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 with parameters g _x=1.98, g _y=2.01, g _z=2.00, D=0.43 cm⁻¹, and E=5.87×10^t-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 E _a=0.044 eV. A model of the orthorhombic Fe³⁺ center in KTaO₃ is discussed within the framework of the kinetic parameters obtained. Fiz. Tverd. Tela (St. Petersburg) 39, 861–864 (May 1997)     

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 Fe _Ta ³⁺ -V_O. 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. Fiz. Tverd. Tela (St. Petersburg) 39, 626–629 (April 1997)     

Theoretical studies of the g factors and local structure for the rhombic Fe center in NaF

The anisotropic g factors and local structure for the rhombic Fe⁺ center in NaF are theoretically studied using the perturbation formulas of the g factors for a rhombically distorted octahedral 3d⁷ cluster. The impurity Fe⁺ is found not to occupy exactly the host Na⁺ site in NaF but to experience the off-center displacement of about 0.28 Å along [1 1 0] axis due to size mismatch substitution. The calculated g factors based on the above impurity displacement show reasonable agreement with the experimental data. The local structure of the Fe⁺ center is discussed.     

Local structure of the trigonal defect center for Cr3+ ions in KMgF3 crystals

Abstract

The zero-field splitting D, the anisotropic g-factors g _∥, Δg(=g _∥ ? g _⊥) and the first excited state splitting Δ(² E) for the trigonal Cr³⁺–V_K center in KMgF₃: Cr³⁺ crystals have been studied from Macfarlane's high-order perturbation formulas. From the studies, the local structure of the trigonal center is obtained. The local lattice distortions (i.e., the displacement directions of the ions in the center) are consistent with the expectation based on the electrostatic interaction.     

Local structure of Gd3+ impurity center at cubic sites in fluorites

A computer controlled ENDOR spectrometer and special software are used to determine the coordinates of¹⁹F nuclei in four anionic spheres of cubic MeF₂:Gd³⁺ (Me = Ca, Sr, Ba, Pb) centers. The computer simulation of Gd³⁺ cubic impurity center in crystals MeF₂ (Me = Ca, Sr, Ba) has been also performed. The analysis of lattice relaxation near the impurity center carried out on the base of ENDOR experiments data has allowed us to obtain potential of interaction Gd³⁺-F^?. For the first time not only radial displacements but also angular displacements of F^? ions of the third sphere have been taken into account and it has been shown that it influences the determination of lattice relaxation around the impurity center essentially. The influence of hydrostatic pressure (up to 30 kbar) on the local structure of the impurity center has also been investigated using computer simulation. The comparison of the experimental and calculated displacements of distant ions gives a reliable test of the validity of theoretical lattice relaxation model and accuracy of calculations of impurity-directed shifts of ligands.     

ODMR and EPR investigations of Fe centers in KTaO3

The analysis of EPR spectra obtained from iron doped KTaO₃ crystals in the as-grown state revealed three dominant iron centers: Fe³⁺-O_I, axial Fe-centers with spinS = 3/2 and rhombic Fe³⁺. By comparison with data from literature possible assignments for the center withS = 3/2 are discussed. For the rhombic species the temperature dependence of the main parameters of the Spin- Hamiltonian was measured. The result makes it most plausible that only one rhombic iron center exists in KTaO₃, in contrast with literature. The understanding of the EPR spectra allows us to assign transitions, observed at very low magnetic fields by optically detected magnetic resonance (ODMR), to this rhombic Fe center. On this basis, the magnetic circular dichroism (MCD) of this defect could be identified using the method of tagged-MCD. This spectrum is compared to the tagged-MCD of Fe³⁺-O₁ and of axial Fe⁴⁺ centers, which may be generated metastably by optical charge transfer. Considerably different structures in the MCD spectra of both Fe³⁺ centers indicate different local surroundings and electronic states.Dedicated to O. F. Schirmer on the occasion of his 60th birthday     

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 Fe _Ta ⁴⁺ -V _O 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 Fe _K ³⁺ -O_i center. These two centers were established to undergo mutual charge transfer, in which the electron released in the photoionization of the Fe _K ²⁺ -O_i center is trapped by the Fe _Ta ⁵⁺ -V _O center. Irradiation by light with a photon energy below 2.05 eV, which is the ionization threshold of Fe _K ²⁺ -O_i, 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 Fe _Ta ⁴⁺ -V _O and Fe _K ³⁺ -O_i 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 Fe_Ta-V _O and Fe_K-O_i complexes, gives rise, nevertheless, to the alignment of the centers along (or at right angles to) the light polarization vector.     

Field-induced splitting of the OH-Stretch absorption line in KTaO3 and KTaO3 :Li

Abstract

We have studied the electric field-induced splitting of a single OH stretch mode in KTaO₃ and KTaO₃ :Li. For the three prominent directions [100], [110] and [111] we observed polarized splitting patterns with up to six components. The line positions exhibit no mirror symmetry with respect to the zero-field frequency. This behaviour can be interpreted by taking into account a field-dependent shift of the Ta⁵⁺ ion and a change of the covalent character of the bond. Li-doped KTaO₃ samples show for [Li] < 2mol% a line broadening, while for [Li] < 3.5 mol% a field dependent phase transition has been observed with evidence for a 90 degree domain structure.     

Ultrasonic investigation in KTaO3

The anomalous ultrasonic attenuationα _c of longitudinal waves propagating along (100) direction in KTaO₃ has been analyzed above the phase transition temperature in the frequency range 150–300 MHz in the paraelectric phase. The attenuation of longitudinal ultrasonic waves in KTaO₃ is primarily due to a strong interaction with thermally-excited phonons in the soft mode. Frequency and temperature variations of attenuation are discussed.     

LiSrAlF6晶体Cr3+掺杂中心电子结构的研究

使用离散变分方法(DVM)对LiSrAlF6晶体中以Cr3+为中心的多种分子团簇的电子结构进行了计算.主要研究了镶嵌势、团簇大小、基组类型对单电子能量本征值、基态原子轨道特性的影响,给出了使计算结果趋于收敛的基本条件.     

Electron impact double ionization of Fe+ and Fe3+

Electron impact double ionization cross-sections for Fe⁺ and Fe³⁺ have been calculated in the modified binary encounter model incorporating the effects of the Coulombic field of the target on the incident electron. Accurate expression of cross-section for energy transfer ΔE given by Vriens and Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. The present results show satisfactory agreement with experimental observations. It is concluded that the discrepancy in the high energy region observed in the present calculations may be attributed to non-inclusion of indirect ionization processes arising from the L-shell.