas Co and O have a predominantly bonding character in the bixbyite structure of the solid solution (Y0.5Co0.5)2O3.