Volume 4 Issue 2 - April 25, 2008
Effects of Cu stoichiometry on the micro structures, barrier-layer structures, electrical conduction, dielectric responses, and stability of CaCu3Ti4O12
TT Fang (Fang, Tsang-Tse)*, LT Mei (Mei, Li-Then), HF Ho (Ho, Hei-Fong)

Department of Materials Science and Engineering

ACTA MATERIALIA 54 (10): 2867-2875 JUN 2006

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Recently the unusual cubic perovskite-related CaCu3Ti4O12 (CCTO) has attracted considerable attention and been much studied because it possesses an extraordinarily high dielectric constant at room temperature of about 104–105 and good temperature stability over a wide temperature range from 100 to 600 K. Both properties are significant for device implementation. Although CCTO possesses such a high permittivity, the intrinsic nature of the polarization has been questioned and essentially is inconsistent with the results based on first-principles calculations. Moreover, unlike the common ferroelectric ceramics, no structural or ferroelectric transition has been observed in CCTO over a wide temperature range, attributed to the tilting of TiO6 octahedra for the accommodation of the unusual square-planar Cu ions on A sites. Structural and spectroscopic investigations of the dielectric responses of CCTO suggest extrinsic effects as contributions from the interface. Also, a barrier-layer structure with semiconducting areas encircled by insulating layers was established to explain the very large capacitance behavior of both polycrystalline and single-crystal CCTO. The dielectric responses of such a structure essentially are characterized by the so-called interfacial polarization, behaving as the well-known Maxwell–Wagner relaxation.

There have been a few reports suggesting the semiconducting nature of CCTO, the origin of which, however, was not justified. Regarding the insulating barrier layer of single crystals, in addition to locally planar twin boundaries, antiphase and compositional-ordering domain boundaries were proposed. The twin boundaries, investigated using X-ray methods, were not fully justified, and recent detailed transmission electron microscopy (TEM) studies did not reveal twin domains of a size larger than 10 nm in CCTO films. The antiphase and compositional-ordering domain boundaries, however, have not yet been observed. For polycrystalline ceramics, although reasons related to oxygen vacancies inducing the insulating barrier layers at the grain boundaries were suggested, the mechanism has still not been fully elucidated. Insulating barrier layers arising from the Maxwell–Wagner relaxation of the depletion layers at the interface between sample and electrode contacts were also considered as possible contributing factors to the anomalously high dielectric constant of CCTO. However, the electrode contact effect was also not conclusive. Other reasons for the electrode contact effect to be discounted are that the dielectric responses of CCTO are strongly affected by dopants and the microstructures observed in this investigation and other reports.

The development of insulating barrier layers at grain boundaries is widely accepted, and the presence of internal barrier layers inside grains or crystals has also recently been discovered. There have also been a few reports on the observation of domains inside grains which, however, did not confirm the domain structures observed using scanning electron microscopy (SEM). Moreover, essentially these domains cannot be considered as a barrier-layer structure, in particular for a single domain boundary inside a grain. Although it has been proposed that the Cu stoichiometry is related to the dielectric responses, electrical conduction, and the internal domains, the real role of Cu stoichiometry in the very large dielectric responses of CCTO still remains unresolved. Recent results for impurity- doped CCTO revealed that the dielectric responses are strongly related to Cu species.

In the present investigation the effects of Cu stoichiometry on the development of the barrier-layer structures, microstructural evolution, electrical conduction, and dielectric behavior of polycrystalline CCTO are studied, and the stability of CCTO is also explored. The following conclusions can be drawn. It is confirmed that the reaction of CuO is sluggish during the formation of CCTO, and the presence of Cu ions at the grain boundaries enhances the densification, induces discontinuous grain growth, and enhances the grain boundary resistivity. The presence of Cu3+ ions has been established from the Cu 2p3/2 XPS spectra, and it has also been confirmed that CCTO is Cu-deficient. Electron hopping between Cu2+ and Cu3+ is proposed as the origin of the semiconducting nature of CCTO. The Cu deficiency of CCTO is fundamental for the formation of insulating barrier-layer structures for developing the interfacial polarization, this is because Cu vacancies form superlattices for the development of strained domains and the presence of Cu ions at grain boundaries. The dielectric constant is lower for CC(2.9)TO, which can be attributed to the smaller grain boundary areas and the difficulty for domains to develop inside fine grains. This further supports the idea that the large grains play a significant role in enhancing the dielectric constant. The dielectric responses of CC(2.9)TO and CCTO(3.1) can also be described by the brick-layer model. They are mainly determined by the thickness and conductivity of the domain boundaries and grain boundaries and increase with increasing temperature. CCTO is not stable and gradually decomposes.
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