Double-Bilayer polar nanoregions and Mn antisites in (Ca, Sr)<sub>3</sub>Mn<sub>2</sub>O<sub>7</sub>.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 35995791.
- Also identified by DOI 10.1038/s41467-022-32090-w and PMC identifier 9395386.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
The layered perovskite Ca<sub>3</sub>Mn<sub>2</sub>O<sub>7</sub> (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 Ca<sub>2.9</sub>Sr<sub>0.1</sub>Mn<sub>2</sub>O<sub>7</sub> using aberration-corrected scanning transmission electron microscopy (S/TEM). In-situ TEM heating experiments show that the db-PNRs can exist up to 650 °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.