Covalency Effects Research Articles

Nephelauxetic effect for the isoelectronic 3d3-ions (Cr3+, Mn4+, Fe5+) in SrTiO3

The exchange charge model of crystal field theory has been used to analyze systematically the ground state absorption spectra of isoelectronic Cr 3+ , Mn 4+ , and Fe 5+ ions in an octahedral coordination in the SrTiO 3 crystal. The parameters of the crystal field acting on the valence electrons of impurity ions are calculated from the available crystal structure data. A special attention is paid to the analysis of dependencies of the crystal field invariants and covalence effects on the impurity ion. It is shown numerically that the covalence effects between the above impurity ions and ligands increase with an increase of the 3d-ion oxidation state.

Fully relativistic analysis of the covalence effects for the isoelectronic 3d3 ions (Cr3+, Mn4+, Fe5+) in SrTiO3

Fully relativistic calculations of the energy levels, absorption spectra, molecular orbitals (MO) compositions, covalence effects and energies of the charge transfer (CT) transitions for three isoelectronic ions Cr3+, Mn4+, Fe5+ in the SrTiO3 (STO) crystal have been performed. The recently developed first-principles approach to the analysis of the absorption spectra based on the first principles discrete variational multi-electron method (DV-ME) (K. Ogasawara et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. As a result, energy levels of the above ions, their absorption spectra and energies of the lowest CT transitions were all calculated. By performing analysis of the MO population, it was shown that the degree of covalence of the chemical bonds between 3d ions and oxygen ions in SrTiO3 increases in the following order: Cr3+→Mn4+→Fe5+, whereas the CT energies monotonically decrease in the same order.

비자성 이온 Ga, In이 치환된 유화물 스피넬의 뫼스바우어 분광학 연구

단일상 <TEX>$FeCr_{2-x}M_xS_4$</TEX> (M=Ga, In; x=0.1, 0.3)에 대하여 x-선 회절기(XRD), 진동 시료 자화율 측정기(VSM), 뫼스바우어 분광기를 이용하여 비자성 이온의 치환효과를 연구하였다. 결정구조는 Rietveld프로그램을 이용하여 공간그룹이 Fd3m[Fe, Ga, In(8a); Cr, Ga, In(160); S(32e) (u, u, u)]인 입방 스피넬 구조임을 확인하였다. 비자성 이온 Ga이 치환된 시료의 경우, Ga이 치환 될수록 격자상수가 10.007에서 <TEX>$9.996\;{\AA}$</TEX>,으로 감소하는 반면, In이 치환된 시료의 경우, In이 치환 될수록 격자상수가 10.029에서 <TEX>$10.092\;{\AA}$</TEX>로 증가함을 확인하였다 VSM측정 결과 Ga과 In이 치환 될수록 Neel온도는 각각 180에서 188K, 173에서 160K로 변화하였는데, 이것은 격자상수의 변화에 따른 사면체 자리(A자리)와 팔면체 자리(B 자리)의 초교환 상호작용 세기의 변화로 해석 할 수 있었다 <TEX>$4.2K\~300K$</TEX>의 온도범위에 걸쳐서 뫼스바우어 스펙트럼을 분석한 결과 Fe이온이 각각 A자리와 B자리에 점유함을 확인 할 수 있었다. 이것은 비자성 이온 Ga과 In 이 A자리의 Fe이온의 비대칭적인 전하 분포를 야기시키는 것으로 해석할 수 있었다. 또한 Cr-S의 결합거리를 비교해 본 결과, <TEX>$FeCr_{2-x}Ga_xS_4$</TEX>(x=0.3)와 <TEX>RFeCr_{2-x}In_xS_4$</TEX> (x=0.3)가 각각 2.41, <TEX>$2.43\;{\AA}$</TEX>로<TEX>$FeCr_{2-x}Ga_xS_4$</TEX>(x=0.3)의 결합거리가 작아 공유 결합력이 커짐에 따라 비대칭적인 전하 분포를 유도함으로 해석할 수 있다. 이는 큰 전기 사중극자를 유도하는 결과와 일치함을 알 수 있었다. The sulphur spinel <TEX>$FeCr_{2-x}M_xS_4$</TEX>(M=Ga, In) have been studied with Mossbauer spectroscopy, x-ray diffraction (XRD), and vibrating sample magnetometer. The XRB patterns for samples <TEX>$FeCr_{2-x}M_xS_4$</TEX>(M=Ga, In: x=0.1, 0.3) reveal a single phase, which the Ga and In ions are partially occupied to the tetrahedral (A) site. The Neel temperature for the Ga substituted samples increases from 180 to 188 K, with increase from x=0.1 to 0.3. While, it decreases from 173 to 160 K, for the In substituted samples of the x=0.1 and 0.3, respectively. The Mossbauer spectra were collected from 4.2 K to room temperature. We have analyzed the Mossbauer spectra using eight Lorentzian lines fitting method for the <TEX>$FeCr_{2-x}In_xS_4$</TEX>(x=0.1) at 4.2 K, yielding the 1311owing results; <TEX>$H_{hf}=146.0kOe,\;{\Delta}E_Q=1.88mm/s,\;\theta=36^{\circ},\;\phi=0^{\circ},\;\eta=0.6$</TEX>, and R=1.9. The Ga ions enter into the both sites octahedral (B) and tetrahedral (A), simultaneously the same amounts of Fe ions migrate from the A to the B site, this result is an agreement with XRD results, too. The <TEX>${\Delta}E_Q$</TEX> of the A and B site in Mossbauer spectra of the samples <TEX>$FeCr_{2-x}Ga_xS_4$</TEX>(x=0.3) are 0.83 and 2.94mm/s, respectively. While they are 0.56 and 2.36mm/s for the <TEX>$FeCr_{2-x}In_xS_4$</TEX>(x=0.3). It is noticeable that the <TEX>${\Delta}E_Q$</TEX> for the Ga doped samples are larger than that of the corresponding In doped samples, in spite of the larger ionic radius for In ions. The bond lengths of Cr-S, for the Ga and In doped samples (x=0.3) are found to be 2.41 and <TEX>$2.43\;{\AA}$</TEX>, respectively. We interpret that the larger covalence effect from the smaller bond length induces a large asymmetric charge distribution. Finally, it gives a large quadrupole interaction.

A Comprehensive Theoretical View of the Bonding in Actinide Molecular Complexes

While usual atomic population methods give a consistent view of trivalent lanthanide or a uranium-ligand bond, the description of the bonding of americium(III), which is a key element for nuclear fuel processes, is a challenge. Neither experimental data nor theoretical calculations have been able so far to evidence covalency effects in the americium-ligand bond. We show herein that the use of more sophisticated methods based on a topological approach (AIM and ELF) gives a consistent view of such an interaction for the first time, showing a weak covalent back-bonding interaction with the CO ligand.

Quantum chemical study of 4f→5d excitations of trivalent lanthanide ions doped in the cubic elpasolite Cs2NaYCl6. Ce3+ to Tb3+

Wave-function-based ab initio calculations on the lowest states of the 4f(n),4f(n-1)5d(t2g)1, and 4f(n-1)5d(e(g))1 configurations of (LnCl6)3- clusters (Ln=Ce to Tb) embedded in the cubic elpasolite Cs2NaYCl6 have been performed, in an attempt to contribute to a comprehensive understanding of the 4f-->5d excitations of lanthanide ions in crystals. Reliable data are provided on the changes of bond lengths and breathing mode vibrational frequencies upon 4f-->5d(t2g) and 4f-->5d(e(g)) excitations, as well as on minimum-to-minimum and vertical absorption and emission transitions, and on the Stokes shifts. The available experimental data are discussed and predictions are made. The stabilization of the 4f-->5d(baricenter) excitation of the doped ions with respect to the 4f-->5d excitations of the free ions, which is a key variable for the understanding of these excitations in solid hosts, is analyzed and found to be due, in two-thirds, to dynamic ligand correlation effects and, in one-third, to orbital relaxation, charge transfer, and covalency effects present in a mean-field approximation.

Comparative crystal field study of Ni 2+ energy levels in NiCl 2, NiBr 2, and NiI 2 crystals

The exchange charge model of crystal field theory has been used to analyze systematically the ground state absorption spectra of octahedrally coordinated Ni 2+ ion in isostructural nickel halide crystals NiCl 2, NiBr 2, and NiI 2. The parameters of the crystal field acting on the Ni 2+ ion are calculated from the available crystal structure data. The obtained energy level schemes are compared with experimental absorption spectra; a good agreement with experimental data is demonstrated. Dependencies of the crystal field invariants and covalence effects on the type of ligands are considered. It is shown numerically that the overlap effects between Ni 2+ ion and ligands increase with an increase of the ligands’ atomic number resulting in the following order of the degree of covalence: NiI 2>NiBr 2>NiCl 2.

EPR studies on Cu 2+ and Co 2+ complexes of tetraaza porphyrin derivatives

The electron paramagnetic resonance (EPR) investigations on 2,9,17,23-tetra-((1,1,2-(tricarbethoxyethyl)) phthalocyanine, 2,9,16,23-tetra-(carboxymethyl)-3,10,17,24-tetrachloro-phthalocyaninato complexes and octakis-(1-naphthyl-methylthio) substituted porphyrazine derivatives coordinated by Co 2+ and Cu 2+ ions have been performed in the temperature range of 10–300 K. The EPR spectra indicate axially symmetric character of ligand field at site of the Co 2+ and Cu 2+ paramagnetic ions consistent with the planar structure of each complex. Especially, the Cu 2+ coordinated compounds show important covalency effects with neighbouring N atoms with decreasing temperature, whereas absorption intensities obey the Curie law for both type paramagnetic ion coordinated complexes. The computer programs have been written to determine Spin-Hamiltonian and EPR linewidth parameters for both solid powder and solution forms of ion coordinated complexes. Orbital energy levels for magnetic electrons were determined from Spin-Hamiltonian parameters as well.

Luminescence properties of red-emitting M 2Si 5N 8:Eu 2+ (M = Ca, Sr, Ba) LED conversion phosphors

The influence of the type of the alkaline-earth ion and the Eu 2+ concentration on the luminescence properties of Eu 2+-doped M 2Si 5N 8 (M = Ca, Sr, Ba) has been investigated. XRD analysis shows that Eu 2+-doped Ca 2Si 5N 8 forms a limited solid solution with a maximum solubility of about 7 mol% having a monoclinic lattice. The Eu 2+ ion can be totally incorporated into Sr 2Si 5N 8 and Ba 2Si 5N 8 forming complete solid solutions with orthorhombic lattices. M 2Si 5N 8:Eu 2+ (M = Ca, Sr) shows typical broad band emission in orange to red spectral range (600–680 nm) depending on the type of M and the europium concentration. Ba 2Si 5N 8:Eu 2+ shows yellow to red emission with maxima from 580 to 680 nm with increasing Eu 2+ content. The long-wavelength excitation and emission is attributed to the effect of a high covalency and a large crystal-field splitting on the 5d band of Eu 2+ in the nitrogen environment. With increasing the europium concentration, the emission band shows a red-shift for all M 2Si 5N 8:Eu 2+ compounds due to changing Stokes shift and the reabsorption by Eu 2+. The conversion (i.e. quantum) efficiency increases going from Ca to Ba and Sr under excitation at 465 nm. In particular, Sr 2Si 5N 8:Eu 2+ with a quantum efficiency of 75–80% and a thermal quenching of only a few percent at 150 °C, demonstrates to be a highly promising red-emitting conversion phosphor for white-LED applications.

Binding of transition metal complexes to guanine and guanine–cytosine: hydrogen bonding and covalent effects

Density functional calculations and Atoms in Molecules analysis are used to investigate the role of covalent and hydrogen bondings in determining the binding of transition metal complexes to guanine, and the subsequent effect on pairing with cytosine. Hydrogen bonding is ubiquitous, and typically contributes ca. 10% to overall binding, a value that varies with the coordination site on guanine, as well as metal and ligands. Early transition metals show a clear preference for the O6 position, while later ones prefer N7, the crossover point coming at the vanadium group. Metallation at N7 causes a redistribution of hydrogen bonding strength between guanine and cytosine, but does not greatly affect the overall pairing energy. In contrast, metallation at O6 strongly reduces the pairing energy, as may be expected given the role of O6 in pairing guanine with cytosine. This effect can be quantified using electron density properties, and seems to be due to both electrostatic repulsion from the positive metal centre and a redistribution of electron density within guanine itself. Qualitative agreement with experimental mass spectroscopic results is obtained.

A covalent fraction model for lanthanide compounds

We formalize the ordinal covalency scale for lanthanide compounds, introduced in a previous work, through an analytical function (the covalent fraction) of the overlap polarizability. The inverse of a parameter appearing in this function is interpreted as a typical average volume of the overlap region. The red shift in the nephelauxetic effect for the 5D 0 → 7F 0 transition is analyzed, for 15 europium compounds, in terms of the ligand field strength parameter, N v, and the covalent fraction. A clear tendency of this shift to increase with the covalent fraction is observed. A similar trend, though less clear, is also observed with respect to N v, which is interpreted as a reflection of covalent effects in the ligand field parameters.

Studies on the EPR Parameters for the Rhombic Co2+ Center in Magnesium Acetate

The electron paramagnetic resonance (EPR) g factors gi (i = x,y, z) and the hyperfine structure constants Ai for the rhombic Co2+ center in magnesium acetate are theoretically studied from the perturbation formulas of these parameters for a 3d7 ion under rhombic symmetry. In these formulas, the contributions from the admixture among different states, covalency effect and rhombic crystal-fields are taken into account. The related crystal-field parameters are determined from the superposition model and the local geometrical relationship of the impurity center. The calculated results show reasonable agreement with the observed values. In addition, the nuclear quadrupole interaction constant Q is analyzed, and its negative sign is verified theoretically.

Jahn-Teller vibronic coupling in II–VI compounds with Cr 2+ ion

A theoretical study of the vibronic Jahn-Teller interaction in a series of II–VI crystals doped with Cr 2+ ion is reported. The vibronic coupling parameters for the tetrahedrally coordinated Cr 2+ ion (high-spin d 4 configuration) are evaluated in the framework of the exchange charge model of the crystal field accounting for the exchange and covalence effects. The calculations demonstrate the leading role of the trigonal vibrations for the 5T 2 term although the contribution of tetragonal modes is also important; the full symmetric and tetragonal vibrations for the 5E term significantly contribute to the phonon broadening of the absorption and emission bands related to the 5E ↔ 5T 2 transition in Cr 2+ ion.

Oxidation of aluminum nanoclusters

The dynamics of oxidation of aluminum nanoclusters ($20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ diameter) is investigated using a parallel molecular dynamics approach based on variable charge interatomic interactions due to Streitz and Mintmire that include both ionic and covalent effects. Simulations are performed for both canonical ensembles for molecular oxygen $({\mathrm{O}}_{2})$ environments and microcanonical ensembles for molecular $({\mathrm{O}}_{2})$ and atomic $({\mathrm{O}}_{1})$ oxygen environments. Structural and dynamic correlations in the oxide region are calculated, as well as the evolution of charges, surface oxide thickness, diffusivities of atoms, and local stresses. In the microcanonical ensemble, the oxidizing reaction becomes explosive in both molecular and atomic oxygen environments due to the enormous energy release associated with $\mathrm{Al}\text{\ensuremath{-}}\mathrm{O}$ bonding. Local stresses in the oxide scale cause rapid diffusion of aluminum and oxygen atoms. Analyses of the oxide scale reveal significant charge transfer and a variation of local structures from the metal-oxide interface to the oxide-environment interface. In the canonical ensemble, oxide depth grows linearly in time until $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, followed by saturation of oxide depth as a function of time. An amorphous oxide layer of thickness $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ is formed after $466\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, in good agreement with experiments. The average mass density in the oxide scale is 75% of the bulk alumina density. Evolution of structural correlation in the oxide is analyzed through radial distribution and bond angles. Through detailed analyses of the trajectories of O atoms and their formation of $\mathrm{O}{\mathrm{Al}}_{n}$ structures, we propose a three-step process of oxidative percolation that explains deceleration of oxide growth in the canonical ensemble.

Investigations Of The G Factors Of Fe+ In Mgo And Cao

The g factors of Fe+ in MgO and CaO are theoretically investigated by the perturbation formula of the g factor of a 3d7 ion in cubic octahedral symmetry based on the cluster approach. By considering the partial quenching of the spin-orbit coupling interaction and the effective Land´e factor due to the dynamic Jahn-Teller effect (DJTE), the experimental g factors of the studied systems are reasonably interpreted. It can be suggested that the small g factors of the Fe+ centers in MgO and CaO can be likely attributed to the DJTE, rather than the covalency effect within the scheme of the static crystalfield model.

The Electronic Structure of the Isoelectronic, Square-Planar Complexes [FeII(L)2]2-and [CoIII(LBu)2]-(L2-and (LBu)2-= Benzene-1,2-dithiolates): An Experimental and Density Functional Theoretical Study

The electronic structures of two formally isoelectronic transition-metal dithiolato complexes [Fe(L)2]2- (1) and [Co(L Bu)2]1- (2) both possessing a spin triplet ground state (St=1) have been investigated by various spectroscopic and density functional methods; H2L Bu represents the pro-ligand 3,5-di-tert-butylbenzene-1,2-dithiol and H2L is the corresponding unsubstituted benzene-1,2-dithiol. An axial zero-field splitting (D) of +32 cm(-1) for 2 has been measured independently by SQUID magnetometry, far-infrared absorption, and variable-temperature and variable-field (VTVH) magnetic circular dichroism spectroscopies. A similar D value of +28 cm(-1) is obtained for 1 on the basis of VTVH SQUID measurements. The absorption spectra of 1 and 2 are found, however, to be very different. Complex 1 is light yellow in color with no intense transition in the visible region, whereas 2 is deep blue. DFT calculations establish that the electronic structures of the [Fe(L)2](2-) and [Co(L)2]1- anions are very different and explain the observed differences in their absorption spectra. On the basis of these spectroscopic and theoretical analyses, 1 is best described as containing an intermediate spin FeII ion, whereas for the corresponding cobalt complex, oxidation states describing a d6 (CoIII) or d7 (CoII) electron configuration cannot be unambiguously assigned. The physical origin of the large zero-field splitting in both 1 and 2 is found to be due to the presence of low-energy spin-conserved d-d excitations which lead to a large Dzz through efficient spin-orbit coupling. Differential covalency effects appear to be of limited importance for this property.

Molecular orbital cluster calculation study of electron correlation and local lattice instability inLa2−2xSr1+2xMn2O7

In order to examine covalence and polaron effects in the bilayer manganite ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$, we have performed molecular orbital cluster calculations. Two types of ${({\mathrm{Mn}}_{2}{\mathrm{O}}_{11})}^{15\ensuremath{-}}$ clusters, one with the manganese aligned in the $a$ direction and the other in the $c$ direction, were embedded in a point charge environment that mimicked the crystal environment of the bilayer manganite at $x=0.40$, and their electronic states were calculated by the unrestricted Hartree-Fock (UHF) and the complete active-space self-consistent field (CASSCF) methods. The CASSCF result for the cluster along the $a$ direction exhibits double-well potential energy surfaces for symmetry-breaking deformations. This indicates small polaron formation in this system. On the other hand, the UHF calculation did not give double-well potential surfaces, showing the importance of the electron correlation for the polaron formation. Significantly large wells are obtained for the in-plane antiphase breathing and in-plane antiphase $\mathrm{O}\mathrm{Mn}\mathrm{O}$ stretching deformations. The double-well barrier for the former is $68\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and that for the latter is $92\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, where the former is close to the experimentally obtained polaron hopping activation energy above ${T}_{\mathrm{c}}$. A similar calculation for the cluster along the $c$ direction exhibits a negligibly small double well, indicating that the polaron effect is very small in the carrier hopping in the $c$ direction within a bilayer. Electronic structures have been investigated using natural orbitals. At a double-well minimum, a localized polaron orbital is seen. In the ground state, a small but significant hole population is found in $p$ orbitals of the bridging oxygen, and a slight electron population is found in the ${e}_{g}$ orbital above the localized polaron orbital. For the cluster along the $a$ direction and without deformation, the first excited state is an electron-transfer state where an electron is moved from the bridging oxygen ${p}_{z}$ to a manganese ${e}_{g}$ orbital. This excited state couples with the ground state by the pseudo-Jahn-Teller effect, thus, the polaron is the ``pseudo-Jahn-Teller polaron.'' Using the natural orbitals, we have calculated magnetic Compton profiles and compared with experiment. Comparison between the experimental and theoretical results suggests the presence of polarons below ${T}_{\mathrm{c}}$. We briefly discuss the implication of this result in relation to the colossal magnetoresistance effect.

Systematics of f-element crystal-field interaction

Systematic behaviors of free-ion and crystal-field interactions are elucidated as a function of N, the number of f electrons in a lanthanide or actinide ion. Experimentally determined values of the free-ion interaction parameters are compared with those calculated based on Hartree-Fock theory. Comparison is also made between the lanthanide series in 4 f N configurations and the actinide series in 5 f N configurations. Variation in intra-ionic electrostatic interaction, spin–orbit coupling, and ion–ligand interaction is analyzed in comparison between the iso- f-electron lanthanide and actinide ions. Based on an exchange-charge model of crystal-field theory, crystal-field parameters of the f-element ions in various crystals are summarized in terms of point charge contribution and covalence effect. A systematic correlation is found between the free-ion parameters and the crystal-field strength. Increase of the crystal-field interaction results in a reduction in the free-ion parameters.

CALCULATION OF EPR PARAMETERSgFACTORS FORCe3+ION INYBa2Cu3O6+xSUPERCONDUCTOR

The perturbation formulae of EPR parameters g factors gx, gyand gzfor the lowest Kramers doublet of 4f1ion in orthorhombic symmetry are established. In these formulae, the contributions of the covalency effects, the admixture between J=7/2 and J=5/2 states as well as the second-order perturbation (which is not considered in the previous works) via crystal-field and orbital angular momentum interactions are all included. According to these formulae, and by using the superposition model, the g-factors gx, gyand gzfor Ce3+center in YBa2Cu3O6+ xsuperconductor are calculated. The results are discussed.

Ab initio calculations of the atomic and electronic structure of layered Ba 0.5Sr 0.5TiO 3 structures

Understanding of the atomic and electronic structure of Ba cSr 1 − c TiO 3 (BST) solid solutions is important for several applications including the non-volatile ferroelectric memories (dynamic random access memory, DRAM). We present results of ab initio calculations of several spatial arrangements of Ba 0.5Sr 0.5TiO 3 solid solutions based on DFT-HF B3PW hybrid method. We calculate the atomic and electronic structure, the effective charges, interatomic bond populations, the electronic density distribution, and densities of states for three layered structures with the same composition. The suggested method reproduces experimental lattice parameters of both pure BaTiO 3 and SrTiO 3. The calculated optical band gaps for the pure SrTiO 3 and BaTiO 3 are in a good agreement with experimental data, much better than those from the standard LDA or HF calculations. In the studied BST structures with the equiatomic composition ( c = 0.5) the gap is reduced by ca. 0.2 eV. The electron density maps demonstrate the covalency effects in the Ti–O bonding. The electron density near the Sr atoms is stronger localized, as compared with the Ba ions.

Electronic Properties of Layered Ba0.5Sr0.5TiO3 Heterostructure: Ab initio Hybrid Density Functional Calculations

Ab initio calculations of the BST heterostructure with equiatomic constituent of Sr and Ba species has been carried out within hybrid functional B3PW involving a hybrid of non-local Fock exchange and Becke's gradient corrected exchange functional combined with the non-local gradient corrected correlation potential by Perdew and Wang. The suggested scheme of calculations reproduces experimental lattice parameters of both pure BaTiO3 and SrTiO3. The calculated optical band gap for the pure SrTiO3 (BaTiO3) is 3.56 (3.46) eV (expt. 3.25 and 3.20 eV, respectively), i.e. agreement is much better than in the standard LDA or HF calculations. In the Ba0.5Sr0.5TiO3 solid solution the gap is reduced by 0.2 eV. The BST upper valence band consists of O 2p atomic orbitals (AO) with a small admixture of Ti 3d, whereas the conduction band bottom consists mostly of Ti 3d orbitals with a small contribution of O orbitals. The Sr (Ba) atomic orbitals make a significant contribution to the higher part of the conduction band. The electron density maps demonstrate the covalency effects in the Ti-O bonding. The electron density near the Sr atoms is stronger localized, as compared with the Ba ions.