Publication No.43



C. Zambrano, A. Sánchez, L. M. Prócel, J. M. Larrea, A. Stashans, 
N. Vinueza, W. Cao, S. Dávalos. Impurity doping of lead titanates. 
A quantum mechanical study - 2001 MRS Fall Meeting, Boston (USA), 
2001. 

Abstract:
	Among ceramic materials, lead titanate, PbTiO3, has shown to posses 
unique properties which make it suitable materials for catalytic, sensor 
and optical applications among others. In this report, we wish to present 
our studies involving the effects of a hydrogen atom added to a PbTiO3 
crystal, and also doping of PbTiO3 with Zr and Si leading to PbZrxTi1-xO3 
(x = 0.125, 0.25, 0.375, 0.50) and PbTiO3:Si, respectively.  These systems 
have been studied at the fundamental level (Hartree-Fock MO theory) using 
an advanced computer code developed for periodic structures. All calculations 
exploit the LUC (large unit cell) model as it is incorporated in the 
computational program SYM-SYM.
	When doping of Zr was studied on PbTiO3, characteristic patterns in 
the atomic displacements were noticeable.  In general, oxygen outward movements 
(0.16 to 0.19 Ĺ) with respect to the Zr impurity occur for both phases; 
additionally, Pb displacements are also observed for the tetragonal phase. 
Asymmetric displacement of oxygens is significant in the tetragonal phase 
where the bonding is shown to be less ionic.  Si-impurity doping was carried 
out considering a Si atom in an interstitial lattice site. The performed 
geometry shows the importance of the coulombic interaction in the material 
since the negatively charged oxygen atoms move towards the impurity, while 
the positively charged Pb and Ti atoms move away from it. Differences  of 
defect behaviour in two crystallographic lattices, cubic and tetragonal, are 
analyzed.
	For the addition of Hydrogen on PbTiO3, the computational analysis 
shows that the hydrogen atom binds preferentially to an oxygen atom in both 
phases, which produces a slight shift of Ti and Pb atomic positions.  A 
reduction in polarization effects in the tetragonal phase was observed.




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