Effects of Cu stoichiometry on the microstructures, barrier-layer structures, electrical conduction, dielectric responses, and stability of CaCu3Ti4O12

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 CaCu2.9Ti4O12, 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 CaCu2.9Ti4O12 and CaCu3.1Ti4O12 can also be described using a brick-layer model. Finally, CCTO is found to be unstable and gradually decomposes.