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Transition metal spinel oxides comprised of earth-abundant Mn and Co have long been explored for their use in catalytic reactions and energy storage. However, understanding functional properties can be challenging due to differences in sample preparation and the ultimate structural properties of the materials. Epitaxial thin film synthesis provides a novel means of producing precisely controlled materials to explore the variations reported in the literature. In this work, MnxCo3−xO4 samples from x = 0 to x = 1.28 were synthesized through molecular beam epitaxy and characterized to develop a material properties map as a function of stoichiometry. Films were characterized via in situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, and polarized K-edge x-ray absorption spectroscopy. Mn cations within this range were found to be octahedrally coordinated, in line with an inverse spinel structure. Samples largely show mixed Mn3+ and Mn4+ character with evidence of phase segregation tendencies with the increasing Mn content and increasing Mn3+ formal charge. Phase segregation may occur due to structural incompatibility between cubic and tetragonal crystal structures associated with Mn4+ and Jahn–Teller active Mn3+ octahedra, respectively. Our results help in explaining the reported differences across samples in these promising materials for renewable energy technologies.
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Probing the structure of vanadium tetracyanoethylene using electron energy-loss spectroscopy
The molecule-based ferrimagnetic semiconductor vanadium tetracyanoethylene (V[TCNE]x, x ≈ 2) has garnered interest from the quantum information community due to its excellent coherent magnonic properties and ease of on-chip integration. Despite these attractive properties, a detailed understanding of the electronic structure and mechanism for long-range magnetic ordering have remained elusive due to a lack of detailed atomic and electronic structural information. Previous studies via x-ray absorption near edge spectroscopy and the extended x-ray absorption fine structure have led to various proposed structures, and in general, V[TCNE]x is believed to be a three-dimensional network of octahedrally coordinated V2+, each bonded to six TCNE molecules. Here, we elucidate the electronic structure, structural ordering, and degradation pathways of V[TCNE]x films by correlating calculations of density functional theory (DFT) with scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS) of V[TCNE]x films. Low-loss EELS measurements reveal a bandgap and an excited state structure that agree quantitatively with DFT modeling, including an energy splitting between apical and equatorial TCNE ligands within the structure, providing experimental results directly backed by theoretical descriptions of the electronic structure driving the robust magnetic ordering in these films. Core-loss EELS confirms the presence of octahedrally coordinated V+2 atoms. Upon oxidation, changes in the C1s-π* peak indicate that C=C of TCNE is preferentially attacked. Furthermore, we identify a relaxation of the structural ordering as the films age. These results lay the foundation for a more comprehensive and fundamental understanding of magnetic ordering and dynamics in these classes of metal–ligand compounds.
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- PAR ID:
- 10594945
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- APL Materials
- Volume:
- 10
- Issue:
- 8
- ISSN:
- 2166-532X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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