Publication No.4



A. Stashans, F. Erazo.
Computer modeling of CaTiO3 crystal doped with Nb - 5th Int. Conf. Comput.
Phys., Kanazawa (Japan), 1999, p1-12.

Abstract:
We have utilized the periodic INDO method developed for crystal calculations 
and the large unit cell (LUC) model to study Nb impurity in the CaTiO3 crystal. 
The calculations have been carried out using the relevant computer code CLUSTERD. 
The cubic and orthorombic crystalline structures were considered.
    The energy band structure was computed self-consistently reproducing completely 
the experimental geometry of both lattices. The analysis of the effective charges 
of the pure crystal suggests the mixed ionic-covalent chemical bonding. The 
orthorombic crystal shows to be more covalent in comparison to the cubic one which 
points out to the occurence of the ferroelectricity in this phase as a result of 
high admixture of Ti 3d states in the upper valence band.
    In order to study the Nb doping we have substituted the Nb atom for one of the 
Ti atoms situated approximately in the center of the LUC which consisted either of 
40 or 80 atoms. The geometry optimization shows that the Nb-surrounding O atoms 
move forward while the Ca atoms outward the inserted impurity. The atomic displacements 
of Ca atoms in the case of the orthorombic structure are larger than in the case 
of the cubic lattice reduring considerably the total energy of the system. We explain 
this effect as a result of the more open orthorombic lattice packing. The importance 
of asymmetric geometry relaxation for the cubic structure is discussed.
    The local one-electron energy level within the forbidden energy gap is found to 
be located around 2.9 eV above the top of the upper valence band which is mainly 
composed of O 2p states. This level originates from the presence of Nb atom. The 
position of the level is found to be sensible to the Nb concentration. The differences 
between two crystalline structures with respect to Nb presence are discussed in light 
of the available experimental data.




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