ABO3-x type perovskite oxides are potential candidates for solid oxide fuel cells. Among them those which crystallize in the orthorhombic brownmillerite-phase (ABO2.5), such as SrCoO2.5, are particularly interesting due to their promising crystal structure which contains ordered channels of oxygen vacancies. Recent experimental work [1,2,3] successfully demonstrated that the two phases, perovskite SrCoO3 and brownmillerite SrCoO2.5, could be reversibly switched without destroying the parent framework.

In this work, we complement the experimental studies with first-principles calculations. The strong correlations in the Co d orbitals are treated within the local spin density approximations of Density Functional theory (DFT) with Hubbard U corrections (LSDA+U), U being an empirical parameter. We also compare our results with the Heyd Scuzeria Ernzerhof (HSE) functional. We first perform a theoretical investigation of the electronic structure and magnetic ground state properties of SrCoO2.5, which is interesting due to variety of valence and spin states of Co.

Consistent with experimental observation, the G type antiferromagnetic structure is found to be the most stable. By mapping the total energies of different magnetic configurations onto a Heisenberg Hamiltonian, we compute the magnetic exchange interaction parameters J, which are then used to compute the spin-wave frequencies and inelastic neutron scattering intensities. Our study of the structural and electronic properties of SrCoO2.5 allows us to select the parameter U used to perform the calculations of oxygen diffusion. The diffusion energy barriers are found to be highly anisotropic which is a direct consequence of the oxygen vacancy channels in SrCoO2.5 for fast oxygen transport. The effect of tensile and compressive strain on the activation energy barriers have also been investigated.

[1] H. Jeen, W. S Choi, M.D. Biegalski, C. M. Folkman, I.C Tung, D.D. Fong, J.W. Freeland, D. Shin, H.Ohta, M.F. Chisholm & H.N. Lee; Nature Materials (2013) doi:10.1038/nmat3736

[2] H. Jeen, W. S. Choi, J. W. Freeland, H. Ohta, C. U. Jung, H. N. Lee; Adv. Mater., 25: 3651-3656. doi: 10.1002/adma.201300531

[3] W.S. Choi, H.Jeen, J.H Lee, S. S. A. Seo, V.R. Cooper, K.M. Rabe, and H.N Lee; Phys. Rev. Lett. 111, 097401 (2013)

Chandrima Mitra

Oak Ridge National Laboratory

Appearances