First-principles calculations on charge states and solubility of impurity defects in BaTiO3

Various impurities have been doped into ferroelectric BaTiO3-based materials, which are used in multi-layer ceramic capacitors, to improve their insulation resistance, reliability, and dielectric properties. However, the relationship between doped impurities and these device properties at the microscopic level remains unexplored. This study investigated the charge states and solubilities of doped impurities (V, Cr, Mn, Fe, Mg, Ca, and Sr) in BaTiO3 using density functional theory. We employed the screened hybrid functional and a rhombohedral lattice model. Owing to a better description of unoccupied states with the hybrid functional, equilibrium charge states and magnetic moments of V, Mn, and Fe impurities were in good agreement with those measured by magnetic susceptibility measurements. Furthermore, impurity levels of transition metal elements were qualitatively consistent with the experimental reports obtained by electron paramagnetic resonance. However, levels of Fe calculated in cubic BaTiO3 did not reproduce those observations. Therefore, a dynamically stable rhombohedral lattice was found to be more suitable than a cubic lattice for calculating the defects in BaTiO3. The solubility of transition metal dopants was highly dependent on the oxygen chemical potential conditions. By analyzing the cohesive energy, we observed a lower (higher) equilibrium charge state of the defect compared with the higher (lower) oxidation number of stable competitive phase enhanced solubility under reducing (oxidizing) conditions. Our calculations revealed that vanadium and manganese dopants were soluble under reducing conditions owing to instabilities of their metal phases and act as effective acceptors in BaTiO3.