Publication No.34



C. Zambrano, A. Sánchez , L. M. Prócel, A. Stashans. 
Study of structural and electronic properties of wide 
band-gap semiconductor PbTiO3 doped with Zr - 21st 
Int. Conf. Def. Semiconductors, Giessen (Germany), 2001, p. pb114.

Abstract:
	Among a variety of ceramic materials, PbTiO3 (PT) and 
PbZrxTi1-xO3 (PZT) have been found to have unique properties for 
memory, sensor, actuation and optical applications. The goal of 
the present work is to study the structural and electronic 
properties of these materials at the fundamental level using an 
advanced quantum-chemical method. The Hartree-Fock MO theory and 
SYM-SYM computer code developed for periodic systems (e.g.: crystal 
computations) are used to investigate PbTiO3:Zr systems for both 
cubic and tetragonal crystalline lattices.  We exploit a LUC model 
consisting of 40 atoms. We have studied Zr doping by substituting 
one to four of Ti atoms by Zr impurities.  This corresponds to the 
impurity concentration x equal to 0.125, 0.25, 0.375 and 0.5. 
	The obtained results show the following pattern in the atomic 
displacements. For the cubic phase and x=0.125 we have obtained 
oxygen outward movements with respect to the Zr impurity along the 
<100> axes by 0.19 Å. The atomic relaxation for the tetragonal phase 
is similar. However, in addition to oxygen outward movements by 
0.19 Å we also find outward Pb displacements by 0.14 Å along the 
<111> directions. Slightly larger relaxation along the ferroelectric 
z-axis is found for the tetragonal phase. In the case of two Zr atoms 
in the cubic phase we find the same atomic displacements as in the 
case of one impurity. In the tetragonal structure the Zr-surrounding 
oxygens move away from the impurity along the <100> directions; but 
these movements are not symmetric, we find shorter displacements of 
0.16 Å along the z-axis compared to 0.19 Å movements along the x and 
y axes. It is worth to mention that the equilibrium geometry is found 
for impurity location along the <110> and <111> directions for the 
cubic and tetragonal phases, respectively. 
	In general, our computations indicate less ionic chemical 
bonding for the tetragonal phase. This is in agreement with experimental 
observations where the tetragonal phase is more covalent due to 
ferroelectricity.




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