Structural Changes in Oxygen-Deficient Double Perovskite, Sr2CaSbO5.5
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In order to develop a more efficient cathode oxygen vacancy conduction fuel cell, it is important to understand the oxygen vacancy structure and conduction in perovskites and double perovskites. Previous research using a neutron pair distribution analysis looked at double perovskite Sr2CaSbO5.5 and found that the local geometry changes significantly when oxygen vacancies are introduced across multiple structures. Here, we looked at energetically different computationally generated structures by employing methods like density functional theory (DFT) using a PBE functional and a generalized gradient approximation (GGA). These methods were implemented in Vienna ab initio simulations package (VASP) to obtain final configurations and their energies. To obtain the lowest energy final configuration, the conjugate gradient (CG) method was utilized. First, Glazer octahedral tilting was performed on cubic Sr2CaSbO6 to generate 23 possible distorted structures. The minimum energy configurations of these structures were found using CG. Then, to the lowest energy Glazer, oxygen vacancies were introduced into the system with either no distortion, a trigonal bipyramid distortion around Sb and Ca ions, a 45 degree rotation around SbO5 or CaO5, or 45 degree octahedral rotation on SbO6 or CaO6. All resulting structures were optimized on VASP. The global minimum energy configuration was determined to have trigonal bipyramidal geometry around the Sb ion. The edge of the SbO5 polyhedra lined up with the edges of the polyhedra around the M ion. Lastly, molecular dynamic trajectories on the Born-Oppenheimer surface showed no conduction or rotation at 1200K for 2000 fs.