Publication No.12 A. Stashans, F. Vargas. F-type Centers in cubic and tetragonal lattices of SrTiO3 crystal. - 14th Int. Conf. Def. Insulat. Mat., Johannesburg (South Africa), 2000, p. 68. Abstract: Using the modified for crystals quantum-chemical method and a periodic large unit cell (LUC) model we study F centers (two electrons trapped by an oxygen vacancy) in SrTiO3 crystal considering its cubic and tetragonal structures. The spatial configurations of F centers, lattice distortion and relaxation energies are calculated. The computations have been carried out in two different LUCs consisting of 40 and 135 atoms, respectively. For 40-atom LUC we have obtained that Ti, Sr and O atoms move outwards the F center symmetrically in the cubic and asymmetrically in the tetragonal phases, respectively. The atomic displacements are equal to 0.13 �, 0.08 � and 0.06 � for the Ti, Sr and O atoms, respectively, in the cubic phase. For the tetragonal phase considerably larger displacements for Sr and O atoms were obtained along the ferroelectric c axis. The relaxation energies are 1.6 eV and 3.2 eV for cubic and tetragonal lattices, respectively. The atomic displacements in the 135-atom LUC are slightly different. In particular, for the cubic lattices the movements of Ti, Sr and O atoms are equal to 0.13 �, 0.16 � and 0.03 �, respectively. The Ti atoms move along <100> directions while the Sr and O atoms change their positions along <110> directions. The atomic displacements are larger along the c axis similarly to the case of 40-atom LUC. However, we found much smaller relaxation energies for F center in 135-atom LUC, which do not exceed the value of 0.8 eV. This can be explained by the fact that F center is more delocalized in the 135-atom LUC. Obviously, the size of 40-atom LUC is not sufficient to describe with high precision structural properties of F centers in this material. The electronic density analysis shows the localization of two electrons on two vacancy-nearest Ti atoms. However, the wave function of these electrons is more diffuse for the case of tetragonal lattice, which points out to the possibility of occurrence of free electrons in this structure.
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