Extrinsic models for the dielectric response of CaCu3Ti4O12

The large, temperature-independent, low-frequency dielectric constant recently observed in single-crystal CaCu3Ti4O12 is most plausibly interpreted as arising from spatial inhomogenities of its local dielectric response. Probable sources of inhomogeneity are the various domain boundaries endemic in such materials: twin, Ca ordering, and antiphase boundaries. The material in, and neighboring, such boundaries can be insulating or conducting. We construct a decision tree for the resulting six possible morphologies, and derive or present expressions for the dielectric constant for models of each morphology. We conclude that all six morphologies can yield dielectric behavior consistent with observations. Thus, present experimental findings are insufficient to establish the internal morphology responsible for the remarkable properties of CaCu3Ti4O12, and we suggest further experiments to distinguish among them.