Incipient Ferroelectric KTaO3 Research Articles

Surface chemistry on a polarizable surface: Coupling of CO with KTaO3(001).

Polarizable materials attract attention in catalysis because they have a free parameter for tuning chemical reactivity. Their surfaces entangle the dielectric polarization with surface polarity, excess charge, and orbital hybridization. How this affects individual adsorbed molecules is shown for the incipient ferroelectric perovskite KTaO3. This intrinsically polar material cleaves along (001) into KO- and TaO2-terminated surface domains. At TaO2 terraces, the polarity-compensating excess electrons form a two-dimensional electron gas and can also localize by coupling to ferroelectric distortions. TaO2 terraces host two distinct types of CO molecules, adsorbed at equivalent lattice sites but charged differently as seen in atomic force microscopy/scanning tunneling microscopy. Temperature-programmed desorption shows substantially stronger binding of the charged CO; in density functional theory calculations, the excess charge favors a bipolaronic configuration coupled to the CO. These results pinpoint how adsorption states couple to ferroelectric polarization.

Probing the dynamic response of ferroelectric and ferroelastic materials by simultaneous detection of elastic and piezoelectric properties

Domain walls and precursors in ferroelectric and ferroelastic materials are of great importance for the optimization of ferroic devices and novel paradigms for domain switching. Any domain wall- or ferroic-precursor-based device requires knowledge about structural phase transitions and the formation and evolution of mesoscopic structures. In this review, Resonant Piezoelectric Spectroscopy (RPS) is shown to be a convenient investigative tool for the characterization of ferroic materials. RPS simultaneously measures elastic and piezoelectric properties related to domain walls and ferroic precursors and correlates the onset of phase transitions and the formation of mesoscopic structures with piezoelectric properties. Examples are given for ferroelastic, ferroelectric, and relaxor materials, including SrTiO3, BaTiO3, and PbMg1/3Nb2/3O3. Comparison of the data with incipient ferroelectric KTaO3 provides insights into the physical meaning of the coherence temperature and the Burns temperature in ferroelectrics, relaxors, and potentially other ferroic systems.

Oxide nanomaterials with properties absent in bulk (Author Review)

A ferroelectric phase can be induced in otherwise nonferroelectric binary oxides like BaO, EuO, and Er2O3 by a strong enough intrinsic surface stress present under the curved surface in nanowires. Our analytical calculations performed within the Landau–Ginzburg–Devonshire theory prove that BaO nanowires can be ferroelectric at room temperatures. Undoped nanoparticles of incipient ferroelectrics SrTiO3 and KTaO3 without any magnetic ions can become ferromagnetic even at room temperatures due to the presence of a new type of magnetic defects, such as oxygen vacancies, inherent to nanostructures. Nanosize Eu x Sr1–x TiO3 wires and tubes are shown to be the new multiferroics with “triple” the antiferrodistortive–ferroelectric–ferromagnetic phase, which is of interest for fundamental study and can be important for their potential applications. We hope that our predictions will stimulate experimental studies of the nanosized binary oxides and perovskites.

Microscopy and the Solid Solubility Limit in K1-xMnxTaO3 Ceramics

Multiferroic materials, combining at least two of three properties: ferromagnetism, ferroelectricity and ferroelasticity in the same phase, have been widely studied nowadays and have tremendous potential for multifunctional applications, although magnetoelectric multiferroics are difficult to obtain. Recently, dielectric and magnetic anomalies were found to be coupled in the incipient ferroelectrics SrTiO3 and KTaO3 doped with Mn on A-site of ABO3 perovskite lattice.

Magnetoelectric Interactions in Mn- and Co-Doped Incipient Ferroelectrics from Density Functional Calculations

We study the interactions between Mn impurities substituting for different cations in the incipient ferroelectric perovskites SrTiO3 and KTaO3. Using the LSDA+ U, we show that magnetic interactions J between off-centered Mn 2+ A ions substituting for Sr in SrTiO3 and inducing ferroelectric instability in the system is negligible. For the nearest and next nearest neighbours |JAA /kB |<5 K. The calculations for KTaO3 with Mn in K site show the stability of Mn in off-central positions shifted by 0.7 Å along the cubic axes. We have obtained that there is only one potential well for MnA located near a MnB in KTaO3:Mn. It corresponds to MnA shift towards the MnB by 0.8 Å. We predict that Co substituting for Sr in SrTiO3 should be stable in off-central positions shifted by 1.0 Å along a cubic axis.

Anion vacancy-driven magnetism in incipient ferroelectric SrTiO3 and KTaO3 nanoparticles

We predict that undoped nanoparticles (size ≤ 10–100 nm) of incipient ferroelectrics without any magnetic ions can become ferromagnetic even at room temperatures due to the inherent presence of a new type of magnetic defects with spin S = 1, namely oxygen vacancies, where the magnetic triplet state is the ground state in the vicinity of the surface (magnetic shell), while the nonmagnetic singlet is the ground state in the bulk material (nonmagnetic core). In particular, for a particle radii R less than the critical radius Rc(T) ferromagnetic long-range order appears in a shell region of thickness 5–50 nm once the concentration of magnetic defects N exceeds the magnetic percolation threshold Np. At vacancy concentrations N &amp;lt; Np and radii R &amp;lt; Rc(T), short-range ferromagnetic order and consequently a spin glass state may appear. For particle radii R &amp;gt; Rc(T), only the paramagnetic phase is possible.

Ab initio calculations of the geometry and electronic structure of point defects in ferroelectrics with a perovskite structure

Ab initio calculations of the optimized geometry and the electronic structure of lattice defects in incipient perovskite ferroelectrics SrTiO3 and KTaO3 are performed in the framework of the density functional theory. The results are presented for the Li+ impurity ion at the A site in the KTaO3 and SrTiO3 ferroelectrics; the Mn2+, Cd2+, Ca2+, Mg2+, and Zn2+ ions at the A site and the Mn4+ and Mg2+ ions at the B site in the SrTiO3 compound; and the MNTi2+-VO and MgTi2+-VO complexes in the SrTiO3 ferroelectric. The results are obtained by the cluster method with allowance made for the structural relaxation initiated by the defect and, for nonisovalent substitutional impurities, with due regard for the charge and spin states of the defect. It is established that the CaSr2+, CdSr2+, MnTi4+, and MgTi2+ ions have a stable central position, whereas the LiK+ ion in the KTaO3 compound and the LiSr+, MnSr2+, and ZnSr2+ defects in the SrTiO3 ferroelectric are off-center ions. The shape of the multiminimum adiabatic potential and the parameters of dielectric relaxators (activation barrier, dipole moment) for polar defects are obtained. The electronic impurity levels are determined for the LiSr+ and MgTi2+ neutral defects.

Mechanisms of off-center ion behavior in oxygen-octahedral perovskite-type ferroelectrics

A mechanism of the off-center impurity-ion behavior induced by the quadratic Jahn-Teller effect on polar distortions and (or) joint action of the local electrostriction and linear Jahn-Teller effect on nonpolar distortions is proposed. The off-center behavior of Ni5+ and Fe4+ ions in the incipient ferroelectric KTaO3 is substantiated. It is shown that, in the limit of neglecting the correlation between off-center Li+ ions, their displacements from KNbO3 lattice sites decrease, whereas, under the conditions of strong correlation, a critical increase in their displacements from lattice sites occurs with approaching the Curie point.

Appearance of ferroelectricity in thin films of incipient ferroelectric

AbstractThe consideration of a size‐induced ferroelectric–paraelectric phase transition for conventional and incipient ferroelectric thin films with perovskite structure was carried out in the phenomenological theory framework of Ginsburg–Landau–Devonshire. The more general form of surface free energy expansion that includes intrinsic surface stress tensor, the surface piezoelectric effect and electrostriction as well as quadratic and quartic powers of surface polarization has been considered. The analytical expressions for the thickness dependence of the transition temperature was derived both for the conventional and incipient ferroelectrics. It was shown that although there is no ferroelectricity in the bulk incipient ferroelectrics it appears in thin film for the negative extrapolation length that is realized, e.g., at positive surface stress coefficients and negative or zero misfit strain. In our consideration we came to the conclusion about thickness‐induced ferroelectricity in incipient ferroelectrics KTaO3 at room temperature for films that were sufficiently thin. The similar surface effects can strongly influence the phase transitions in the thin films of another incipient ferroelectric, SrTiO3. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Theoretical phase diagram of ultrathin films of incipient ferroelectrics

A first-principles-based scheme is used to compute the temperature-versus-misfit strain “Pertsev” phase diagram of ultrathin films of incipient ferroelectric KTaO3. The results suggest that, at variance with the bulk material, KTaO3 ultrathin films cannot be described as quantum paraelectrics. Rather, the behavior of the films is largely determined by surface/interface effects that favor ferroelectricity and the imperfect screening of the depolarizing fields. This leads to Pertsev phase diagrams that are qualitatively similar to those of normal ferroelectrics such as BaTiO3.

Second-harmonic-generation microscopic observations of polar state in Li-dopedKTaO3under an electric field

Incipient ferroelectric KTaO3 with off-center Li impurity of the critical concentration of 2.8 mol% was investigated in order to clarify the dipole glass state under electric field. Using optical second-harmonic generation (SHG) microscope, we observed a marked history dependence of SHG intensity through zero-field cooling (ZFC), zero-field heating (ZFH), field heating after ZFC (FH/ZFC) and FH after field cooling (FH/FC). These show different paths with respect to temperature: In the ZFC/ZFH process, weak SHG was observed at low temperature, while in the FH/ZFC process, relatively high SHG appears in a limited temperature range below TF depending on the field strength, and in the FC and FH/FC processes, the SHG exhibits ferroelectric-like temperature dependence: it appears at the freezing temperature of 50K, increases with decreasing temperature and has a tendency of saturation. These experimental results strongly suggest that dipole glass state or polar nano-clusters which gradually freezes with decreasing temperature is transformed into semi-macroscopic polar state under the electric field. However at sufficiently low temperature, the freezing is so strong that the electric field cannot enlarge the polar clusters. These experimental results show that the polar nano-cluster model similar to relaxors would be more relevant in KTaO3 doped with the critical concentration of Li. Further experiments on the anisotropy of SHG determine that the average symmetry of the field-induced polar phase is tetragonal 4mm or 4, which is also confirmed by the X-ray diffraction measurement.

Thermal Treatment and Light Illumination Effects in EPR Spectra of KTaO3:Cu

In order to understand the manner of incorporation of oxygen vacancies into perovskite structure oxides, KTaO3 crystals containing Cu2+ impurity ions were investigated following thermal treatments in atmosphere of H2, air, and vacuum. The results of thermal treatments were controlled by EPR. The influence of light illumination on copper centres in these crystals was studied also. The thermal treatment and light illumination effects in EPR spectra of Cu impurity centres are studied for the incipient perovskite-like ferroelectric KTaO3 single crystals. It was found that thermal treatment at ∼950°C in hydrogen atmosphere and in vacuum strongly depresses EPR signal of both types of Cu2+(1) and Cu2+(2) axial centres substituting Ta5+ in the B-sublattice positions. Subsequent high-temperature treatment in air restores EPR signals of Cu2+ centres, but changes the ratio between Cu2+(1) and Cu2+(2) centres concentration. These effects, together with UV light illumination effects observed for T < 80 K, are connected with the possibility of the Cu2+ ←→ Cu+ thermal and photo-induced transformations of the Cu impurity centres.

Ferroelectric phase transition induced by charge transfer vibronic excitons in ferroelectric oxides

It is shown theoretically that bi-polaronic excitons (charge transfer vibronic excitons – CTVE) in ferroelectric oxides can induce a significant positive shift of the ferroelectric phase transition temperature under strong enough optical or X-ray pumping (producing CTVEs in ∼1016 cm–3 concentration range). Incipient ferroelectrics KTaO3 and SrTiO3 could serve as a good example. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Oxygen‐related intrinsic defects and charge transfer vibronic excitons localized on a such type defects: optical manifestation of long term and metastable states in pure KTaO 3 crystals

Light absorption at low temperatures in nominally pure incipient ferroelectric KTaO3 is investigated. In such an absorption broad bands peaking approximately at 1.23 eV, at 1.53 eV and at 2.26 eV (the latter in the quenched sample) are observed in reduced KTaO3. Additional UV-illumination in the band gap region at low temperature gives rise to broad light-induced absorption bands in the same region: approximately at 1.14 eV, at 1.48 eV, and at 2.17 eV. The states active in ∼2 eV region absorption manifest long-term metastable behaviour. These observations are being considered on the basis of oxygen-related intrinsic defects (double and single negatively charged interstitial oxygen ions as well as double and single positively charged oxygen vacancies). The model proposed includes pair dipole-type intrinsic defects, which consist in oxygen interstitial ions and oxygen vacancies and manifest metastable behaviour. The latter dipole defects can be treated as Charge Transfer Vibronic Excitons localized on the pair defects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural effects of Li and Nb codoping in a KTaO3 matrix: A Raman investigation

Raman investigations on single crystals of K1−xLixTa1−yNbyO3 were performed to identify structural effects arising from the simultaneous presence of off-centre Li and Nb ions in the incipient ferroelectric KTaO3. Two soft-mode transitions were evidenced at 45K and 30K in a sample with x = 0.0014 and y = 0.024 after both zero-field and field cooling. These temperatures are close to the maximum and to the extrapolated Curie temperature for nonrelaxing permittivity reported in the same system. A sample with x = 0.0028 and y = 0.013 displayed a ferroelectric transition just below 30K only after field cooling, also being consistent with permittivity data.

Low temperature dielectric behavior in iron doped incipient ferroelectric KTaO3

The dielectric spectroscopy has been applied to study low temperature behavior in the incipient ferroelectric KTaO3 doped with Fe. The measurements were performed in helium-3 cryostat. In nominally pure KTaO3 no peculiarities in the temperature-frequency behavior of electric permittivity ε′ and ε″ have been detected. Below 5 K the saturation of ε′ and ε″ takes place. It is demonstrated that low iron doped (0.01%) KTaO3, crystals exhibit dielectric dispersion of ε′ and ε″, suggesting glass-like behavior below 5K. Electric permittivity dependence on dc electric field confirms the existence of short-range order. The temperature dependence of the first order non-linear dielectric permittivity was detected. Substitution the K+ ions by off-center Fe+ in KTaO3 is considered as the source of induced polar clusters of the short range order (glass or improper glass) at low temperatures for KTaO3:0.01% Fe.

Percolative clusters in KTaO3:Li

Percolation of polarization is considered in solid solutions of incipient ferroelectric KTaO3: Li. The Li impurities were surrounded by a sphere of the interaction radius. The latter was thought to be proportional to the correlation radius, whichcan be represented through the dielectric susceptibility, the temperature dependence of which is well known. This temperature dependence determines the dependence of the average size of the percolative clusters on temperature. It was shown that, in the Hyper-Rayleigh scattering case, the SHG intensity is just proportional to the average size that gives an opportunity to check the idea of existing the percolative clusters by straight comparison of the experimental and theoretical result.

SHG properties of pure and doped incipient ferroelectrics KTaO3 and SrTiO3 under applied electric fields

The investigation of the incipient ferroelectric systems KTO and STO at low temperature with optical second harmonic generation (SHG) using a Nd:YAG-laser shows a similar behaviour for both crystal systems. The SHG intensity S increases drastically upon reduction and conversely decreases upon oxidation. Oxygen defect related inversion-symmetrybreaking-centers interacting with dipolar centers introduced by doping (f.i. in Li+ doped KTO) result in strong changes of the dipolar properties. The static and dynamic behaviour of the SHG intensity under applied electric field confirms the dipolar nature of the centers and allows us to estimate the activation energy for reorientation. The SHG signal S of nominally pure, but containing oxygen defect-related-dipole-centers, in KTO first is approximately proportional to E. For Li-doped KTO the proportionality-coefficient b increases strongly with increasing Lidoping. At moderately higher E-fields the SHG intensity S appears to be saturated in agreement with recent theoretical model calculations.

The extra brillouin doublets and central peak of KTaO3: Second sound vs. two-phonon difference scattering

Low-T Brillouin spectra of the incipient ferroelectric KTaO3 exhibit a broad central peak (CP), and additional Brillouin doublets (BD), that can both be related to phonon-density fluctuations. On the basis of new high-resolution neutron data obtained of low-lying phonon branches, we analysed the phonon-kinetics mechanisms that are possibly the origin of these unusual features. Firstly, transverse acoustic (TA) phonons whose normal damping is faster than the BD frequency can produce hydrodynamic second sound. Secondly, two-phonon difference scattering from low damping thermal transverse phonons contribute to the spectra with either a sharp or a broader doublet, depending on the phonon group velocity and anisotropy of dispersion surfaces. The position of the observed sharp doublet is consistent with both mechanisms, but a comparison of the computed and experimental anisotropies favours the second process.

Possible nature of the red luminescence in incipient ferroelectric KTaO3

The red luminescence at 686 nm in KTaO3 is investigated in pure as-grown and doped (Al, Ba, Cr, Fe, Er, Li, Mg, Ni, Rh, V, Zr) crystals from different growth laboratories. In contrast to earlier results [1,2] which led to the conclusion that the main center should be an intrinsic defect Ta3+ + O2- vacancy, we found that the red luminescence to be sample dependent, together with a dependence on growth procedure. Moreover a strong dependence on any kind of heat treatment (oxidation or reduction) leading to a decreasing or vanishing of the red luminescence after above treatments, was found. Therefore excited metastable defect states have to be discussed for the origin of the red luminescence. The (i) two hole polarons localized near a Ta3+, (ii) Ta3+-O0-center (a vibronic exciton with double charge transfer) and at last (iii) a pair of Ta3+, one in a Ta5+ site and the other in a K+ site are discussed as possible centers connected with the red luminescence.