Publication No.67



S. Serrano, C. Duque, P. Medina, A. Stashans. Oxygen-
vacancy defects in PbTiO3 and BaTiO3 crystals: a quantum
chemical study - SPIE Proceedings, 2003, Vol. 5122, p. 287-294.

Abstract:
	We study effects produced by an O vacancy and F center in cubic 
and tetragonal lead titanate (PbTiO3) crystals as well as barium titanate 
(BaTiO3) (001) surface. Displacements and charges of defect-surrounding 
atoms, lattice distortion and relaxation energies are carefully analyzed. 
It is found that the predominant cause of atomic movements in PbTiO3 
around an O vacancy is the Coulomb interaction while in the case of F 
center also changes in the chemical bonding within the atomic planes 
should be taken into consideration in order to explain atomic movements. 
We also observe a phenomenon known as bi-stability of the fundamental 
state, which occurs due to the rotation of some cationic planes. The 
obtained vibronic energy barriers for bi-stability are found to be 
around 1.0 eV. In the case of the BaTiO3 crystal the computed average 
atomic movements around the O vacancies are around 0.12 � and 0.15 � 
for the cubic and tetragonal lattices, respectively. In the case of F 
center we observe somewhat smaller lattice distortion. In the latter 
case, however, we find a considerable redistribution of electronic 
charge leading to a polarization of the defect-surrounding lattice, 
especially if the F centre is situated within the Ti-O2 plane. In 
general, there is a reduction of the ferroelectric dipole moment for 
the tetragonal phase of the crystal due to the F center presence on 
BaTiO3 (001) surface.




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