Publication No.44



E. Patiño, A. Stashans, R. Nieminen.  Quantum chemical study of 
effects produced by Nb- and La-doping in BaTiO3 - Key Engin. Mater., 
2002, Vol. 206/213, p. 1325-1328.

Abstract:
	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 Å, the oxygen displacements 
are asymmetric along only one crystallographic axis by 0.02 Å. In the 
tetragonal phase only defect-nearest oxygens move towards the La impurity 
by 0.06 Å 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.




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