Distinct roles between complex defect clusters and insulating grain boundary on dielectric loss behaviors of (In3+/Ta5+) co-doped CaCu3Ti4O12 ceramics

The effects of (In3+/Ta5+) co-dopants on the crystalline structure, microstructure, dielectric response, and electrical properties of CaCu3Ti4-x(Ta0.5In0.5)(x)O-12 ceramics are investigated. By increasing co-doping concentration, the lattice parameter is enlarged, while the mean grain size (98.6 -> 11.3 mu m) and the dielectric permittivity (epsilon' approximate to 1.0 x 10(5) -> 3.5 x 10(4)) are significantly reduced, while the low-frequency loss tangent greatly increases. The grain boundary (GB) resistance (R-gb) and capacitance (C-gb) are significantly reduced, corresponding to the variations in tan delta and epsilon', respectively. The decrease in R-gb is attributed to a decrease in the conduction activation energy of the GBs from 0.705 to 0.537 eV, while the electrical properties of the grains slightly change. Surprisingly, tand values in a low-temperature range (e.g., tan delta similar to 0.03 at -60 degrees C and 40 Hz) of the CaCu3Ti4-x(Ta0.5In0.5)(x)O-12 ceramics are much lower than that of the CaCu3Ti4O12 ceramic (tan delta similar to 0.10) even though the low-temperature R-gb values of the co-doped ceramics are greatly decreased by similar to 2-4 orders of magnitude. In a low-temperature range, free electrons can be localized in defect clusters, e.g., In-2(3+) - V-O - Cu2+/Cu+/Ti3+, resulting in a low tand. In a relatively high-temperature range, tan delta is controlled by R-gb.