Calculations for antiferrodistortive phase of SrTiO3 perovskite: hybrid density functional study

The electronic and atomic structure of SrTiO3 crystals below the antiferrodistortive phase transition observed at 105 K is calculated using the hybrid B3PW functional as implemented in the ab initio CRYSTAL-2003 computer code. Such a combination of non-local exchange and correlation permits the calculation for the first time of the TiO6 octahedron rotational angle and the ratio c/a of tetragonal lattice constants in excellent agreement with experimental data. The level splitting of the bottom of the conduction band is found to be very small, <1 meV. The predicted phase-transition induced change of the optical gap from indirect to direct is confirmed by experimental photoconductivity data

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

The first-principles treatment of the electron-correlation and spin-orbital effects in uranium mononitride nuclear fuels

The DFT+U calculations were employed in a detailed study of the strong electron correlation effects in promising nuclear fuel -- uranium mononitride (UN). A simple method for solving the multiple minima problem in DFT+U simulations and insure obtaining the correct ground state is suggested and applied. The crucial role of spin-orbit interactions in reproduction of the U atom total magnetic moment is demonstrated. Basic material properties (the lattice constants, the spin- and total magnetic moments on U atoms, magnetic ordering, and the density of states) were calculated varying the Hubbard U-parameter. Varying the tetragonal unit cell distortion, the meta-stable states have been carefully identified and analyzed. The difference of the magnetic and structural properties obtained for the meta-stable and ground states are discussed. The optimal effective Hubbard parameter Ueff =1.85 eV reproduces correctly the UN anti-ferromagnetic ordering, and only slightly overestimates the experimental total magnetic moment of U atom and the unit cell volume.JRC.E.3-Materials researc

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