Revisit of the AC conductivity behavior and the colossal dielectric constant of CaCu3Ti4O12

The frequency dependences of AC conductivity and dielectric constant spectra of polycrystalline CCTO are reappraised. The physical significance of the related spectra has been solved based on the present model constructed by the super-linear and the extended power laws, which is composed of five relaxations: region I ': super-linear power law relaxation, region I: grain intra-clusters relaxation and grain inter-clusters relaxation, and region II: overall intra-clusters relaxation and overall inter-clusters relaxation. It is found that the existence of super-linear power law is related to the electrons hopping in the asymmetric double-well potential caused by the lattice strain. The frequency-independent conductivity in Jonscher's power law equation is redefined, denoted as quasi-DC conductivity at a critical frequency as inter-clusters become percolation clusters between blocks. The blocks can be either grain boundary (region I) or electrode (region II). The obtained activation energy of the quasi-DC conductivity is high ascribed to the electrons surmounting the barriers caused by lattice strain for region I and grain boundary barrier for region II. In comparing the evaluated values of the activation energy of temperature dependence of loss peak frequency and the quasi-DC conductivity, it implies that the occurrence of the loss peak may be attributed to the most charge carriers surmounting the largest barrier arising from lattice strain or grain boundary in order to form the percolation path. The defect structure of the grain boundary barrier of CCTO is proposed.