Publication No.5



A. Stashans, J. Coloma.
Computer modeling of cubic and tetragonal phases of PbTiO3, PbZrxTi1-xO3 and 
PbMgxTi1-xO3 - 5th Int. Conf. Comput. Phys., Kanazawa (Japan), 1999, p1-14.

Abstract:
Among a variety of ferroelectric materials, PbTiO3, PbZrxTi1-xO3 and PbMgxTi1-xO3 
have been found to have unique properties for memory, sensor, actuation and optical 
applications.
    Using the modified for crystals INDO method we have made the parametrization of 
the perfect PbTiO3 crystal considering both cubic and tetragonal phases. The reproduced 
electronic band structure shows mixed ionic-covalent bonding in this material with the 
conduction band bottom composed of Pb 6p states and the upper valence band top having Pb 
6s character. The effective charges on atoms are: q(Ti) = 2.48e, q(Pb) = 1.27e and q(O) 
= -1.25e. In the tetragonal phase we observe considerable admixture of Pb 6p states in 
the upper valence band which points out to the ferroelectric behaviour of this material 
below the 490 C.
    In order to simulate Zr and Mg doping into the PbTiO3 crystal we used 40- and 
80-atom large unit cells (LUCs) and substituted one of the Ti atoms for Zr or Mg atom. 
The carried out optimization of geometry showed the displacements towards the impurity 
of six defect-nearest oxygens by 0.1 � and 0.06 � in the case of Zr- and Mg-doping, 
respectively. The atomic movements are only slightly larger for the tetragonal phase. 
The non-symmetric atomic displacements were considered taking into account (i) different 
magnitude of displacements along the three crystallographical axes and (ii) rotation 
of oxygens around the impurity atom. In the case of Mg-doping the local one-electron 
energy level was found in the forbidden energy gap located 0.4 eV below the bottom of 
the conduction band. The chemical character of this level is mainly O 2p in nature and 
it obviously originates due to the Mg presence in the crystal. The effect of impurity 
concentration upon the electronic band structure properties is analized.




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