Garnet-type Li7La3Zr2O12 (LLZO) is considered as a promising solid electrolyte. Nb-doped LLZO ceramics exhibit significantly improved ion conductivity. However, how to prepare the Nb-doped LLZO ceramics in a simple and economical way, meanwhile to investigate the relationship between process conditions and properties in Li7-xLa3Zr2-xNbxO12 ceramics, is particularly important. In this study, Li7-xLa3Zr2-xNbxO12 (LLZNxO, x = 0, 0.2, 0.4, 0.6) ceramics were prepared by conventional solid-state reaction. The effect of sintering process on the structure, microstructure, and ionic conductivity of LLZNxO (x = 0, 0.2, 0.4, 0.6) ceramics was investigated. Due to the more contractive Nb-O bonds in LLZNxO ceramics, the cubic structures are much easier to form and stabilize, which could induce the decreased preparation time. High-performance garnet LLZNxO ceramics can be obtained by optimizing the sintering process with lower calcining temperature and shorter holding time. The garnet samples with x = 0.4 calcined at 850 °C for 10 h and sintered at 1250 °C for 4 h exhibit the highest ionic conductivity of 3.86 × 10−4 S·cm−1 at room temperature and an activation energy of 0.32 eV, which can be correlated to the highest relative density of 96.1%, and good crystallinity of the grains.

La2/3Cu3Ti4O12 ceramics were prepared by the same method of solid-state reaction as CaCu3Ti4O12 ceramics. The structure and dielectric responses for La2/3Cu3Ti4O12 and CaCu3Ti4O12 ceramics were systematically investigated by X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy, and impedance analyzer. Compared with CaCu3Ti4O12 ceramics, La2/3Cu3Ti4O12 ceramics with higher density and refined grain exhibit a high dielectric constant (ε′ ~ 104) and two dielectric relaxations in a wide temperature range. The dielectric relaxation below 200 K with an activation energy of 0.087 eV in La2/3Cu3Ti4O12 ceramics is due to the polyvalent state of Ti3+/Ti4+ and Cu+/Cu2+, while the dielectric relaxation above 450 K with higher activation energy (0.596 eV) is due to grain boundary effects. These thermal activated dielectric relaxations with lower activation energy in La2/3Cu3Ti4O12 ceramics both move to lower temperatures, which can be associated with the enhanced polyvalent structure in La2/3Cu3Ti4O12 ceramics. Such high dielectric constant ceramics are also expected to be applied in capacitors and memory devices.