Stability of the Ce2O3 phases:: A DFT+U investigation

We report a first-principles investigation of the energetics and structure properties of CeO1.50 in the hexagonal (La2O3), cubic (bixbyite), and monoclinic structures. Our calculations are based on density functional theory within the local density approximation (LDA), generalized gradient approximation (GGA), LDA+U, and GGA+U functionals. The hexagonal (cubic) structure is 53 meV/CeO1.50 (57 meV/CeO1.50) lower in energy than the cubic (hexagonal) structure using LDA+U (GGA+U), which is consistent (in disagreement) with experimental observations. Thus, these results might indicate a superior description of cerium oxides by the LDA+U functional. We found that V-0(1.50)CeO,hexagonal approximate to V-0(2)CeO,fluorite, while V-0(1.50)CeO,cubic is 4%-9% larger than V-0(2)CeO,fluorite, where V-0 is the equilibrium volume per f.u. Therefore, only the results for CeO1.50 in the cubic structure can explain the volume expansion of CeO2 upon reduction conditions, which supports experimental observations of a cubiclike structure for partially reduced CeO2. The volume expansion is due to the change in the oxidation state of the Ce atoms from CeIV+ in CeO2 to CeIII+ in CeO1.50 without changes in the lattice structure.