Nanoscale Lamellar Monoclinic Li₂MnO₃ Phase with Stacking Disordering in Lithium-Rich and Oxygen-Deficient Li_1.07Mn_1.93O_4−δ Cathode Materials

The powdered crystalline samples of nominal composition Li_1.07Mn_1.93O_4−δ have been investigated by transmission electron microscopy (TEM) combined with X-ray powder diffraction (XRD) at room temperature. As suggested by the TEM observation, the dominant phase of the particles is a cubic spinel Li_1+αMn_2−αO_4−δ with space group <i>Fd</i>3̅<i>m</i>. A monoclinic Li₂MnO₃ phase with <i>C</i>2/<i>m</i> space group was also identified. Furthermore, the occurrence of nanoscale rotational twinning domains in Li₂MnO₃ with 120° rotation angles, stacked along the [103]_m/[111]_c (“m” and “c” represent the monoclinic and cubic descriptions, respectively) axis was also observed. These nanoscale rotational twining domains are responsible for the pseudo-3-fold axis and their formation is supported by the superstructure reflections in selected-area electron-diffraction (SAED) patterns. Similar patterns were reported in the literature but may have been misinterpreted without the consideration of such domains. Consistent with the TEM observation, the XRD results reveal the increasing percentage of monoclinic Li₂MnO₃ with increasing annealing time, associated with more oxygen vacancies. In addition, the electron beam irradiation during TEM studies may cause the nucleation of nanoscale cubic spinel Li–Mn–O crystallites on the monoclinic Li₂MnO₃ grains. These results provide the detailed structural information about the Li_1.07Mn_1.93O_4−δ samples and advance the understanding of corresponding electrochemical properties of this material as well as other layer structured cathode materials for lithium-ion batteries

Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO₃ perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO₃ network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO₃ network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation

A Polar Corundum Oxide Displaying Weak Ferromagnetism at Room Temperature

Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO₃ (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO₃ has a weak ferromagnetic ground state below 356 Kthis is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO₃

A Polar Corundum Oxide Displaying Weak Ferromagnetism at Room Temperature

Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO₃ (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO₃ has a weak ferromagnetic ground state below 356 Kthis is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO₃

A Polar Corundum Oxide Displaying Weak Ferromagnetism at Room Temperature

Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO₃ (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO₃ has a weak ferromagnetic ground state below 356 Kthis is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO₃