Publication No.21 H. Pinto, R. Nieminen, A. Stashans. DFT and semi-empirical studies of acceptor centres in BaTiO3 - XXXIV Annual Conf. Finnish Phys. Soc., Helsinki (Finland), 2000, p. 202. Abstract: Recently computational chemistry tools have been found to be very useful in resolving theoretical problems in condensed matter. By means of first-principles electronic structure calculations based on the density- functional theory (DFT) within the local-density approximation and the so-called intermediate neglect of differential overlap (INDO) quantum-chemical method developed for crystal calculations we have studied impurity doping in the BaTiO3 crystal.In the DFT calculations, we have used Vanderbilt-type ultrasoft pseudopotentials as well as a 600 eV kinetic-energy cutoff. On the other hand, the INDO method has been found very reliable in a number of defect studies in solids. For the present investigation we used 90-atom supercells and to sample de Brillouin zone (BZ) we used the Gamma-point that show reliable for this type of crystal. 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 localization 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, a small rotation of the four oxygens situated in the x-y plane by 0.8 degrees was observed.
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