Grain Size-Dependent Defect and Domain Evolution in Lead Titanate-Based Relaxor Ferroelectrics

Ferroelectric materials are widely used in diverse applications, where their performance is strongly dominated by grain size. Here, dense Er-doped lead titanate-based relaxor ferroelectrics were synthesized via spark plasma sintering, enabling precise grain size control from 0.9 to 11.1 μm. Fine-grained ceramics exhibit high defect concentrations and internal stress, stabilizing the tetragonal phase and resulting in weak, disordered polarization with low domain wall density and constrained mobility. At intermediate grain sizes, dense nanodomain networks with narrow walls (∼150 nm) form, allowing sharp and reversible polarization switching. Coarse-grained ceramics develop hierarchical, web-like domains with thicker walls (∼400 nm), reducing the pinning effect and enhancing wall mobility. Both saturation and remanent polarizations increase with grain size up to 5.4 μm before plateauing, while the piezoelectric coefficient rises by 200%, reaching 723 pC N^-1. These results demonstrate grain-size engineering as an effective route to optimize domain wall structure and relaxor ferroelectric performance.