Hydration, oxidation, and reduction of GdBaCo₂O_5.5 from first-principles

Structural, magnetic and chemical properties of GdBaCo₂O_5.5 regarding hydration, oxidation and reduction are studied by density-functional calculations in the DFT + U formalism. Besides the orthorhombic Pmmm structure, a lower energy, tilted, distorted structure is found. The hydrated configurations obtained by water dissociation in the oxygen vacancies of the GdO_0.5 plane exhibit strong distortions with respect to the Pmmm structure. Simulation of protonic defects provides the energy landscape of incorporated protons, which preferentially bind to oxygens of the CoO₂ planes, suggesting their possible bidimensional diffusion in this plane. We also studied oxygen incorporation (oxidation) in the oxygen vacancies of the GdO_0.5 planes, and oxygen removal (reduction) from BaO, CoO₂ and GdO_0.5 planes. The oxidized compound, GdBaCo₂O_5.75, is rather p-type metallic, while the reduced compound, GdBaCo₂O_5.25, remains an insulator, due to electronic localization (Co³⁺ + e^- → Co²⁺). Taking Pmmm as the reference, both hydration at high water concentration (one H₂O per 38-atom supercell) and oxidation are found exothermic. However, if the distorted structure is chosen as the reference, these reactions become endothermic, at least at the high water/oxygen concentration studied. Reduction is, in both cases, endothermic. Nevertheless, negative formation energies of the protonic defects suggest the possibility of hydration at lower water concentration.