Electron/hole pinning effects, localization, and dielectric properties in (Nb,Cu) co-doped SnO₂ ceramics

Dielectric ceramics with charge carrier pinning effects have exhibited high dielectric permittivity and low dielectric loss. However, the detailed mechanism has not yet been fully clear, such as the types of carriers, certain parameters within the pinning effect, etc. To address these issues, a prototype high dielectric permittivity system, i.e. (Nb,Cu) co-doped rutile NbCuSn_1-O₂ (x = y = 0, 0.01, 0.02, 0.03, 0.04, 0.05) ceramics, was explored. The findings indicate that the charge transfer and distribution were inferred by the combination of Bader charge difference (BCD) and the iso-surface of the excitation state. Moreover, the pinning effect was expressed by the hole delocalization index (HDI) and the electron delocalization index (EDI) at the specific ionic position. The ceramics with a low doping level (x = y ≤ 0.03) exhibited a large dielectric permittivity (>10³) and a loss of 0.1 (1 kHz, room temperature). The dielectric permittivity decreased while the loss increased with increasing doping levels. A series of characterization studies combined with theoretical calculations suggested that the ceramic with a low doping level (x = y ≤ 0.03) was dominated by the electron pinning effect, while the hole pinning effect was observed at the higher doping level. This work provides new insights into the charge pinning defect dipole models to illustrate the dielectric properties of ceramics.