We describe a material, cubic CaCu₃Ti₄O₁₂, which exhibits a large dielectric response, the temperature-dependence of which has not been seen, to our knowledge, in any existing material. This compound possesses a low-frequency dielectric constant,ε∼10⁴[1], which is only weakly varying in the temperature range 100–400 K. Below, however, there is an abrupt 100-fold reduction in the value ofε. X-ray diffraction and thermodynamic data argue against an explanation in terms of ferroelectricity, i.e. the collective ordering of local dipole moments. Both the low-frequency dielectric response as well as Raman scattering data suggest the existence of highly polarizable relaxational modes with a characteristic gap energy of 28 meV.

There has been much recent interest in a so-called “giant-dielectric phenomenon” displayed by an unusual cubic perovskite-type material,CaCu 3 Ti 4 O 12 ;  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, Science293, 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 onCaCu 3 Ti 4 O 12   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.CaCu 3 Ti 4 O 12   is an attractive option to the currently usedBaTiO 3   -based materials which require complex, multistage processing routes to produce IBLCs of similar capacity.

CaCu₃Ti₄O₁₂ (CCTO) was prepared by the solid state technique. The sample was calcined at 900 °C/12 h and sintered at 1050 °C/24 h, then subjected to XRD to ensure CCTO formation. The microstructure was observed by SEM. XRD results identified both samples as single phase CCTO, whereas the microstructure shows abnormal grain growth and large pores. Sintering was studied in the temperature range of 950–1050 °C for 3–12 h. Increasing sintering temperature enhances the density and secondary formation of Cu₂O. A clear grain boundary and dense microstructure were observed. The results show that the sample sintered at 1040 °C/10 h yields a clearly uniform grain size with the highestε_r (33,210).

Abstract

CaCu₃Ti₄O₁₂ (CCTO) ceramics have been processed by solid state reaction and sintered at 1100 °C for different times. A clear increase of the dielectric constant of the material up to values of 6 × 10⁴ has been observed with the sintering time. This increase is accompanied by a limited grain growth and intergranular Cu-oxide phase thickness reduction. The disappearance of the Cu-oxide phase is caused by the incorporation of Cu cations into the grains, contributing to the increase of the dielectric constant. Raman spectroscopy shows the decrease of TiO₆ octahedral rotational modes with the sintering time due to the incorporation of Cu cations into the CCTO grains. XANES measurements show that the Cu main oxidation state is Cu²⁺ and does not change with the sintering time. The fitting of the experimental dielectric constant to the Internal Barrier Layer Capacitance (IBLC) model reveals the change of the intergranular phase dielectric constant, caused by a compositional change due to the incorporation of Cu into the CCTO grains.

The effect of processing on the dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) was researched. CaCu₃Ti₄O₁₂ was prepared using conventional ceramic solid state reaction processing techniques. Powders mixed via mortar and pestle yielded CCTO with a room temperature permittivity of 11,700 and a loss of 0.047, which is similar to reported values in the literature. However, attrition-milled powders led to CCTO with permittivities close to 100,000 which are in the same range reported for single crystal CCTO. Increasing sintering temperature in the range from 990 to 1050 °C led to an increase in both the dielectric constant (714 to 82,450) and loss (0.014 to 0.98). Increasing sintering times also led to substantial improvements in permittivity. Grain size and density differences were not large enough to account for the enhancement in dielectric constant. The colossal effective dielectric constant of close to one million at room temperature was measured after annealing in flowing argon at 1000 °C. Based on the observations, it is believed that the primary factor affecting dielectric behavior is the development of internal defects. It appeared that higher defect concentration within the ‘core’ of a grain results in higher conductivity of the core and therefore, higher effective dielectric constant but also higher loss. The correlation between grain size and dielectric constant was also observed.

Abstract

The presence of Cu³⁺ ions in CaCu₃Ti₄O₁₂ (CCTO) has been determined using X-ray photoelectron spectroscopy and the Cu deficiency of CCTO confirmed. Electron hopping between Cu²⁺ and Cu³⁺ is proposed as the origin of the semiconducting nature of CCTO. A new model of CCTO with Cu deficiency being fundamental for the development of a barrier-layer structure is proposed. The enhancement of the densification rate, the occurrence of discontinuous grain growth, and the increase of grain boundary resistivity are found to be related to the presence of Cu ions at grain boundaries. For samples of CaCu_2.9Ti₄O₁₂, the lower dielectric constant can be attributed to the smaller grain boundary areas and the difficulty in the development of domains inside the fine grains. The dielectric responses of CaCu_2.9Ti₄O₁₂ and CaCu_3.1Ti₄O₁₂ can also be described using a brick-layer model. Finally, CCTO is found to be unstable and gradually decomposes.