No abstract

Infrared reflectivity spectra of lead zirconate ceramics were measured in the frequency region of 20-3000 cm-1 and temperature range from 10 to 900 K. The data, extended to lower frequencies by coherent source submillimetre, time-domain terahertz transmission and microwave dielectric spectroscopies, were fitted with the factorized model of the dielectric function. Besides the polar optical phonons, one of which slightly softens in the vicinity of the antiferroelectric phase transition, a central-mode type dispersion was revealed in the 5-25 cm-1 (1011-1012 Hz) range. This central mode is found to be responsible for the strong dielectric anomaly in the paraelectric phase, whereas the lattice phonon contribution does not exceed ε'~300 in the vicinity of Tc. We attribute the origin of the central mode to the lattice disorder caused by strongly anharmonic hopping of Pb ions.

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.

Investigations of impurity centers, electrical resistivity and microstructure of BaTiO 3 ceramics doped with rareearth ions Y, La, Nd, Sm, Dy and Lu at concentrations x~0.001±0.005 were carried out. Electron paramagnetic resonance, X-ray diffraction and electron microscopy were used for measurements. The most intense EPR lines were shown to belong to paramagnetic complexes Fe 3z ±V O and Ti 3z ±Ln 3z (Ln~rare-earth ion, V O~o xygen vacancy). A change in symmetry of the center Fe 3z ±V O at the transition temperature from the ferroelectric to paraelectric phase has been revealed for the ®rst time. Measurements of the dependence of EPR line intensities and electrical resistivity with rare-earth ion concentrations were performed. The observed correlation in their behaviour showed an essential role of the identi®ed paramagnetic complexes in the appearance of BaTiO 3 ceramic semiconducting properties and the positive temperature coef®cient of resistance (PTCR) effect. The latter effect was at a maximum for x#x c where x c #0.002±0.003 is the critical rare-earth ion concentration which determines the excess charge compensation mechanism. Up to x c , the rare earths investigated, (except for the small ion Lu), substitute for barium, and the main compensation mechanism is an electronic mechanism. At high concentrations (xwx c ) in the case of large ions (e.g. La), substitution is at barium sites, with the creation of titanium vacancies, whereas intermediate ions (e.g. Y) begin to substitute for titanium. The in¯uence of impurities on the BaTiO 3 microstructure, including the grain sizes, is discussed.

ron spin resonance, lattice position and dynamic properties of Mn 2+ ions were studied was tric ACS numbers: 61.72.-y, 76.30.Fe, 77.84.Dy Using elect in 0.5 and 2 % manganese doped SrTiO 3 ceramics prepared by conventional mixed oxide method. The measurements showed that Mn 2+ ions substitute preferably up to 97 % for Sr if the ceramics is prepared with a deficit of Sr ions. Motional narrowing of the Mn 2+ ESR spectrum observed when temperature increases from 120 K to 240-250 K that was explained as a manifestation of off-center position of this ion at the Sr site. From the analysis of the ESR spectra the activation energy E a = 86 mV and frequency factor 1/τ 0 ≈ (2-10)⋅10 -14 1/s for jumping of the impurity between symmetrical off-center positions were determined. Both values are in agreement with those derived previously from dielectric relaxation. This proves the origin of dielec anomalies in SrTiO 3 :Mn as those produced by the reorientation dynamics of Mn 2+ dipoles. P 1