Publication No.55



S. Serrano, C. Duque, P. Medina, A. Stashans. Oxygen-vacancy 
defects in PbTiO3 and BaTiO3 crystals: a quantum chemical 
study - 3rd Int. Conf. Advanced Opt. Mater. Devices, 
Riga (Latvia), 2002, p. 40. 

Abstract:
	We study effects produced by an oxygen vacancy and F-centre 
(two electrons trapped at an oxygen vacancy) in cubic and tetragonal 
PbTiO3 crystals as well as BaTiO3 (001) surface. Displacements of 
defect-surrounding atoms, changes in atomic charges, and lattice 
relaxation energies due to the defect presence are carefully analysed. 
	In the case of the PbTiO3 it is found that in the case of 
the oxygen vacancy, the Coulomb interaction has a predominant role 
in atomic movements. However, in some cases of F centre, one has to 
consider also the importance of chemical bonding within the atomic 
planes, especially Ti-O interaction. The delta SCF absorption energy 
for the F-center is computed in the cubic lattice of the crystal 
giving value of 2.36 eV. A phenomenon known as bi-stability of the 
fundamental state is found in the PbTiO3 crystal due to the rotation 
of some atomic planes. We find for the first time in lead titanate 
the occurrence of this phenomenon observed before in some III-V 
semiconductors and ionic crystals. As follows from our studies this 
effect occurs due to the rotation of some cationic planes if the 
F centre is present. The estimated vibronic energy barriers for the 
bi-stability is found to be around 1.0 eV, which is close to 
magnitudes found before in another crystals [1].
	The average atomic movements around the O vacancies in the 
BaTiO3 crystal are found to be around 0.12 � and 0.15 � in the cubic 
and tetragonal lattices, respectively. 
	The atomic displacements around the F centre are found somewhat 
of a smaller magnitude preserving the same pattern. We also observe 
some interesting effects for F centre studies on the tetragonal BaTiO3 
(001) surface. First of all, due to the F centre we notice a considerable 
electronic charge redistribution signalising a polarisation of the 
defect-surrounding lattice. For the F centre situated within the Ti-O 
plane the direction of polarisation is along the ferroelectric c axis 
while for the defect situated within the Ba-O plane the polarisation 
occurs within the xy plane. In general, there is a reduction in the 
ferroelectric dipole moment of the crystal. 

[1] A. S. Shcheulin, A. I. Ryskin, K. Swiatek and J. M. Langer, Phys. 
    Lett. A, 222, 107 (1996).




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