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Some point defects (i.e., oxygen vacancies) create deep trapping levels in the bandgap of the paraelectric phase ferroelectric crystals. Under applied DC field the traps release electrons via the Poole-Frenkel mechanism and become charged. The electric field of a point charge polarizes the crystal locally reducing its permittivity. In this paper a simple theory is proposed for calculating the DC field dependent apparent (measureable) permittivity of a paraelectric crystal with point charge defects. It is shown that the apparent permittivity of a paraelectric crystal may be sufficiently lower as compared with the defectless crystal. This reduction is in addition to the possible reduction of the apparent permittivity associated with the interfacial 'dead' layers and strain. [ABSTRACT FROM AUTHOR]

In polycrystalline BiMn2O5, a broad thermal stimulated depolarization current curve has been observed in the range from 10 K to 300 K and the pyroelectric coefficient determined. In magnetic susceptibility measurements reported in the literature, features appearing in the pyroelectric coefficient could also be identified for the same temperatures, suggesting a connection between electric and magnetic data above the Néel temperature. A detailed study of the dielectric constant from 240 K to 700 K for an extended range of frequencies revealed a broad maximum at low frequencies, characteristic of relaxor ferroelectrics, following Vogel-Fulcher relation. A freezing temperature of the polar nanoregions Tf = 512 K has been determined. This high temperature ferroelectric behavior is attributed to the Bi3+ in the distorted BiO8 cage. [ABSTRACT FROM AUTHOR]

Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature- dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric- to-antiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we propose that the commonly observed two dielectric anomalies are attributed to thermal evolution of ferroelectric polar nanoregions of R3c and P4bm symmetry, which coexist nearly throughout the entire temperature range and reversibly transform into each other with temperature. [ABSTRACT FROM AUTHOR]

In ferroelectric superlattices (SLs), the short-range interactions and intermixing between contacting dissimilar materials can lead to the formation of interfacial nanolayers with physical properties different from those of both constituents. We demonstrate theoretically that such nanolayers can give rise to a sizable dependence of the SL permittivity on the period even in the paraelectric regime. The results of dielectric measurements performed for coherent Ba0.8Sr0.2TiO3-Ba0.4Sr0.6TiO3 SLs are qualitatively consistent with the theoretical predictions. At the same time, the experimental data indicate that other factors also contribute to the variation of the permittivity with the period in ferroelectric superlattices and multilayers. [ABSTRACT FROM AUTHOR]

We investigate the dependence of Monte Carlo ( MC) calculations for two-dimensional ferroelectrics on random-number generators by using the fixed seeds and variable time and date seeds. The MC results obtained by using different seeds are elucidated using auto-correlation functions and the temperature dependence of transverse dielectric constants. The dielectric properties of the pseudo-spin Ising model obtained using the Metropolis statistical methods are discussed by the quality of randomness. [ABSTRACT FROM AUTHOR]

Ferroelectric epitaxial Pb(Zr,Ti)O3 (PZT) layers were grown by pulsed laser deposition on SrTiO3/GaAs templates fabricated by molecular beam epitaxy. The templates present an excellent structural quality and the SrTiO3/GaAs is abrupt at the atomic scale. The PZT layers contain a- and c-domains, as shown by X-Ray diffraction analyses. Piezoforce microscopy experiments and macroscopic electrical characterizations indicate that PZT is ferroelectric. A relative dielectric permittivity of 164 is extracted from these measurements. [ABSTRACT FROM AUTHOR]

The "lone" 6s electron pair often plays a key role in determining the structure and physical properties of compounds containing sixth-row elements in their lower oxidation states: Tl+, Pb2+, and Bi3+ with the [Xe]4f145d106s² electronic configuration. The lone pairs on these ions are associated with reduced structural symmetries, including ferroelectric instabilities and other important phenomena. Here we consider the isoelectronic auride Au- ion with the [Xe]4f145d106s² electronic configuration. Ab initio density functional theory methods are employed to probe the effect of the 6s lone pair in alkali-metal aurides (KAu, RbAu, and CsAu) with the CsCl structure. The dielectric constants, Born effective charges, and structural instabilities suggest that the 6s lone pair on the Au- anion is stereochemically inert to minor mechanical and electrical perturbation. Pressures greater than 14 GPa, however, lead to reorganization of the electronic structure of CsAu and activate lone-pair involvement and Au-Au interactions in bonding, resulting in a transformation from the cubic CsCl structure type to an orthorhombic Cmcm structure featuring zigzag Au--Au chains. [ABSTRACT FROM AUTHOR]

Nanocrystalline powders of the composition Pb(Zr0.52Ti0.48)O3 were obtained by Mechanical alloying (high-energy ball milling). X-ray diffraction studies show that these compounds are completely into the perovskite phase. Detailed studies of electrical and mechanical properties of PZT as a function of temperature (and frequency) showed the high permittivity of 20653 at Curie transition temperature. Temperature variation of longitudinal modulus and internal friction of these ceramics at 104 kHz frequency were studied in the wide temperature range of 30°C-420°C. The internal friction measurements showed sharp stress induced relaxation peaks in the present composition corresponding to those temperatures where the minima were noticed in temperature variation of longitudinal modulus behavior. This dielectric and internal friction behaviour was explained in the light of polaron hopping mechanism and structural phase transitions in the present piezoelectric compositions. [ABSTRACT FROM AUTHOR]