Synergistic Enhancement of Ferroelectricity in Eu/Fe Codoped Bi0.5Na0.5TiO₃ Thin Films: Mechanistic Insights via Luminescent Probing and Defect Engineering

Defect engineering is a pivotal strategy for optimizing the polarization intensity in ferroelectrics. However, the synergistic effects of codoping transition metals and rare-earth ions into ferroelectric systems remain underexplored, and the resulting atomic-scale coupling mechanisms are not fully deciphered. This study demonstrates a synergistic enhancement of ferroelectric polarization in lead-free Bi0.5Na0.5TiO3 (BNT) thin films through Eu/Fe codoping while elucidating the underlying mechanism via luminescent probing and simulation calculation. Remarkably, codoping achieves a 130.9% enhancement in saturated polarization (Ps). The synergistic enhancement derives from the formation of [Eu3+-Fe3+] defect pairs and the amplification of [TiO6] octahedral distortion. This local structural asymmetry variations are characterized by in situ luminescent probes of Eu3+ and first-principles calculation. It is confirmed that the coupling mechanism is related with the [Eu3+-Fe3+] defect-pair-induced electron density redistribution and dipole-dipole coupling energy transfer process. These findings provide atomic-scale insights into rare-earth/transition-metal coupling mechanisms and advance defect engineering strategies for high-performance multifunctional ferroelectric devices.