Double-Bilayer polar nanoregions and Mn antisites in (Ca, Sr)3Mn2O7

The layered perovskite Ca3Mn2O7 (CMO) is a hybrid improper ferroelectric candidate proposed for room temperature multiferroicity, which also displays negative thermal expansion behavior due to a competition between coexisting polar and nonpolar phases. However, little is known about the atomic-scale structure of the polar/nonpolar phase coexistence or the underlying physics of its formation and transition. In this work, we report the direct observation of double bilayer polar nanoregions (db-PNRs) in Ca2.9Sr0.1Mn2O7 using aberration-corrected scanning transmission electron microscopy (S/TEM). In-situ TEM heating experiments show that the db-PNRs can exist up to 650 degrees C. Electron energy loss spectroscopy (EELS) studies coupled with first-principles calculations demonstrate that the stabilization mechanism of the db-PNRs is directly related to an Mn oxidation state change (from 4+ to 2+), which is linked to the presence of Mn antisite defects. These findings open the door to manipulating phase coexistence and achieving exotic properties in hybrid improper ferroelectric. The competition between the polar and nonpolar phase in the prototypical hybrid improper ferroelectric crystal Ca3Mn2O7 leads to exotic properties. Here, the authors directly imaged the crystal at atomic resolution to understand its nanostructure and discovered the double bilayer polar nanoregion.

Automated summary

In this study, the authors directly observed double bilayer polar nanoregions in Ca2.9Sr0.1Mn2O7 using aberration-corrected scanning transmission electron microscopy. Electron energy loss spectroscopy studies and first-principles calculations revealed that the stabilization mechanism of the nanoregions is directly related to a change in the oxidation state of manganese, which is linked to the presence of manganese antisite defects.