Publication No.20 P. Sánchez, H. Pinto, A. Stashans. Computer Modelling of Crystalline Ternary Compounds Doped with Impurities - 3rd Europ. Conf. Comput. Chem., Budapest (Hungary), 2000. Abstract: Recently computational chemistry tools have been found to be very useful in resolving theoretical problems in condensed matter. By means of the periodic INDO quantum-chemical method developed for crystal calculations [1] we have studied impurity doping in BaTiO3 and SrTiO3 crystals. This method has been found very reliable in a number of defect studies in solids [2,3]. The large unit cell model (LUC) was used for the present investigation. When we substituted an Al atom for one of the Ti atoms, we found that the hole is trapped by two oxygens situated at both sides of Al atom along the z-direction. This result is in accordance with the expectations since the Al impurity has a negative effective charge with respect to the perfect crystal. Therefore, the hole localisation on these two oxygens has mainly electrostatic origin. The results obtained for the Sc doped BaTiO3 crystal were not very different. Similarly, two oxygens situated along the x-direction trap the hole. In addition, when the automated geometry relaxation was carried out for the six Sc-nearest O atoms and the Sc atom itself, it was observed a small rotation of the four oxygens situated in the x-y plane by 0.8 degrees. In La-doped SrTiO3 we found that for its tetragonal lattice the relaxation of the atoms presents a rotation around the x-axis, and a bigger displacement along the z-axis. Besides, it can be observed that the one electron of the La-atom jumps to the conduction band. We think that this effect is part of the superconductivity phenomenon. 1. A. Shluger, E. Stefanovich, Phys. Rev. B, 1990, 42, 9664. 2. A. Stashans, J. L. Calais, E. Kotomin, AIP Conf. Proc. 330, 1995, 176. 3. S. Lunell, A. Stashans. L. Ojamäe, H. Lindström, A. Hagfeldt, J. Am. Chem. Soc., 1997, 119, 7374.
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