Quantum chemical study of effects produced by Nb- and La-doping in BaTiO3

The effects produced in BaTiO3 due to Nb and La doping have been studied. The investigation has been performed using a quantum chemical method developed for crystals and it is based on the Molecular Orbital theory. The geometry relaxation due to the Nb impurity shows a symmetric character in both phases, cubic and tetragonal, i.e., outward defect-closest barium displacements and inward oxygen displacements. The relaxation energy, as a difference for fully relaxed and unrelaxed systems, is found to be equal to 1.69 eV and 5.44 eV for the cubic and tetragonal phases, respectively. It is found that the extra electron belonging to Nb transfers from the local energy level within the band gap to the conduction band. In the case of the La impurity, the lattice distortion shows a non-symmetric pattern in the cubic phase. Although titaniums move away symmetrically by 0.07 Angstrom, the oxygen displacements are asymmetric along only one crystallographic axis by 0.02 Angstrom. In the tetragonal phase only defect-nearest oxygens move towards the La impurity by 0.06 Angstrom while the titaniums remain motionless. The calculated relaxation energy for this phase is found to be equal to 0.84 eV and 0.52 eV for the cubic and tetragonal phases, respectively. The extra electron in the case of La-doping remains within the band gap.