  The origins of an ultrahigh dielectric constant in polycrystalline
CaCu3Ti4O12 (CCTO) was studied using the combination of impedance spectroscopy,
electron microscopy, and scanning probe microscopy (SPM). Impedance spectra
indicate that the transport properties in the 0.1 Hz .. 1 MHz frequency range
are dominated by a single parallel resistive-capacitive (RC) element with a
characteristic relaxation frequency of 16 Hz. Dc potential distributions
measurements by SPM illustrate that significant potential drops occur at the
grain boundaries, which thus can be unambiguously identified as the dominant RC
element. High frequency ac amplitude and phase distributions illustrate very
weak contrast at the interfaces, which is indicative of strong capacitive
coupling. These results demonstrate that the ultrahigh dielectric constant
reported for polycrystalline CCTO materials are related to the grain boundary
behavior.

Materials Science