Enhanced hybrid improper ferroelectricity in Fe/Nb cosubstituted Ca3Mn2O7 ceramics

The prototypical Ruddlesden-Popper compound Ca3Mn2O7 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 Ca3Mn2O7 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 Ca3Mn2O7 are reported, and easy polarization switching at room temperature is achieved in Ca-3[Mn-0.5(Fe0.5Nb0.5)(0.5)](2)O-7. The ferroelectric phase transition occurs directly from I4/mmm to A2(1)am at a temperature far above room temperature without intermediate nonpolar Acaa phase. The distinct transition pathway forms the alternating 180 degrees-type ferroelectric domains rather than the irregular 90 degrees-type ferroelastic domains stacked along [001], resulting in easy polarization switching at room temperature. Moreover the enhanced ferroelectric polarization (P-r similar to 2.0 mu C/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.