AFe 1/2 B 1/2 O3(A-Ba, Sr, Ca; B-Nb, Ta, Sb) ceramics were synthesized and temperature dependencies of the dielectric permittivity were measured at different frequencies. The experimental data obtained show very high values of the dielectric permittivity in a wide temperature interval that is inherent to so-called high-k materials. The analyses of these data establish a Maxwell-Wagner mechanism as a main source for the phenomenon observed.

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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.

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