Publication No.57



A. Sánchez, C. Zambrano, L. M. Prócel, A. Stashans. Structural and 
electronic properties of PZT - 3rd Int. Conf. Advanced Opt. 
Mater. Devices, Riga (Latvia), 2002, p. 42. 

Abstract:
	The PbZrxTi1-xO3 (PZT) crystals are very important technological 
materials exhibiting interesting characteristics that make them useful 
for a wide range of applications. The purpose of this work is to describe 
at the fundamental level (through the Hartree-Fock MO theory) the 
electronic and structural properties of this material. The SYM-SYM 
computational program SYM-SYM is used to investigate the PbZrxTi1-xO3. 
By replacing from one to four of the eight Ti atoms by Zr - which 
corresponds to impurity concentrations of 0.125, 0.25, 0.375 and 0.5 
- we study the effects of the Zr impuriities on both cubic and 
tetragonal PZT lattices.
	It is found that in the cubic phase there are outward oxygen 
displacements (with respect to the Zr atoms) by 0.19 Ĺ, along the <100> 
axes - regardless of the Zr concentration. In the tetragonal phase, 
different patterns are observed for the lattice relaxation. For x equal 
to 0.125, the oxygen movements are similar to that of the cubic phase. 
However, there are also outward Pb displacements by 0.14 Ĺ along the 
<111> axes. Oxygen movements are also found when x is 0.25, 0.375 or 
0.5, though they are not symmetric: the displacements are shorter along 
the z-axis (0.16 Ĺ) than those along the other axes (0.19 Ĺ). In addition, 
if x is 0.375 or 0.5, there are outward Pb movements similar to those 
reported for x =0.125. The equilibrium geometry for both cubic and 
tetragonal phases corresponds to the case when none of the Zr impurities 
is the closest to another Zr atom. The impurities are located along the 
<110> directions, so they actually resemble a tetrahedron structure 
when x=0.5.
	It is worth to mention that, according to our calculations, 
covalent chemical bonding becomes more prevalent in the tetragonal phase 
because of ferroelectricity in conformity with the available experimental 
results and increases with the impurity concentration.




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