There has been much recent interest in a so-called “giant-dielectric phenomenon” displayed by an unusual cubic perovskite-type material, CaCu3Ti4O12; however, the origin of the high permittivity has been unclear [M. A. Subramanian, L. Dong, N. Duan, B. A. Reisner, and A. W. Sleight, J. Solid State Chem. 151, 323 (2000); C. C. Homes, T. Vogt, S. M. Shapiro, S. Wakimoto, and A. P. Ramirez, Science 293, 673 (2001); A. P. Ramirez, M. A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, and S. M. Shapiro, Solid State Commun. 115, 217 (2000)]. Impedance spectroscopy on CaCu3Ti4O12 ceramics demonstrates that they are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. The giant-dielectric phenomenon is therefore attributed to a grain boundary (internal) barrier layer capacitance (IBLC) instead of an intrinsic property associated with the crystal structure. This barrier layer electrical microstructure with effective permittivity values in excess of 10 000 can be fabricated by single-step processing in air at ∼1100 °C. CaCu3Ti4O12 is an attractive option to the currently used BaTiO3-based materials which require complex, multistage processing routes to produce IBLCs of similar capacity.

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Single phase La-doped BaTiO 3 with the formula Ba 1Ϫx La x Ti 1Ϫx/4 O 3 : 0рxр0.20 was prepared by solid state reaction of oxide mixtures at 1350°C, 3 days, in O 2 . The tetragonal distortion in undoped BaTiO 3 decreased with x and samples were cubic for xу0.05. Both the tetragonal/cubic and orthorhombic/tetragonal transition temperatures decreased with x, but at different rates, and appeared to coalesce at xϳ0.08. The value of the permittivity maximum at the tetragonal/cubic phase transition in ceramic samples increased from ϳ10 000 for xϭ0 at 130°C to ϳ25 000 for xϭ0.06 at ϳϪ9°C. At larger x, the permittivity maximum broadened, showed ''relaxor''-type frequency dependent permittivity characteristics and continued to move to lower temperatures. Samples fired in O 2 were insulating and showed no signs of donor doping whereas air-fired samples were semiconducting, attributable to oxygen loss.

A detailed investigation using variable temperature powder neutron diffraction demonstrates that BiFeO 3 undergoes a phase transition from the ferroelectric α phase

Tetragonal tungsten bronzes (TTBs), an important class of oxides known to exhibit ferroelectricity, undergo complex distortions, including rotations of oxygen octahedra, which give rise to either incommensurately or commensurately modulated superstructures. Many TTBs display broad, frequency-dependent relaxor dielectric behavior rather than sharper frequency-independent normal ferroelectric anomalies, but the exact reasons that favor a particular type of dielectric response for a given composition remain unclear. In this contribution the influence of incommensurate/commensurate displacive modulations on the onset of relaxor/ferroelectric behavior in TTBs is assessed in the context of basic crystal-chemical factors, such as positional disorder, ionic radii and polarizabilities, and point defects. We present a predictive crystal-chemical model that rationalizes composition–structure–properties relations for a broad range of TTB systems.

The behavior of the Pm3m-R3c phase transition in LaAlO 3 ͑T C = 813 K from differential scanning calorimetry measurements͒ has been studied using temperature-dependent measurements of the crystal structure, dielectric relaxation, specific heat, birefringence, and the frequencies of the two soft modes ͑via Raman spectroscopy͒. While all these experiments show behavior near T C consistent with a second-order Landau transition, there is extensive evidence for additional anomalous behavior below 730 K. Below this temperature, the two soft mode frequencies are not proportional to each other, the spontaneous strain is not proportional to the square of the AlO 6 rotation angle, and anomalies are seen in the birefringence. Twin domains, which are mobile above 730 K, are frozen below 730 K. These anomalies are consistent with biquadratic coupling between the primary order parameter of the transition ͑AlO 6 rotation͒ and a second process. From the dielectric results, which indicate a smooth but rapid increase in conductivity in the temperature range 500-800 K, we propose that this second process is hopping of intrinsic oxygen vacancies. These vacancies are essentially static below 730 K and dynamically disordered above 730 K. The interaction between static vacancies and the displacive phase transition is unfavorable. A similar anomaly may be observed in other aluminate perovskites undergoing the same transition.

The electrical properties of two single-phase, lanthanumdoped BaTiO 3 compositions, x ‫؍‬ 0.03 and x ‫؍‬ 0.20, in Ba 1-x La x Ti 1-x/4 O 3 were investigated by impedance spectroscopy after heat treatment in oxygen, argon, and air at 1350°C. Samples heated in oxygen were electrically insulating, whereas those heated in argon lost oxygen and were semiconducting at room temperature, irrespective of x. Samples heated in air showed intermediate electrical properties and also were electrically inhomogeneous; the two compositions showed different electrical behaviors, and a model for each, based on oxygen nonstoichiometry within the ceramics, is proposed. Oxygen deficiency in samples sintered in air was avoided by heating at 1200°C, instead of 1350°C. Alternatively, oxygen lost from ceramics heated in air at 1350°C was regained by postannealing in oxygen at 1350°C.

Temperature dependent impedance spectroscopy enables the many contributions to the dielectric and resistive properties of condensed matter to be deconvoluted and characterized separately. We have achieved this for multiferroic epitaxial thin films of BiFeO 3 ͑BFO͒ and BiMnO 3 ͑BMO͒, key examples of materials with strong magnetoelectric coupling. We demonstrate that the true film capacitance of the epitaxial layers is similar to that of the electrode interface, making analysis of capacitance as a function of film thickness necessary to achieve deconvolution. We modeled non-Debye impedance response using Gaussian distributions of relaxation times and reveal that conventional resistivity measurements on multiferroic layers may be dominated by interface effects. Thermally activated charge transport models yielded activation energies of 0.60± 0.05 eV ͑BFO͒ and 0.25± 0.03 eV ͑BMO͒, which is consistent with conduction dominated by oxygen vacancies ͑BFO͒ and electron hopping ͑BMO͒. The intrinsic film dielectric constants were determined to be 320± 75 ͑BFO͒ and 450± 100 ͑BMO͒.