Unusual dielectric properties of CaCu₃Ti₄O₁₂

The A-site ordered perovskite CaCu3Ti₄O₁₂ has been extensively studied since the discovery of the huge dielectric constant by Subramanian and the coworkers. This material is clearly different from conventional ferroelectrics, because the dielectric constant comes from neither structure phase transitions nor soft phonons. The huge value is thus most likely to be extrinsic, and can be explained by a barrier layer capacitor model. An important feature is that the dielectric constant is well controlled in the forms of solid solutions and composites. In this article, we will review various aspects of CaCu₃Ti₄O₁₂ such as the basic physical properties, the impurity effects, and the composites with other dielectrics to show how unusual they are. We further introduce some related oxides that have unconventional physical properties originated from the peculiar crystal structure of AAB₄O₁₂.