Centers In KTaO3 Research Articles

Studies of the g factors and the local structure of the orthorhombic Ni+ center in KTaO3 crystal

The local structure and the g factor (gx, gy, and gz) of the Ni+ center in KTaO3 are theoretically studied using the perturbation formulas of the g factors for a 3d9 ion in orthorhombically elongated octahedra. The orthorhombic field parameters are determined from the superposition model and the local geometry of the system. In view of the covalency, the contributions from the ligand orbital and spin–orbit coupling interactions are taken into account from the cluster approach. In the calculations, the orthorhombic center is attributed to Ni+ occupying the host Ta5+ site, associated with the nearest-neighboring oxygen vacancy VO along the c-axis. Furthermore, the planar Ni+–O2− bonds are found to experience the relative variation ΔR (≈0.076Å) along the a- and b-axis, respectively, due to the Jahn–Teller effect and the size mismatching substitution of Ta5+ by Ni+. Meanwhile, the effectively positive VO can make the central Ni+ displace away from VO along the c-axis by about 0.20Å. The calculated g factors based on the above local distortions show good agreement with the experimental data.

Investigations of spin‐Hamiltonian parameters and defect structures for two tetragonal Cu2+ centers in KTaO3 crystal

AbstractBased on the defect models that the two tetragonal Cu2+ centers in KTaO3 are due to Cu2+ ions at Ta5+ sites associated, respectively, with one and two oxygen vacancies (V0) along C4 axis because of charge compensation, the spin‐Hamiltonian parameters (g factors g‖, g⊥and hyperfine structure constants A‖, A⊥) of both Cu2+ centers in KTaO3 are calculated from the perturbation theory method (PTM) and the complete diagonalization (of energy matrix) method (CDM). The calculated results from both theoretical methods are not only close to each other, but also in reasonable agreement with the experimental values. This suggests that both methods are effective in the explanations of spin‐Hamiltonian parameters for 3d9 ions in tetragonal symmetry. From the calculations, the defect models are confirmed and the defect structural data are obtained for both Cu2+ centers in KTaO3. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical Studies on the Gyromagnetic Factors and the Hyperfine Structure Constants for the Tetragonal Copper Center in KTaO3

The gyromagnetic factors g∥, g⊥ and the hyperfine structure constants A∥ and A⊥ of the tetragonal Cu2+ center in KTaO3 are theoretically studied in this work. Based on the analyses of the electron paramagnetic resonance results of this center, it is found that the impurity Cu2+ occupies the octahedral Ta5+ site, associated with a nearest-neighbouring oxygen vacancy VO along the C4 axis. Due to the electrostatic repulsion of VO, Cu2+ is displaced away from VO by ΔZ(≈ −0.29 Å ) along the C4 axis. The theoretical values of the g and A factors based on the above defect structure and the impurity displacement agreee reasonably with the experimental data.

Theoretical Studies of the EPR Parameters and the Local Structure of the Tetragonal Fe+ Center in KTaO3

The EPR parameters (zero-field splitting D and g factors g‖ and g⊥) and the local structure for the tetragonal Fe+ center in KTaO3 are theoretically studied by using the perturbation formulas of the EPR parameters for a 3d7 ion in tetragonally distorted dodecahedra. Based on these studies, we find that the impurity Fe+ may not locate on the regular dodecahedral K+ site but suffer a large off-center displacement ΔZ (≈ 0.43 Å) along one of the 100 〈or C4〉 axes, which is responsible for the large tetragonal distortion of the impurity center. The displacement ΔZ obtained in this work is comparable with that (≈ 0.46 Å ) of a similar monovalent Li+ on K+ site of KTaO3 obtained from the nuclear quadrupole shift and can be regarded as reasonable. The calculated g factors, particularly the anisotropy Δg (= Kg⊥ −g‖) based on the above displacement, agree with the observed values.

Optical alignment of Cu2+ axial centers in KTaO3: Spectral dependence of the effect

The CuTa2+-VO axial centers in crystalline KTaO3 were found to undergo alignment under the action of polarized light. The sign of the effect is shown to change depending on the wavelength of the aligning light. A parallel study of the spectral response of photoconductivity of the same samples led to the conclusion that the alignment of the copper centers is driven not by reorientation but rather by an anisotropic recharging of the centers, which involves both the conduction and valence bands of the crystal. This interpretation was supported by a study of the kinetics of thermal destruction of the copper center alignment.

Anisotropic photoionization as a mechanism of axial iron center alignment in KTaO3

An EPR study has revealed light-induced recharging and optical alignment of the FeTa4+-VO tetragonal complexes in KTaO3. The data on the optical creation and destruction of this center by light of different polarizations and wavelengths are discussed together with similar results obtained for the FeK3+-Oi center. These two centers were established to undergo mutual charge transfer, in which the electron released in the photoionization of the FeK2+-Oi center is trapped by the FeTa5+-VO center. Irradiation by light with a photon energy below 2.05 eV, which is the ionization threshold of FeK2+-Oi, reverses this process. In both cases, the absorption cross section depends on the orientation of the center axis relative to the light polarization vector. As a result, the FeTa4+-VO and FeK3+-Oi tetragonal centers in KTaO3 acted upon by polarized light undergo orientation-sensitive light-induced recharging and the defects with the given charge state are no longer characterized by an equally probable distribution of the orientations of their axes over the three 〈100〉 directions. This mechanism, which does not involve real reorientations of the FeTa-VO and FeK-Oi complexes, gives rise, nevertheless, to the alignment of the centers along (or at right angles to) the light polarization vector.

EPR investigations of copper centers in KTaO3 single crystals

Copper centers in KTaO3:Cu single crystals were studied by Electron Paramagnetic Resonance. We evidenced two types of single Cu2+ centers and one exchange-coupled Cu2+-Cu2+ pair centre. The spectra of pair centers show tetragonal symmetry and the exchange interaction in the pairs is ferromagnetic (S=1).

Electric-field-induced orientation of iron tetragonal centers in KTaO3

Fe K 3+ -O i 2− impurity centers in a KTaO3 sample to which a dc electric field E=75 kV/cm is applied are shown to be oriented at temperatures T≥120 K. In these conditions, the effective local field acting on the electric dipole moment of a center exceeds the applied field by a factor 7.6.

EPR studies of Cu2+ in K1−xLixTaO3

Electron paramagnetic resonance (EPR) of copper centers in KTaO3:Li:Cu single crystals have been studied. Superhyperfine (SHF) structure of an exchange pair Cu2+-Cu2+ center, associated with the eight surrounding nearest-neighbor potassium ions was found. The EPR signal behaviour with the temperature may characterize the dielectric properties of crystals at the microwave frequency.

Local structure of the rhombic Fe3+ center in KTaO3

A study is reported of the effect of high-temperature annealing in oxygen, an inert gas, and water vapor on EPR spectra of the Fe3+ center of different local symmetries in the incipient ferroelectric KTaO3. An analysis of the relations obtained permits one to propose and substantiate a model of the rhombic Fe3+ center, by which Fe3+ substitutes for Ta5+ near two oxygen vacancies (Fe3+-2VO). Calculations of the crystal-field parameters performed within the Newman superposition model showed in the rhombic center the Fe3+ ion is displaced along [011] from the Ta5+ position it occupies within the tetrahedron formed by four oxygens, to a distance of about 0.25 A. Some of the recent results obtained in second-harmonic light scattering in iron-doped KTaO3 samples are interpreted. It is shown that, within the temperature range of 4.2–300 K, rhombic Fe3+ centers are static electric dipoles, and that they cannot therefore be a source of dielectric losses in KTaO3 at T≈40 K, as suggested earlier in some publications.

EPR and dielectric relaxation of Fe3+ in KTaO3

EPR and the method of dielectric losses have been used to investigate Fe3+ centers of axial and orthorhombic symmetry in KTaO3 single crystals. The EPR spectrum of orthorhombic-symmetry Fe3+ obtained in the 8-mm wavelength range at T=77 K is described by the spin-Hamiltonian \(\hat H = \beta (g_x H_x S_x + g_y H_y S_y + g_z H_z S_z ) + DS_z^2 + E(S_x^2 - S_y^2 )\) with parameters gx=1.98, gy=2.01, gz=2.00, D=0.43 cm−1, and E=5.87×10t-2 cm−1. From the dielectric measurement data we have obtained the following parameters of the relaxation of the orthorhombic Fe3+ centers in KTaO3: characteristic relaxation frequency τ 0 −1 =2.33×1012 Hz and activation energy Ea=0.044 eV. A model of the orthorhombic Fe3+ center in KTaO3 is discussed within the framework of the kinetic parameters obtained.

Observation of superhyperfine structure in the ESR spectra of two tetragonal Fe centers in KTaO3 and a new model for the center

Superhyperfine structure is observed in the lines of the ESR spectra of two tetragonal iron centers in the KTaO3 crystal, namely, “Fe 4/2” and FeTa3+-VO. Analysis of the superhyperfine structure of the former of these centers shows that the iron ion replaces Ta5+ and is found in the charge state 5+. Justification is given for assuming that the tetragonal symmetry of the center is due to the displacement of Fe5+ from the Ta5+ site along a 〈100〉 direction to an off-center position.

Erratum: Optical alignment of axial Fe centers in KTaO3

Erratum: Optical alignment of axial Fe centers in KTaO3

“Local phase transitions” and related relaxation processes in incipient ferroelectrics with a perovskite-like structure

Abstract The review of recent experimental and theoretical results presented here shows the important role of the local phase transitions (LPT) phenomena in low temperature dynamics of weakly doped KTaO3 and SrTiO3 incipient ferroelectrics. An unusual behaviour of zero-phonon R-luminescence line of Cr3+ centers in KTaO3 and SrTiO3 and radio-frequency dielectric permittivity and losses of weakly doped KTaO3 are discussed in detail. The important role of LPT for local electronic impurity centers as well as for impurity clusters (pairs) of interacting relaxators is demonstrated. A new mechanism of off-center effect for impurity centers at the absence of local charge compensation is proposed.

Optical alignment of axial Fe centers in KTaO3.

Axial ${\mathrm{Fe}}^{3+}$-${\mathrm{O}}_{\mathrm{I}}$ centers in ${\mathrm{KTaO}}_{3}$ can be aligned using linearly polarized ${\mathrm{Ar}}^{+}$-ion laser light with \ensuremath{\lambda}=456 nm. The alignment is observed directly via the change of the characteristic line pattern of the optically detected magnetic resonance (ODMR) of these centers. The ODMR is monitored by the magnetic circular dichroism (MCD) of absorption and is unambiguously attributed to ${\mathrm{Fe}}^{3+}$-${\mathrm{O}}_{\mathrm{I}}$, because the angular dependence of the ODMR lines agrees well with known electron paramagnetic resonance results. By observing the strength of the ODMR as a function of the photon energy, the MCD spectrum and, hence, the absorption bands of ${\mathrm{Fe}}^{3+}$-${\mathrm{O}}_{\mathrm{I}}$ centers in ${\mathrm{KTaO}}_{3}$ are identified. These bands are also found in the spectrum of the linear dichroism (LD), which is produced by photoalignment of the axial centers. The results lead to the interpretation that the orientation of ${\mathrm{Fe}}^{3+}$-${\mathrm{O}}_{\mathrm{I}}$ has a local, molecular character and is similar to the case of ${\mathit{F}}_{\mathit{A}}$ centers in alkali halides.

Rhombic Fe3+ centers in KTaO3

AbstractThe EPR spectrum of rhombic Fe3+ paramagnetic centers in KTaO3 both nominally pure and with Li and Nb impurities is investigated. The absolute values and signs of the parameters of the spin‐Hamiltonian describing the observed EPR spectrum are determined on the basis of the angular and temperature dependences of the mentioned spectrum. The model of a rhombic iron center involving the Fe3+ ion substituted for a Ta5+ ion with a nearby interstitial bivalent positive ion in [110] direction for charge compensation is suggested.