Experimental evidence is provided that colossal dielectric constants Ју1000, sometimes reported to exist in a broad temperature range, can often be explained by Maxwell-Wagner-type contributions of depletion layers at the interface between sample and contacts or at grain boundaries. We demonstrate this on a variety of different materials. We speculate that the largest intrinsic dielectric constant observed so far in nonferroelectric materials is of order 10 2 .Materials exhibiting a colossal dielectric constant ͑CDC͒ ЈϾ10 3 have recently gained considerable attention. CDC behavior is of technical importance for applications using high-electronic materials, such as random access memories based on capacitive elements. Fundamental interest was initiated by the observation of CDC behavior in some high-T c parent compounds. 1,2 Indeed, CDC behavior may indicate a colossal polarizability, which was invoked in early polaronic and bipolaronic models as a possible mechanism for high-T c superconductivity. 3 During the last decade, similar observations of CDC behavior have been reported in an increasing number of materials, such as transition-metal oxides. 4 -6 Large dielectric constants are expected for ferroelectrics in a narrow temperature range close to T c or for systems with hopping charge carriers yielding a dielectric constant that diverges towards low frequencies. However, in various recent reports 1,2,4 -6 giant values of the dielectric constant were claimed to persist over broad temperature ranges and, when plotted as a function of frequency, revealing an almost constant low-frequency value and a steplike decrease of the dielectric constant towards higher frequencies. This steplike decrease, which is accompanied by a loss peak in the imaginary part of the permittivity, Љ, shifts exponentially to higher frequencies with increasing temperature, characteristic of Debye-like dipolar relaxation with a thermally activated relaxation rate. Several intrinsic physical interpretations have been given. Examples include almost incipient ferroelectricity in high-T c materials, 2 highly polarizable relaxation modes, 5 or a relaxorlike slowing down of dipolar fluctuations in nano-sized domains. 6 However, in Ref. 7 it was suspected that extrinsic effects may play a role in the CDC reported in Ref. 6.In the present paper we provide evidence that many of these observations are not intrinsic in origin and we speculate that most, if not all, of the CDC's reported so far are based on Maxwell-Wagner-type extrinsic effects. 8 We will promote the notion that the most natural explanation of apparent CDC's is contact effects and that in ceramic samples grain boundary effects may play a similar role and further ''enhance'' the dielectric constant. At these interfaces ͑metal-to-insulator contacts, intergrain boundaries͒ depletion layers are formed yielding Maxwell-Wagner-type relaxations when measured by standard dielectric techniques that use metallic electrodes and two-point contact configurations. Thus, while some of the reports may ...

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The temperature dependence of the dielectric nonlinearities in a PMN single crystal and in 9/65/35 PLZT ceramics has been determined by measuring the first and third harmonic response as well as the dielectric behavior as a function of the dc electric field. In zero field a paraelectric-to-glass, and, in a high enough dc field, a glass-to-ferroelectriclike crossover in the temperature dependence of the nonlinear response have been observed. Both crossovers agree with the predictions of the spherical random-bond-random-field model. Relaxors thus undergo in zero field a transition to a spherical glass, while above the critical field a transition into a ferroelectric state occurs.

New lead-free relaxors have been produced from the K0.5Na0.5NbO3–SrTiO3 (KNN-STO) system. The solid solubility within the studied range of compositions (1 - x) K0.5Na0.5NbO3–xSrTiO3 was observed for x up to 0.33. A pseudo-cubic perovskite structure was determined for x = 0.15 to 0.25. The high density and the uniform distribution of fine grains and pores were confirmed by the translucency of these ceramics. The 0.85KNN-0.15STO composition reaches the dielectric permittivity of above 3000 at room temperature. Dielectric spectroscopy measurements revealed that, as with lead-based complex perovskites, the cationic distribution disorder is reflected in relaxorlike properties, thus suggesting possible applications based on this environmentally friendly lead-free ceramic system.