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Transition metal spinel oxides comprised of Earth-abundant Mn and Co havelong been explored for their use in catalytic reactions and energy storage.However, understanding of functional properties can be challenging due todifferences in sample preparation and the ultimate structural properties of thematerials. Epitaxial thin film synthesis provides a novel means of producingprecisely-controlled materials to explore the variations reported in theliterature. In this work, Mn$_{x}$Co$_{3-x}$O$_{4}$ samples from x = 0 to x =1.28 were synthesized through molecular beam epitaxy and characterized todevelop a material properties map as a function of stoichiometry. Films werecharacterized via in situ X-ray photoelectron spectroscopy, X-ray diffraction,scanning transmission electron microscopy, and polarized K-edge X-rayabsorption spectroscopy. Mn cations within this range were found to beoctahedrally coordinated, in line with an inverse spinel structure. Sampleslargely show mixed Mn$^{3+}$ and Mn$^{4+}$ character with evidence of phasesegregation tendencies with increasing Mn content and increasing Mn$^{3+}$formal charge. Phase segregation may occur due to structural incompatibilitybetween cubic and tetragonal crystal structures associated with Mn$^{4+}$ andJahn-Teller active Mn$^{3+}$ octahedra, respectively. Our results help toexplain the reported differences across samples in these promising materialsfor renewable energy technologies.

Jahn-Teller-driven Phase Segregation in Mn$_{x}$Co$_{3-x}$O$_{4}$ Spinel Thin Films