Intercalation-Induced Oxygen Hole Polarons in Layered Transition Metal Oxide Na_2-xMn₃O₇

Transition metal oxides have been studied for decades due to their complex interplay of charge, spin, and lattice degrees of freedom, which can give rise to a range of exotic $phenomena{^1}$. They have also found practical applications as cathode materials for energy storage due to their ability to achieve high energy $density{^2}$. Intercalation of alkali ions into these materials can result in injection of charge and formation of polarons, quasiparticles consisting of an excess charge that self-traps by coupling with a virtual phonon cloud. Here, we perform electrochemical deintercalation, or removal of sodium atoms, in layered $Na_{2-x}Mn{_3}O{_7}$, which consists of alternating sheets of Na atoms and edge-sharing $MnO{_6}$ octahedra. Previous work in this system demonstrated the formation of metastable oxygen hole polarons lasting up to $7 days{^3}$, indicating that the polarons are stabilized against O-O dimerization by a kinetic $barrier{^4}$. Density functional theory (DFT) predicted an unusual “split” polaron state, in which holes are shared by neighboring oxygen atoms via a very weak covalent bond4. We validate this prediction using ground-state Quantum Monte Carlo calculations, performing an exhaustive study across DFT functionals and parameters. We further characterize this state through a combination of spectroscopic techniques, including resonant inelastic x-ray scattering, optical absorption, and vibrational spectroscopy. Finally, we discuss the implications of this state for further investigation of metastable polaron configurations and future design of energy materials.

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