Effect of grain size on phase transitions and dielectric properties in AgNbO3 ceramics

In ferroelectric ceramics, the grain size can influence both structure and properties. In this work, three AgNbO3 ceramics, with average grain sizes ranging from 230 nm to 2.7 μm, were fabricated. Rietveld refinement was performed for their structures using the polar space group Pb21m, consistent with their ferrielectric nature, as evidenced by the E1 peaks observed in current–electric field (I–E) loops. As the grain size increases, the ferrielectric distortion b/a increases, while the cell volume V decreases. All transition temperatures, between the M1-M2a, M2a-M3, and M3-O phases, shift to lower values, and the permittivity peaks associated with these transitions become broader with decreasing grain size. This can be attributed to small and dynamic polar structures with a wide size distribution in fine-grained ceramics, and relatively large sized polar structures with clearer domain walls in coarse-grained ceramics, as supported by transmission electron microscopy results. Under high electric field, double hysteresis loops indicative of antiferroelectric behavior were observed. With decreasing grain size, the field-induced polarization decreases, and both forward and backward reversal fields (EF and EB) increase, which can be attributed to the increased restriction from grain boundaries.