Density functional modeling for a perovskite SrTi$_{1-x}$Co$_{x}$O$_3$ system: Beyond GGA+U functional

Understanding the exact mechanisms of spin-stabilization according to the oxygen stoichiometry and substituting metal-transition ions is essential to finding new perovskites-based technologies to augment silicon-based devices with room-temperature spintronic materials. We perform ab-initio modeling for the SrTi$_{1-x}$Co$_{x}$O$_3$ system with x = 0.0, 0.25, 1.0, and by using a HSE06 hybrid functional. Electronic structure for x=(0.0,1.0) predicts 3.1 eV band gap and a metallic behavior, respectively, predicting lattice parameters in agreement with experiments. Different positions for the cobalt ions are taken into account for x=0.25, and predicted ground states and Co energy-correlations suggest the structurally stabilized Co-site positions as fingerprints of whether possible intrinsic magnetic ordering or other phenomena is giving rise to the macroscopic magnetism. Passing from Stoichiometric SrTi$_{0.75}$Co$_{0.25}$O$_3$ to 1-O-vacancy SrTi$_{0.75}$Co$_{0.25}$O$_{3-\delta}$ produces a lattice parameter expansion due to the valence state distribution of the substituting B (ABO$_3$) ions, which is turn lead us to a band gap expansion. The stabilized valence spin states of the magnetic ions are discussed in comparison with the ligand-field theory.