EPR of a non-Kramers iron ion in KTaO3

The EPR spectrum of the non-Kramers iron ion Fe⁴⁺ (S=2) in a KTaO₃:Fe crystal appearing after illumination of the sample in the visible has been detected and studied. Because of the large initial splitting (|D|=4.15 cm⁻¹), only transitions within the |±1〈 and |±2〈 doublets are seen experimentally. Superhyperfine structure in the spectrum of a non-Kramers ion in perovskites has been detected for the first time. A structure is proposed for the center responsible for the new EPR spectrum, which represents a complex of a Fe₄₊ ion substituting for Ta with an oxygen vacancy at the nearest anion site. Fiz. Tverd. Tela (St. Petersburg) 41, 1424–1427 (August 1999)     

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⁺-2V _O). 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 Å. 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.     

Reentrant Dipole Glass-Like Ordering in Weakly Concentrated KTaO3:Nb

Recently we reported that a very small admixture of Li^? dipole impurities into moderately concentrated KTaO₃:Nb leads to an unusual sequence of ferroelectric-type phase transitions with a reentrant glass phase formation at the lowest temperatures [1], Here, we present dielectric permittivity and Raman scattering study for nearly the same composition as used in Ref. [1], but without Li admixture, for KTa_0.982Nb_0.018,O₃ (KTN1.8). It is found that the presence of strong Li dipole defects is not an essential condition for the reentrant glass formation. Above T_c (27 K) KTN1.8 behaves as a conventional soft mode displacive type ferroelectric in paraelectric phase. However, below T_c a crossover to the order-disorder polar microregion dynamics and a state at which a reentrant glass phase coexists with long-range ordering occurs.     

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)     

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.     

Infrared and ESR spectroscopy of KTaO3 : Er

Infrared reflection and ESR spectra of KTaO₃ : Er³⁺ single crystals have been investigated. The frequency of the lowest optical phonon mode TO₁ has been found to increase as compared to undoped KTaO₃ crystals, which indicates the suppression of the ferroelectric instability of the system. The ESR spectra have revealed the presence of two Er³⁺ centers of different (cubic and non-cubic) symmetries in KTaO₃ : Er³⁺.     

Photoluminescence and photoinduced charge transfer in KTaO3:Cr,Li

Kinetics and spectral composition of the luminescence of a KTaO₃:0.1%Cr,10%Li sample (molar charge percentages) have been studied within a broad temperature range (2–300 K). The luminescence spectra of KTaO₃:0.1%Cr,10%Li have been found to have a strong narrow R-line characteristic of Cr³⁺ ions, with a clearly pronounced vibronic wing. Whereas the R-line width in Li-codoped samples is only higher by 120% than that of crystals doped only with chromium, the R-luminescence intensity in the presence of lithium increases by several orders of magnitude. It is shown that the luminescence excitation, besides the intracenter mechanism, is dominated by a process involving photoinduced charge transfer, which results in specific kinetics of the photoluminescence. The luminescence excitation model considered in the work is based, in particular, on the substantial role played in this process by electronic polaron capture by the Li⁺ dipole center caused by the existence of a characteristic shallow electron trap. Fiz. Tverd. Tela (St. Petersburg) 39, 2172–2178 (December 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)     

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     

Protons and other defects in Fe-doped KTaO3

The properties of defects in Fe-doped KTaO₃, both in the form of single crystals and polycrystalline ceramics, have been investigated for a wide range of Fe concentrations. The techniques employed included infrared (IR) absorption, electron paramagnetic resonance (EPR), and ac electrical conductivity together with complex-impedance analysis. Samples were pretreated (at 900° C) in water vapor to introduce protons which take the form of OH^· _O defects, and were also treated at various oxygen partial pressures. A calibration of the OH^– IR absorption band was carried out with the aid of a deuterium nuclear-probe method. EPR showed cubic and axial Fe³⁺ spectra, but only the axial spectrum appeared for crystals with high Fe concentrations. Pre-dominantly proton conductivity was observed for samples treated in reducing atmospheres, and a proton-hopping activation energy of 0.73 eV was deduced. For samples treated in high oxygen pressures, however, hole conduction dominated. Evidence for proton interaction effects was also found, but the nature of the traps is not clear.     

Li and Nb codoping in dielectric properties and phase transitions of KTaO3: Reentrant dipolar glass and long-range dipole ordering

Abstract

The highly polarizable perovskite-type oxide, KTaO₃ doped simultaneously with Li⁺ and Nb⁵⁺ (K_1?xLi_xTa_1?yNb_yO₃, KLTN), reveals unexpected properties and ordering effects. Studies of the dielectric permittivity ?'(T, f) (10—300K, 100Hz-1 MHz) for x = 0.0014 and y = 0.024 show collective dipolar ordering effects with a transition from paraelectric into a mixed phase (coexisting dipole-glass-like and long-range ordered ferroelectric phases) taking place near 39 K. At 15 K another phase transition into a reentrant dipolar glass-like state is observed. Such a sequence of transitions and the existence of a reentrant glass state are unknown for electrical dipolar systems.     

Metal–insulator transition induced by non-stoichiometry of surface layer and molecular reactions on single crystal KTaO3

In the study we present results on topography, morphology, chemical composition, electronic structure and electrical properties of the (100) surface layer of KTaO₃ single crystal caused by sputtering with Ar⁺ ion beam with energy of 1 keV. Several surface sensitive techniques, i.e. X-ray photoelectron spectroscopy (XPS), local conductivity of atomic force microscopy (LC-AFM), and Kelvin Probe Force Microscopy (KPFM) were used. The observed changes in the electronic structure were explained as a result of the chemical decomposition of the surface layer. A correlation between the electronic states which appeared in the energy gap and the changes in charge state of Ta ions was found. The activation energy related to averaged local conductivity temperature dependence was estimated from Arrhenius plot. It was also found, that variations in the local contact potential difference (LCPD) indicated changes in the chemical composition in nano-scale. The chemical reconstruction of the KTaO₃ surface modified by Ar⁺ ion beam was deduced. This non-homogeneity corresponded to 2-D non-homogeneity of the local electric conduction (LC-AFM), which occurred within nano-areas after sputtering. Chemical reactivity of the modified surface with CO₂ and O₂ was observed. The reversibility of the Ar⁺ induced loss of oxygen non-stoichiometry was observed after the sample was exposed to various doses of O₂. The successful reversibility occurred after oxidation process at high temperature, i.e. 300 °C.     

Photorefractive properties for holographic application of Ce:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

Ce:Fe:LiNbO₃ crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 48.6 and 58 mol% Li₂O. The Ce, Li and Nb concentrations in the grown Ce:Fe:LiNbO₃ crystals were analyzed by the inductively coupled plasma atomic emission spectrometer (ICP-AES). It was found that as the [Li]/[Nb] ratio increases in the melt, the [Li]/[Nb] ratio in the crystal and the distribution coefficients of Ce ions increase also. The photorefractive properties of the Ce:Fe:LiNbO₃ crystals were experimentally studied by the two-wave coupling method. The results show that as the [Li]/[Nb] ratio increases, the dynamic range decreases, but the photorefractive sensitivity and the signal-to-noise ratio improve. In a coherent volume 0.192 cm³ of a Ce:Fe:LiNbO₃ crystal with [Li]/[Nb] ratio of 1.2, 3800 holograms with 800×600 pixels have been successfully multiplexed in a compact volume holographic data storage system.     

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.     

Defect driven ferroelectricity and magnetism in nanocrystalline KTaO3

Nominally pure nanocrystalline KTaO₃ was thoroughly investigated by micro-Raman and magnetic resonance spectroscopic techniques. In all samples the defect driven ferroelectricity and magnetism are registered. Both ordering states are suggested to appear due to the iron atoms and oxygen vacancies. The concentration of defects was estimated to be 0.04 and 0.06-0.1 mol%. Note that undoped single crystals of KTaO₃ are nonmagnetic and have never exhibited ferromagnetic properties. The results enable us to refer a nanosized KTaO₃ to the class of multiferroics and assume that it could perform the magnetoelectric effect at T<29 K. It was also established that the critical concentration of impurity defects necessary to provoke the appearance of the new phase states in the material strongly correlates with the size of the particle; as the size of the particle decreases, the critical concentration decreases as well.     

Low energy phonon spectrum and its parameterization in pure KTaO<Subscript>3</Subscript> below 80 K

High resolution data on low energy phonon branches (acoustic and soft optic) along the three principal symmetry axes in pure KTaO₃ were obtained by cold neutron inelastic scattering between 10 and 80 K. Additional off-principal axis measurements were performed to characterize the dispersion anisotropy (away from the and axes). The parameters of the phenomenological model proposed by Vaks [28] are refined in order to successfully describe the experimental low temperature (10 < T < 100 K) dispersion curves, over an appreciable reciprocal space volume around the zone center ( rlu). The refined model, which involves only 4 temperature-independent adjustable parameters, is intended to serve as a basis for quantitative computations of multiphonon processes. Received: 29 September 1999 and Received in final form 6 January 2000     

锂铌比对铪铁铌酸锂晶体全息存储性能的影响

以掺杂4 mol%Hf⁴⁺的LiNbO₃:Fe:Hf系列晶体（[Li]/[Nb]比变化）为研究对象,研究了系列晶体的可见吸收光谱,在632.8nm的写入光下晶体的衍射效率、灵敏度和抗光散射能力在不同[Li]/[Nb]下的变化规律.研究发现Hf⁴⁺的浓度达到阈值浓度后,随着[Li]/[Nb]比的增大,晶体的可见吸收边会发生红移,而且晶格中[Fe²⁺]/[Fe³⁺]也会增加,这就导致随着[Li]/[Nb]比的增加,样品的衍射效率逐渐减小,写入时间缩短,灵敏度增大.同时,在晶体中,随着[Li]/[Nb]的增大,陷阱中心Fe²⁺Li数量增大会使得晶体抗光散射能力减弱.     

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.     

Optical properties of LiNbO3:Cr crystals co-doped with germanium oxide

Lithium niobate (LiNbO₃) crystals doped with chromium ions show a clear green colouring reflecting the absorption profile of the dominating [Cr]_Li defect centres. A significant change in its colouration takes place when it is co-doped with other valency impurities such as Mg²⁺, Sc³⁺ and W⁶⁺, above a certain threshold concentration. This concentration singularity has been attributed to the formation of [Cr]_Nb centres coexisting with the [Cr]_Li centres.In this work, we extended the investigation on the effect of co-dopant ions in Cr:LiNbO₃ to tetravalent cation such as GeO₂. A singularity in the relative intensity of the ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ absorption band was observed for a concentration of ～1.5 mol%, compared with 4.5 mol% for Mg²⁺. The photoluminescence emission spectra also reveal a new emission band, at a lower energy than the [Cr]_Li centre, corresponding to this threshold concentration. A charge compensation model is proposed to explain the role of cation impurities and results are compared with those of other valence impurities.