Anomalous Dielectric Nonlinearity in Niobium and Aluminum Co-doped SrTiO₃ Ceramics with Giant Permittivity and Low Dielectric Loss

High dielectric nonlinearity is a basic requirement for various microwave devices, such as tunable oscillators, phase shifters, varactors, etc. This study focuses on the dielectric nonlinearity of Sr(Nb_0.5Al_0.5)_xTi_1-xO₃ paraelectric ceramics, a giant permittivity and low dielectric loss system. The anomalous dielectric nonlinearity (permittivity increases as the DC bias electric field intensifies) was observed under a low electric field (∼5 kV/cm) for the first time, which cannot be explained by previously proposed theories that were applied for ferroelectrics and relaxor ferroelectrics. The abnormal phenomenon occurred at the same temperature region as the polarization and hopping conduction between Ti³⁺ and Ti⁴⁺. Furthermore, the Ti³⁺ concentration decreased and the size of the defect dipole increased after the sample was polarized. Compared with being sintered in air, the samples sintered in nitrogen feature a high Ti³⁺ concentration, which reduce the starting temperature of anomalous dielectric nonlinearity. It can be proposed that the anomalous dielectric nonlinearity was ascribed to the electronic relaxation polarization and/or produced large defect dipoles of Ti³⁺ under a low electric field. These fundamental understandings will provide an innovative way to adjust the dielectric nonlinearity of materials.