Enhanced hybrid improper ferroelectricity in Fe/Nb cosubstituted Ca 3 Mn 2 O 7 ceramics

The prototypical Ruddlesden‐Popper compound Ca 3 Mn 2 O 7  has been predicted to possess hybrid improper ferroelectricity, where the polarization is induced by the condensation of two oxygen octahedral distortion modes. Nevertheless, it is a big challenge to switch the polarization at room temperature in Ca 3 Mn 2 O 7 since the presence of intermediate nonpolar Acaa phase generally leads to the complex domain morphology. Here, the effects of Fe/Nb cosubstitution on hybrid improper ferroelectricity in Ca 3 Mn 2 O 7 are reported, and easy polarization switching at room temperature is achieved in Ca 3 [Mn 0.5 (Fe 0.5 Nb 0.5 ) 0.5 ] 2 O 7 . The ferroelectric phase transition occurs directly from I 4/ mmm to A 2 1 am at a temperature far above room temperature without intermediate nonpolar Acaa phase. The distinct transition pathway forms the alternating 180°‐type ferroelectric domains rather than the irregular 90°‐type ferroelastic domains stacked along [001], resulting in easy polarization switching at room temperature. Moreover the enhanced ferroelectric polarization ( P r ~2.0 μC/cm 2 ) is obtained due to the increased anti‐ferrodistortive displacements of Ca cations at A ‐site, arising from the larger amplitudes of oxygen octahedral distortions. Chemical pressure is emphasized here for the tunability of phase transition, domain morphology, and ferroelectric characteristics, and it provides a useful approach for designing and creating high‐performance improper ferroelectrics.