Paramagnetic dipole centers inKTaO3:Electron-spin-resonance and dielectric spectroscopy study

Abstract

Electron-spin-resonance ͑ESR͒ and dielectric spectroscopy techniques have been applied to study the dipole centers connected with Mn 2ϩ and Fe 3ϩ impurity ions substituted for K ϩ in the incipient ferroelectric KTaO 3 . It was shown that the reorientations of paramagnetic dipole complexes Fe 3ϩ -O I (O I is the interstitial oxygen͒ give rise to dielectric losses near TӍ185 K at the frequency 1 kHz. Their activation energy E r ϭ0.34 eV was obtained both from dielectric and ESR measurements. The peak of dielectric losses at TӍ55 K was observed in KTaO 3 doped by low concentrations of Mn 2ϩ (nϷ0.01 at. %͒. At nտ0.3 at. % a pronounced peak of dielectric susceptibility similar to that in KTaO 3 :Li was revealed in addition to the aforementioned losses peak. The studies of electric field and temperature dependence of Mn 2ϩ ESR intensities with respect to the local Lorentz field had shown that the dipole moment 1.4 eÅ can be associated with Mn 2ϩ ion. The origin of this dipole moment was shown to be off-center displacement ⌬Ӎ0.9 Å of Mn 2ϩ from the K ϩ site in one of six ͓001͔-type directions. The relaxation rate of these dipoles after the polarizing electric field switching off was directly measured by the ESR method. It was described by Arrhenius law with activation energy E r ϭ0.104 eV, which is close to the value obtained from dielectric measurements. Possible sources of dielectric losses in nominally pure KTaO 3 single crystals in the vicinity of TϷ40 K are analyzed and discussed on the basis of both present work data and earlier results collected from the literature.

Influence of point defects in KTaO3 on low-temperature dielectric relaxation

“…It has been established that electronic/orbital interactions in Mn-doped KTaO 3 cause the dopant Mn 2+ ions to off-center along the [1 0 0] direction from the potassium Vykoff 1b position for as much as 0.9 Å. [13][14][15] This means that the Mn 2+ ion occupies a position somewhat in the middle of the adjacent oxygen planes and can be regarded as a sort of Frenkel defect. 7 We studied the sample with 10% Na-substitution, for which the synthesis yielded a single-phase reaction product (Fig.…”

“…[13][14][15] This local symmetry distortion has been associated with the large Mn displacement rather than to the charge compensation and cation vacancy appearance, which may only occur in distant ligand spheres. In the case of Na + substitution (1.39 Å for CN = 12) some local symmetry change would also be expected due to a slight Na off-centering.…”

EPR parameters and defect structures of the off-center Ti3+ ion on the Sr2+ site in neutron-irradiated SrTiO3 crystal

“…So, we suggest that the Ti 3+ off-center displacement from Sr 2+ site is not along one of C 2 axes, but along one of C 4 axes, thus, the symmetry and symmetry axes for the low-symmetry Ti 3+ centers in both phases of SrTiO 3 are consistent with those in the experiments. This displacement direction is the same as that of Mn 2+ ion in similar perovskite crystal KTaO 3 (where because of its smaller radius, Mn 2+ ion does not occupy the ideal K + site, but is shifted by about 0.9 Å along one of six [ first-principles study [3], the density functional theory in generalized gradient approximation (GGA) [4] and the dipole moment study [5]) and can be regarded as possible. In order to confirm this suggestion and to explain reasonably the anisotropic g factors and hyperfine structure constants A for the two Ti 3+ centers in both phases of SrTiO 3 crystals (note: these EPR parameters have not been explained reasonably until now), in this paper, we calculate these EPR parameters form the high-order perturbation formulas of EPR parameters for 3d 1 ions in tetragonal and rhombic symmetries.…”

The Local Order in KTaO3Admixtured by the Ions of Li+

“…The observed dependences show an influence of Li + on the dielectric properties of the crystal KTaO 3 . For the concentration x = 0.03 the temperature dependence of ε is typical of the relaxors, with characteristic dielectric dispersion [14][15][16][17]19], with the fast increase in the electric permittivity ε value with lowering temperature till getting the value of the real part of the electric permittivity ε . In the low temperatures (T < 40 K) changes of the electric permittivity ε were not observed for this concentration.…”

“…In the small amount of the admixture (x < 0.022) the induced electric dipole interacts with each other by the elasticity coupling. This coupling leads to the local electric order [12][13][14][15][16][17][18][19].…”