14 Externe wissenschaftliche Einrichtungen
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Item Open Access Ab initio study of point defects in the bulk and on surfaces of an SrTiO3 crystal(2009) Alexandrov, Vitaly; Maier, Joachim (Prof. Dr.)The main goal of the present thesis was the theoretical study of substitutional iron impurities and oxygen vacancies present in different charge states in the bulk and on the SrTiO$_{3}$ (001) surface by means of first-principles simulations. Our first step was to examine basic properties of perfect perovskite crystals. We found for SrTiO$_{3}$ and SrFeO$_{3}$ parent compounds that the best agreement with experimental data on lattice constant, bulk modulus, cohesive energy and optical band gap is provided by the hybrid Hartree-Fock and the density functional theory (DFT) approach in the linear combination of atomic orbitals approximation with the so-called B3PW functional which was therefore chosen as a main tool in the study. In order to study SrFe$_{x}$Ti$_{1-x}$O$_{3}$ solid solutions, we carried out a series of calculations for various iron contents (50, 6.25, 3.70, 3.125, and 1.85 at.\%) in doped SrTiO$_{3}$. We found that the Jahn-Teller (JT) distortion around an Fe$^{4+}$ ion is the largest for the most dilute solid solution, becomes less pronounced for 50\% iron doping, and disappears in a pure SrFeO$_{3}$. This tendency in changing the magnitude of the JT distortion agrees well with the EXAFS experimenatal data extrapolated to the dilute defect limit. Also, the electronic structure calculations indicate that SrFe$_{x}$Ti$_{1-x}$O$_{3}$ containing more than 50\% iron is metallic, and that its conductivity is caused by a strong mixing of O $2p$ and Fe $3d$ ($e_{g}$) states in the pre-Fermi energy region. We found that the iron impurity insertion energy of 1.79 eV (1.85\% iron content) is very close to that for 6.25\% iron, and this energy decreases down to 1.59 eV for 50\% and lastly to 1.57 eV for a pure SrFeO$_{3}$. Calculations of iron impurities on the SrTiO$_{3}$ (001) surfaces revealed that the Fe$^{4+}$ ion has the propensity to segregate from the bulk to both SrO and TiO$_{2}$ facets, with the segregation energies of 0.32 and 0.48 eV, respectively. The Mulliken population analysis indicates that the presence of Fe$^{4+}$ ion in the subsurface plane significantly reduces the charge of the nearest oxygen atom at the topmost SrO surface (in a comparison to the substituted Ti$^{4+}$), thus severely diminishing its basic properties. We also examined the neutral and positively single-charged oxygen vacancies ($F$ and $F^{+}$ centers), both in the bulk and on the SrTiO$_{3}$ (001) surfaces. It was found that the neutral vacancy has an even higher tendency than Fe$^{4+}$ ions to surface segregation, being $\thicksim$1.0 eV for SrO and 1.4 eV for TiO$_{2}$ surfaces. The defect energy level becomes much more shallow when going from the bulk (0.77 eV below the bottom of the conduction band at the $\Gamma$-point of the BZ) to SrO (0.27 eV) and TiO$_{2}$ surfaces (very shallow level, almost degenerate with the CB within an accuracy of the method). Our simulations of the bulk charged $F^{+}$ center show that it has a deeper energy level (1.20 eV) than the neutral defect revealing also a lower vacancy formation energy. The charged nature of the center results in a more pronounced relaxation around the defect with a repulsive interaction with neighboring titanium atoms. This relaxation becomes even stronger at the surface. Thus, a common feature of both types of vacancies is the more shallow energy levels on the surfaces compared to the bulk, particularly, for the TiO$_{2}$ facet, while the effect is less pronounced for the $F^{+}$ center. We simulated additionally the diffusion of oxygen species by means of the density functional theory combined with the nudged elastic band (NEB) method. We have shown that the calculated activation barrier for diffusion of oxygen vacancy along the TiO$_{2}$ surface is almost by a factor of three smaller than in the bulk (0.14 vs. 0.38 eV). Adsorption energy of oxygen atom atop Ti ion for the TiO$_{2}$ facet is as large as 2.13 eV being considerably higher than that atop Sr ion on SrO facet (0.57 eV). Moreover, the creation of surface oxygen vacancy nearby the O atom adsorbed atop Ti ion leads to a significant increase in the O$_{ads}$-Ti binding energy. Because of such a strong adsorbate-adsorbent bonding, penetration of the adsorbed O atom into the surface layer could occur predominantly when the very mobile surface oxygen vacancy meets adsorbed oxygen atom. Simulation of a drop of the adsorbed O atom into the oxygen vacancy nearby reveals a distinguishable but extremely small activation barrier, $\thicksim$0.01 eV. Thus, we predict almost no-barrier soaking of the adsorbed O atom into the surface layer, fast surface diffusion of the oxygen vacancies and a much slower diffusion in the bulk. Considerable part of our study was devoted to the examination of oxygen vacancies (neutral and charged) as important ionic charge carriers under confinement conditions. This aspect is of paramount importance in nanoionic systems in which the boundary zones overlap and not only the density but in addition the nano-size spacing at interfaces becomes a key factor.