Effect of excess oxygen on crystal structures and dielectric responses of Nd2NiO4+δ ceramics

The crystal structures and dielectric properties were characterized for Nd2NiO4+δ ceramics with different contents of excess oxygen. The results of Rietveld refinement for XRD patterns showed that the structure of Nd2NiO4+δ ceramics was partly changed from orthorhombic to tetragonal phase after annealing the as-sintered sample under the flow of nitrogen. Temperature-stable giant dielectric responses were determined in both ceramics, while dielectric constant, dielectric loss and ac conductivity decreased after annealing the as-sintered ceramics in the flow of nitrogen, which was consistent with the tendency of the content of excess oxygen. A low temperature dielectric relaxation was observed in the annealed ceramics, which was not found in the as-sintered one. The analysis of impedance spectrum indicated that the activation energy increased with decreasing the content of the excess oxygen, and the value of grain interior was similar to that of low temperature dielectric relaxation in the annealed sample. The present work confirmed the correlation between the giant dielectric response and small polaronic hopping process in the oxygen-excess Nd2NiO4+δ ceramics.