First principles calculations of oxygen vacancy formation in barium-strontium-cobalt-ferrite

Abstract

Perovskite-type barium-strontium-cobalt-ferrite (BSCF) is a potentially significant material in the development of electrodes for solid oxide fuel cells and electrolysis cells, primarily because of its large oxygen vacancy concentration and mobility. Using density functional theory (DFT) with the DFT + U approach, we perform first principles calculations of oxygen vacancy formation in bulk BSCF with the composition Ba_0.5Sr_0.5Co_0.75Fe_0.25O_3−δ, in which a range of oxygen deficiencies (0.125 ≤ δ ≤ 0.875) is investigated. Contrary to previous DFT studies for δ = 0.125, DFT + U predicts that the non-stoichiometric structure is more stable than its stoichiometric counterpart with δ = 0, and that this originates from a significantly different atomic and electronic structure following the introduction of the Hubbard U term in the calculations. As more vacancies are created there is no tendency for ordering and the total vacancy formation energy becomes positive and increases as the oxygen deficiency increases. Using ab initio thermodynamics, the heat of formation of BSCF as a function of oxygen partial pressure and temperature is determined and the results are in broad agreement with available stoichiometry measurements.