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

TL;DR

This study investigates how Jahn-Teller effects induce phase segregation in Mn-Co spinel thin films, revealing structural incompatibilities that influence their properties for energy applications.

Contribution

It provides a detailed analysis of phase segregation mechanisms driven by Jahn-Teller effects in epitaxial Mn-Co spinel films, using advanced characterization techniques.

Findings

Phase segregation increases with Mn content.

Mn$^{3+}$ and Mn$^{4+}$ coexist in the films.

Structural incompatibility causes phase segregation.

Abstract

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 of 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, Mn$_{x}$Co$_{3-x}$O$_{4}$ 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 Mn$^{3+}$ and Mn$^{4+}$ character with evidence of phase segregation tendencies with increasing Mn content and increasing Mn$^{3+}$ formal charge. Phase segregation may occur due to structural incompatibility between cubic and tetragonal crystal structures associated with Mn$^{4+}$ and Jahn-Teller active Mn$^{3+}$ octahedra, respectively. Our results help to explain the reported differences across samples in these promising materials for renewable energy technologies.