Record-Low Coercive Field and Stable Curie Temperature in Bulk Hybrid Improper Ferroelectrics via Synergistic A/B-Site Co-Substitution

Hybrid improper ferroelectrics (HIFs) have attracted increasing scientific attention due to their unique coupling between structural distortion and polarization, offering a highly promising platform for ferroelectrics and single-phase multiferroics with intrinsic electric field-controlled magnetism. While, the intrinsically high coercive fields ( E c ) have been remained as a big issue. In the present work, we demonstrate a synergistic A / B -site co-substitution strategy in La 2 Sr 1- x Ca x (Sc 1- x Fe x ) 2 O 7 ceramics to reshape the ferroelectric energy landscape. Consequently, room-temperature ferroelectricity with a remarkably reduced E c to approximately 6 kV/cm, a record low value for bulk HIFs, was achieved in La 2 Sr 0.7 Ca 0.3 (Sc 0.7 Fe 0.3 ) 2 O 7 ceramics. This reduction is primarily attributed to a significant structural evolution within the ferroelectric domains, specifically the A -site cation disorder induced by co-substitution triggers a distinct transition from coarse stripe domains to refined stripe nanodomains. Moreover, the correlation between Curie temperature and tolerance factor of La 2 Sr 1- x Ca x (Sc 1- x Fe x ) 2 O 7 breaks the linear relationship previously established in A -site cation-disordered La 2 SrSc 2 O 7 , demonstrating that the co-substitution of Ca 2+ and Fe 3+ cations plays a significant role in thermodynamically stabilizing the ferroelectric phase. Furthermore, A weak short-range magnetic order is observed in La 2 Sr 0.7 Ca 0.3 (Sc 0.7 Fe 0.3 ) 2 O 7 ceramics. This work highlights the importance of A / B -site co-substitution in designing novel HIFs and discovering single-phase multiferroic materials.