Ab initio Calculations for SrTiO_3 (100) Surface Structure

Results of detailed calculations for SrTiO_3 (100) surface relaxation and the electronic structure for the two different terminations (SrO and TiO_2) are discussed. These are based on ab initio Hartree-Fock (HF) method with electron correlation corrections and Density Functional Theory (DFT) with different exchange-correlation functionals, including hybrid (B3PW, B3LYP) exchange techniques. Results are compared with previous ab initio plane wave LDA calculations. All methods agree well on both surface energies and on atomic displacements. Considerable increase of Ti[Single Bond]O chemical bond covalency nearby the surface is predicted, along with a gap reduction, especially for the TiO_2 termination

Tendencies in abo3 perovskite and srf2, baf2 and caf2 bulk and surface f‐center ab initio computations at high symmetry cubic structure

This research was partly funded by the Latvian Council of Science project No. LZP‐ 2020/2‐0009 (for R. Eglitis), as well as the ERAF Project No. 1.1.1.1/18/A/073. We express our gratitude for the financial support from Latvian–Ukraine cooperation Project No. Latvia–Ukraine LV‐ UA/2021/5. The Institute of Solid State Physics, University of Latvia (Latvia), as the Centre of Excellence has received funding from the European Unions Horizon 2020 Framework Pro‐ gramme H2020‐WIDESPREAD01‐2016‐2017‐Teaming Phase2 under Grant Agreement No. 739508, project CAMART2.We computed the atomic shift sizes of the closest adjacent atoms adjoining the (001) surface F‐center at ABO3 perovskites. They are significantly larger than the atomic shift sizes of the closest adjacent atoms adjoining the bulk F‐center. In the ABO3 perovskite matrixes, the electron charge is significantly stronger confined in the interior of the bulk oxygen vacancy than in the interior of the (001) surface oxygen vacancy. The formation energy of the oxygen vacancy on the (001) surface is smaller than in the bulk. This microscopic energy distinction stimulates the oxygen vacancy segregation from the perovskite bulk to their (001) surfaces. The (001) surface F‐center created defect level is nearer to the (001) surface conduction band (CB) bottom as the bulk F‐center created defect level. On the contrary, the SrF2, BaF2 and CaF2 bulk and surface F‐center charge is almost perfectly confined to the interior of the fluorine vacancy. The shift sizes of atoms adjoining the bulk and surface F‐centers in SrF2, CaF2 and BaF2 matrixes are microscopic as compared to the case of ABO3 perovskites. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Published under the CC BY 4.0 license.Latvian Council of Science project No. LZP‐ 2020/2‐0009; ERAF Project No. 1.1.1.1/18/A/073; Latvian–Ukraine cooperation Project No. Latvia–Ukraine LV‐ UA/2021/5. The Institute of Solid State Physics, University of Latvia (Latvia), as the Centre of Excellence has received funding from the European Unions Horizon 2020 Framework Pro‐ gramme H2020‐WIDESPREAD01‐2016‐2017‐Teaming Phase2 under Grant Agreement No. 739508, project CAMART2

Computer Modeling of Point Defects, Impurity Self-Ordering Effects and Surfaces in Advanced Perovskite Ferroelectrics

The calculated optical properties of basic point defects - F-type centers and hole polarons - in KNbO_3 perovskite crystals are used for the interpretation of available experimental data. The results of quantum chemical calculations for perovskite KNb_xTa_(1-x)O_3 solid solutions are presented for x=0, 0.125, 0.25, 0.75, and 1. An analysis of the optimized atomic and electronic structure clearly demonstrates that several nearest Nb atoms substituting for Ta in KTaO_3 - unlike Ta impurities in KNbO_3 - reveal a self-ordering effect, which probably triggers the ferroelectricity observed in KNb_xTa_(1-x)O_3. Lastly, the (110) surface relaxations are calculated for SrTiO_3 and BaTiO_3 perovskites. The positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized using the classical shell model. Strong surface rumpling and surface-induced dipole moments perpendicular to the surface are predicted for both the O-terminated and Ti-terminated surfaces

First principles and semi-empirical calculations of atomic and electronic structure for the (100) and (110) perovskite surfaces

We present and discuss results of the calculations for BaTiO_3 and SrTiO_3 surface relaxation with different terminations using a semi-empirical shell model (SM) as well as ab initio methods based on Hartree-Fock (HF) and Density Functional Theory (DFT) formalisms. Using the SM, the positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized. This permits us determination of surface rumpling and surface-induced dipole moments (polarization) for different terminations of the (100) and (110) surfaces. We also compare results of the ab initio calculations based on both HF with the DFT-type electron correlation corrections, several DFT with different exchange-correlation functionals, and hybrid exchange techniques. Our SM results for the (100) surfaces are in a good agreement with both our ab initio calculations and LEED experiments. For the (110) surfaces O-termination is predicted to be the lowest in energy

Calculations of Perovskite Polar Surface Structures

Results of calculations for the (110) polar surfaces of three ABO3 perovskites — STO, BTO and LMO — are discussed. These are based on ab initio Hartree-Fock method and classical Shell Model. Both methods agree well on both surface energies and on near-surface atomic displacements. A novel model of the "zig-zag" surface termination is suggested and analyzed. Considerable increase of the Ti[Single Bond]O chemical bond covalency nearby the surface is predicted for STO