Investigation of micro- and nanoscale barrier layer capacitance mechanisms of conductivity in CaCu3Ti4O12via scanning probe microscopy technique

Abstract

In this work we disclose micro- and nanoscale origins of the unusually high dielectric constant characteristic of CaCu₃Ti₄O₁₂ (CCTO) ceramic by using the Scanning Probe Microscopy (SPM) technique. Two main mechanisms responsible for the colossal dielectric constant specific to the CCTO compound have been revealed. There is a microscale barrier layer capacitance (MBLC) mechanism, attributed to the potential grain-to-grain barriers, and a nanoscale barrier layer capacitance (NBLC) mechanism, attributed to the potential barriers created by the structural defects such as twinning or slip planes. Using the contact spreading resistance mode of SPM, we have found two types of surface morphology which, being originated from planar defects, can be related to the NBLC mechanism. A clear confirmation of NBLC as the origin of the huge dielectric constant in CCTO has been obtained via the local current–voltage dependence measurements. By using this method, we have found the existence of two sources of conductivity (charge transfer and charge hopping) which simultaneously contribute to the NBLC mechanism. These sources (providing semiconducting and n-type conducting behavior, respectively) have been associated with the different stacking faults predicted for CCTO. The present work promotes a general understanding of anomalous colossal dielectric constant behavior in CCTO material at the macro- and nanoscale levels.