First principles study on the ferroelectricity of the perovskite ABO3 ferroelectrics

In order to understand well the different ferroelectric behaviour of quantum paraelectrics and ferroelectrics and the origin of the ferroelectricity of the solid solution KTa0.5Nb0.5O3(KTN),we calculated the electronic structure of CaTiO3,BaTiO3 and KTN by first principles calculation.From total energy analysis,it is shown that,with increasing cell volume,the crystals (CaTiO3,SrTiO3) will have a ferroelectric instability.For BaTiO3,the ferroelectricity will disappear as the cell volume is decreased.From the density of states analysis,it is shown that the hybridization between B d and O p is very important for the ferroelectric stability of ABO3 perovskite ferroelectrics.This is consistent with the analysis of band structure.     

Ground state and properties of ferroelectric superlattices based on crystals of the perovskite family

The crystal structure of the ground state of ten free-standing ferroelectric superlattices based on crystals with the perovskite structure (BaTiO₃/SrTiO₃, PbTiO₃/SrTiO₃, PbTiO₃/PbZrO₃, SrZrO₃/SrTiO₃, PbZrO₃/BaZrO₃, BaTiO₃/BaZrO₃, PbTiO₃/BaTiO₃, BaTiO₃/CaTiO₃, KNbO₃/KTaO₃, and KNbO₃/NaNbO₃) was calculated from first principles within the density functional theory taking into account criteria for stability of the structures with respect to acoustic and optical distortions. It was shown that the ground state in all the considered superlattices corresponds to the ferroelectric phase. It was found that the polarization vector has a tendency toward a tilt to the plane of the superlattice layers, which makes it possible to decrease the electrostatic and elastic energy in the superlattices consisting of materials with different ferroelectric properties. The importance of the inclusion of structural distortions due to unstable phonons at the Brillouin zone boundary, which, in a number of cases, lead to significant changes in ferroelectric and dielectric properties of the superlattices, was demonstrated.     

Thermoelectric transport coefficients of n-doped CaTiO3, SrTiO3 and BaTiO3: A theoretical study

Boltzmann transport equations and density functional theory calculations were employed to calculate the thermoelectric transport coefficients of CaTiO₃, SrTiO₃ and BaTiO₃. It was found that BaTiO₃ has the largest Seebeck coefficient and power factor. Then the transport coefficients were analyzed using the ‘Tight Binding Model’. The band narrowing, caused by the increasing lattice constants from CaTiO₃ to BaTiO₃, was the main reason for the increasing Seebeck coefficients and the decreasing electrical conductivity. The calculated electrical conductivity and electronic thermal conductivity were in line with the Wiedemann-Franz law and the Lorenz factor was determined to be 2.45 for these oxides as degenerate semiconductors. Our theoretical results are helpful for seeking high performance thermoelectric oxides.     

Influence of the A ion on the K -edge absorption spectra of A TiO3 ( A = Ba, Sr, or Ca) crystals with perovskite structure

The near-edge structure of the Ti K-edge absorption spectra was calculated for BaTiO₃, SrTiO₃, and CaTiO₃ crystals by the full multiple scattering method. The results obtained are in agreement with the experimental data. The numerical experiment revealed the mechanisms forming the structure of the main edge of the spectra. Original Russian Text ? N.M. Novikovskii, E.S. Nazarenko, A.A. Novakovich, R.V. Vedrinskii, 2008, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2008, Vol. 72, No. 4, pp. 462–464.     

Effect of domain wall on the dielectric properties of the BaTiO3/SrTiO3 superlattices

Based on a phenomenological thermodynamical theory, the effect of the domain wall on the dielectric properties of the polydomain BaTiO₃/SrTiO₃ superlattices with 180° electric domains in the BaTiO₃ layer is investigated. Theoretical analysis indicates that complete polarization suppression and the largest dielectric response take place at approximately 72% and 53% of the critical volume fraction of the SrTiO₃ layer for the domain wall energy parameter A=3×10⁷ and 5×10⁷, respectively. The dielectric properties largely depend on both the volume fraction of the SrTiO₃ layer and the domain wall energy parameter. Moreover, the gigantic dielectric response which occurs in single-domain BaTiO₃/SrTiO₃ superlattices cannot appear due to the 180° polydomain state in the BaTiO₃ layer, which is in agreement with the previous prediction. The high domain wall energy parameter results in the stabilization of the paraelectric state in the BaTiO₃ layer, however, the enhancement of the domain wall energy parameter within a certain scale increases the dielectric properties greatly.     

Electronic structure of the valence band for perovskite-type titanium double oxides studied by XPS and DV-Xα cluster calculations

XPS spectra of the valence bands for the perovskite-type titanium double oxides BaTiO₃, SrTiO₃ and CaTiO₃ have been measured and analyzed by means of DV-Xα calculations for the TiO₆ embedded cluster model. The theoretical photoelectron spectra modulated by the photoionization cross-sections are in good agreement with experiment. The XPS results show that the O 2p valence band is constructed of two peaks whose spacing becomes larger in the order BaTiO₃ < SrTiO₃ < CaTiO₃. The DV-Xα results indicate that the greater part of the lower-energy peak is attributed to the levels which have O 2p orbitals pointing to Ti cations and are thus stabilized by the electrostatic potential concomitant with a decreased TiO bond distance. The electrostatic potential also reduces the O 2p-Ti 3d mixing and, together with the repulsion of the electron cloud between the Ti and O ions, makes the TiO bond more ionic.     

Quantitative analysis of UV excitation bands for red emissions in Pr-doped CaTiO3, SrTiO3 and BaTiO3 phosphors by peak fitting

A quantitative spectral analysis of the ultraviolet (UV) broad excitation bands, which are located in the range 300-400 nm, for red emissions at around 610 nm in Pr-doped CaTiO₃, SrTiO₃:Al and BaTiO₃:Mg phosphors has been carried out using a peak fitting technique. The obtained results demonstrate that the UV broad band of CaTiO₃:Pr consists of four primary excitation bands centered around 330, 335, 365 and 380 nm and those of both SrTiO₃:Al and BaTiO₃:Mg consist of three primary bands centered around 310, 345 and 370 nm. Based on the behavior patterns and the values of the respective primary excitation bands’ parameters, i.e. center gravity (λ_top), maximum height (I_max) and full-width at half-maximum (FWHM), the UV-to-red relaxation processes in these titanate phosphors can be explained to be essentially the same, except for the existence of an additional relaxation pathway via electron-trap states in CaTiO₃:Pr, which gives a characteristic shape of its UV excitation spectrum in the wavelength range of >360 nm.     

Vibronic Frenkel exciton in a soft-mode crystal

The pair electron-hole Jahn-Teller effect, which is realized on degenerate states of a vibronic Frenkel exciton (VFE) in soft-mode crystals, is considered. It is shown that the vibronic effect in a VFE can be significantly enhanced in crystals having soft lattice modes. The occurrence of strong local order-disorder fluctuations in this situation is predicted. The manifestations of VFEs localized at impurities in soft matrices (Cr³⁺ in SrTiO₃, Fe⁵⁺ in KTaO₃, and Mn⁵⁺ in TbVO₄ and TbPO₄) are considered. The possibility of fluctuation effects related to VFEs in nominally pure soft-mode crystals (SrTiO₃ and BaTiO₃) is discussed.     

Structural and electronic properties of Fe-doped BaTiO3 and SrTiO3

We have performed first principles calculations of Fe-doped BaTiO3 and SrTiO3. Dopant formation energy, structure distortion, band structure and density of states have been computed. The dopant formation energy is found to be 6.8eV and 6.5eV for Fe-doped BaTiO3 and SrTiO3 respectively. The distances between Fe impurity and its nearest O atoms and between Fe atom and Ba or Sr atoms are smaller than those of the corresponding undoped bulk systems. The Fe defect energy band is obtained, which mainly originates from Fe 3d electrons. The band gap is still an indirect one after Fe doping for both BaTiO3 and SrWiO3, but the gap changes from Γ-R point to Γ-X point.     

First-principles calculations and Bader analysis of oxygen-deficient induced magnetism in cubic BaTiO3−x and SrTiO3−x

The possibility of oxygen deficient induced magnetism in cubic BaTiO₃ (BTO) and SrTiO₃ (STO) perovskites is investigated by first-principles calculations, using the projector-augmented-wave method, within the generalised gradient approximation (GGA) and generalised gradient approximation with on-site effect (GGA + U), for the exchange correlation potential. For non-stoichiometric BaTiO_3?x and SrTiO_3?x, supercells were created in order to have two vacancy concentrations, i.e. x?=?0.125, 0.083. Spin charge distributions and magnetic moments associated with each ion, including local density of states projected in Bader atoms, were analysed by performing a full Bader charge analysis. Results show that oxygen vacancies could induce magnetism in BaTiO_3?x with x?=?0.125 and x?=?0.083 under GGA and GGA + U approximations. For SrTiO_3?x with x?=?0.125, ferromagnetism is induced with GGA, whereas with GGA + U the non-magnetic state is retained. On the other hand, with x?=?0.083, ferromagnetism is induced under GGA and GGA + U.     

用正电子湮没研究钙钛矿结构压电陶瓷中的点缺陷

用正电子湮没寿命谱技术对有意掺杂的或被杂质替代的钙钛矿结构压电陶瓷中的点缺陷进行了研究.掺杂了La³⁺的PbTiO₃，掺杂了Sr²⁺，Cd²⁺和La³⁺的Pb(Mg_1/3Nb_2/3)_0.375Ti_0.375Zr_0.25O₃，掺杂了Ca²⁺的BaT 关键词：     

Ab initio calculations of Born charges in ferroelectrics with a perovskite structure

A method has been proposed for calculating Born effective charges in compounds with a cubic perovskite structure. The method is based on the first-principles calculation of individual contributions from the short-range interaction and the intercell dipole-dipole interaction to the Born tensor Z _ii (s) for crystalline dielectrics. It has been shown that the contribution from the short-range interaction Z _ii ^sr (s) to the Born tensor components can be derived from ab initio calculations performed for polyatomic clusters. The results of the calculations of the short-range interactions Z _ii ^sr (s) for the cubic phases of the BaTiO₃, SrTiO₃, CaTiO₃, PbTiO₃, BaZrO₃, PbZrO₃, KNbO₃, and KTaO₃ compounds with the use of the electronic structure calculations within the Hartree-Fock approximation and the density functional theory are presented. For the BaTiO₃, SrTiO₃, CaTiO₃, PbTiO₃, KNbO₃, and KTaO₃ compounds, the components of the complete Born tensor have been also calculated. The obtained values of Z _ii (s) are in good agreement with the results of the calculations in terms of the linear response theory and the Berry phase approach.     

Novel polymeric potassium complex: Its synthesis,structural characterization,photoluminescence and electrochemical properties

In this paper, we obtained a novel poly(vanillinato potassium) complex (PVP) as a single crystal and characterized by analytical and spectroscopic methods. A single crystal of the PVP was obtained from the acetone solution. X-ray structural data show that crystals contain polymeric K⁺ complex of vanillin. Each potassium ion in the polymeric structure is identical and seven-coordinate, bonded to two methoxy, two phenoxy and three aldehyde oxygen atoms from four vaniline molecules. Two aldehyde oxygen atoms are bridging between potassium ions. It crystallizes in the monoclinic system, space group P2₁/c, with lattice parameters a=9.6215(10) Å, b=17.4139(19) Å, c=9.6119(10) Å, β=100.457(2)° and Z=4. Thermal properties of the PVP were investigated by TGA, DTA and DSC methods. The electrochemical properties of the complex were studied in different solvents and at various scan rates. The luminescence properties of the complex in different solvents and at different pH values have been investigated. The results show that the complex exhibits more efficient luminescence property in CH₃CN and n-butanol.     

Atomic and electronic structures of Mg-doped perfect SrTiO3 and crystals containing oxygen vacancies from first principles

The electronic properties of Mg-doped perfect SrTiO₃ and crystals containing oxygen vacancies systems are investigated by first principles calculation. Dopant formation energy results show that the Mg atoms preferentially enter the Sr site in SrTiO₃. Substitution of Ti by Mg brings some acceptor levels, which introduces the p-type conductivity of SrTiO₃. Creation of oxygen vacancies in SrTiO₃ introduces donor levels, which can contribute to the n-type conductivity. In SrTiO₃ containing oxygen vacancies system, a self-compensation effect will occur when Ti is substituted by Mg, and the system undergoes n-type to p-type transition.     

UPS and XPS studies of the chemisorption of O2, H2 AND H2O on reduced and stoichiometric SrTiO3(111) surfaces; The effects of illumination

About one monolayer of Ti³⁺ species is detectable at the surface of reduced SrTiO₃(111) single crystals by XPS and UPS. O₂, H₂ and H₂O have been adsorbed in the dark and the decrease on the concentration of the Ti³⁺ species has been monitored as a function of the gas exposures. Subsequent band gap illumination partially restores the Ti³⁺ initial concentration in the cases of O₂ and H₂ exposures but not in the case of H₂O. The Ti³⁺ photogeneration on the oxygen covered surface is associated with oxygen photodesorption as indicated by XPS and UPS. UPS measurements give evidence for surface hydroxylation resulting from water and hydrogen adsorption. The activity of the stoichiometric SrTiO₃(111) crystal face for O₂ and H₂ adsorption is very low when compared with the reduced SrTiO₃ samples.     

Ferroelectricity in SrTiO3 nanocrystalline disks

SrTiO₃ and CaTiO₃ conventional bulk materials are incipient ferroelectrics. In this note, we report for the first time that ferroelectricity could occur in SrTiO₃ nanocrystalline disks even at room temperature. The peak in the temperature dependence of permittivity for a CaTiO₃ nanocrystalline disk at a low temperature is also observed. The observed ferroelectricity (or permittivity peak) in SrTiO₃ (or CaTiO₃) nanocrystalline disks could be attributed to the strain effect.     

Charge transfer vibronic excitons: Charge-transfer lattice-instability effects

Two aspects for investigating bipolaronic excitons [charge transfer vibronic exciton (CTVE)] in ionic-covalent solids are considered. These aspects are predictions of the charge-transfer lattice instability corresponding to charge transfer and lattice anharmonicity, as well as to repulsion between different types of CTVEs in the order parameter field. Oxide crystals (SrTiO₃, BaTiO₃, K₃Na(CrO₄)₂) are considered as an example.     

Atomic and electronic structure of oxygen vacancies and Nb-impurity in SrTiO<Subscript>3</Subscript>

We present the results of a first-principle full-potential linearized augmented plane wave (FLAPW) method to study the effect of defects on the electronic structure of SrTiO₃. In addition, the relaxation of nearest neighbor atoms around those defects were calculated self-consistently. The calculations were performed using the local (spin) density approximations (L(S)DA), for the exchange-correlation potential. SrTiO₃ was found to experience an insulator-to-metal transition upon the formation of oxygen vacancies or the substitution of Nb at the Ti site. The formation of oxygen divacancy disclosed additional states below the conduction band edge. The crystalline lattice relaxation showed displacements of atoms in rather large defective region. The magnitudes of atomic movements, however, were not large, normally not exceeding 0.15 Å. Our results were compared to the available experimental observations.     

Anomalous behavior of the dielectric response of quantum paraelectric CaTiO3 with iron impurities

We report on the studies of lattice dynamics and crystal structure of natural perovskite with the general formula CaTiO₃:0.01Fe by dielectric spectroscopy and X-ray diffraction in a wide temperature range. In contrast to the “chemically pure” synthetic perovskite, the lattice dynamics of the Fe-containing natural sample exhibits a structural instability in the vicinity of 240 K. Our studies of the behavior of magnetic moment of CaTiO₃:0.01Fe demonstrate that in the temperature region from 4.3 to 300 K this material is paramagnetic. No changes in the crystal symmetry have been observed in the structural studies, which can be explained by the small degree of lattice distortions, as has been observed previously for isotopically substituted SrTiO ₃ ¹⁸ . Conductivity measurements and analysis of the dielectric response dispersion of CaTiO₃:0.01Fe have shown that the dispersion is due to the Maxwell–Wagner mechanism and can be attributed to oxygen vacancies present in the mineral.     

Observation of ferroelectricity induced by defect dipoles in the strain-free epitaxial CaTiO3 thin film

CaTiO₃ is a well-known incipient ferroelectric material that does not undergo a ferroelectric phase transition in spite of the intriguing dielectric constant behavior. Especially, unlike a prototypical incipient ferroelectric SrTiO₃, the paraelectric state of CaTiO₃ cannot be easily destroyed by small perturbations, including cation doping and epitaxial strain. We present that a nearly strain-free epitaxial CaTiO₃ film grown at a low oxygen partial pressure exhibits polarization–voltage hysteresis loops and the distinct difference of piezoresponse force microscopy phase signals, implying that a ferroelectric phase is induced. Such results are shown even at room temperature. We suggest that the observed ferroelectric behavior in CaTiO₃ film comes from the defect dipoles composed of vacancies inside the film. Using electron-probe microanalysis and optical absorption spectra measurements, we found that CaTiO₃ film has considerable Ca and O vacancies, forming the localized defect state in electronic structure. This work highlights the importance of vacancies and their clusters, such as defect dipoles, in understanding the electronic properties of perovskite oxide thin films, including ferroelectricity.