Quantitative evaluation of annular bright-field phase images in STEM

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 Ca_2.9Sr_0.1Mn₂O₇ using aberration-corrected scanning transmission electron microscopy (S/TEM). In-situ TEM heating experiments show that the db-PNRs can exist up to 650 °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.

Towards 3D Mapping of BO6 Octahedron Rotations at Perovskite Heterointerfaces, Unit Cell by Unit Cell

The rich functionalities in the ABO3 perovskite oxides originate, at least in part, from the ability of the corner-connected BO6 octahedral network to host a large variety of cations through distortions and rotations. Characterizing these rotations, which have significant impact on both fundamental aspects of materials behavior and possible applications, remains a major challenge at heterointerfaces. In this work, we have developed a unique method to investigate BO6 rotation patterns in complex oxides ABO3 with unit cell resolution at heterointerfaces, where novel properties often emerge. Our method involves column shape analysis in ABF-STEM images of the ABO3 heterointerfaces taken in specific orientations. The rotating phase of BO6 octahedra can be identified for all three spatial dimensions without the need of case-by-case simulation. In several common rotation systems, quantitative measurements of all three rotation angles are now possible. Using this method, we examined interfaces between perovskites with distinct tilt systems as well as interfaces between tilted and untilted perovskites, identifying an unusual coupling behavior at the CaTiO3/LSAT interface. We believe this method will significantly improve our knowledge of complex oxide heterointerfaces.