Microstructure, chemical composition, and dielectric response of CaCu₃Ti₄O₁₂ ceramics doped with F, Al, and Mg ions

Ceramics with nominal chemical composition CaCu₃Ti₄O₁₂ (CCTO), CaCu₃Ti_3.96Al_0.04O_11.96F_0.04 (CCTOAF), and Ca_0.98Mg_0.08Cu_2.94Ti_3.96Al_0.04O_11.96F_0.04 (CCTOMAF) were prepared by the solid-state reactions technique. Using SEM, EDX, XPS, EPR, NMR, and complex impedance spectroscopy, the microstructure, elements distribution, chemical composition of grains and grain boundaries, and the dielectric response of ceramics were investigated. In the ССТО, CCTOAF, and CCTOMAF series, the average grain size increases, the degree of copper segregation at the grain boundaries is inversely related to grain size, and the dielectric loss decreases from 0.071 to 0.047 and 0.030, respectively, while dielectric permittivity ε' at 1 kHz is 5.6 × 10⁴, 7.1 × 10⁴, and 4.3 × 10⁴, respectively. Additives of Al, Mg, F and milled particles (ZrO₂, Al₂O₃, and SiO₂) can either partially introduce into the perovskite structure or form low-melting eutectics at the grain boundaries, causing abnormal grain growth. The presence of copper ions in various oxidation states, as well as evidence of exchange spin interactions between them, was confirmed in all samples.