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.
The dielectric parameters and electric conductivity of (NH2(CH3)2)2CoCl4 crystals are investigated. Existence of the ferroelectric phase was observed below Tc = 258 K. The occurrence of phase transitions at 244, 314, and 435 K is confirmed. The investigated crystals also manifest anomalously high conductivity commensurate with that in semiconductors. Its value as well as region of existence strongly depend on the preceding history of the sample. It is shown that the anomalous conductivity is connected with translocation of protons which participate in the formation of hydrogen bonds.
The mixed crystal Rb~-"(NH4)"HzAs04 (RADA) has been investigated for several values of x(x =0, 0.12, 0.15, and 0.20) by measuring the complex dielectric permittivity along the a tetragonal axis in the temperature range from 3 to 300 K and frequency range from 1 Hz to 30 kHz. We find coexistence of ferroelectric and proton-glass order in RADA for x =0.12 and probably at x 0.15. The long-range ferroelectric order is not destroyed by the onset of proton-glass ordering. This behavior is similar to that of other proton-glass systems and of the magnetic Ising glass systems reported by Wong etal. in Feo»Mg0. 45C12 and by Yoshizawa etal. in Fe Mn]-Ti03.
EPR studies of superconducting fullerites Me&, (Me = K and Rb) are performed for temperatures ranging frpm 4 to 300 K and compositions, of x = 2 to 5. A paramagnetic center is an ion radical fullerene C,, which may occur as a cation C;, or anion Cyh6 of different valency. The EPR signal of C;, weakly depends on temperature whereas the signal of C& in a complex is strongly dependent both on temperature and composition. A model of a paramagnetic center related to a wandering electron (hole) localized on the C,, fullerene ball is presented.
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