Atomic Structure, Lattice Properties and Phase Transitions

Strong electronic correlations and lattice-charge coupling in transition metal oxides can give rise to a diverse range of exotic phenomena, including superconductivity and colossal magnetoresistance. These phenomena and a range of other properties have often been linked to polarons, quasiparticles consisting of an excess charge that self-traps by coupling with a virtual phonon cloud. Here, we use electrochemical deintercalation to generate metastable oxygen hole polarons in layered Na_2-xMn₃O₇, which consists of alternating sheets of Na atoms and edge-sharing MnO₆ octahedra. We validate a previous computational prediction of an unusual “split” polaron state, in which holes are shared by neighboring oxygen atoms via a very weak covalent bond, using ground-state Quantum Monte Carlo calculations. This state screens the repulsion between sodium vacancies, which can improve the electrochemical cycling stability by preventing oxygen dimer formation. We further characterize this state through a combination of spectroscopic techniques, including resonant inelastic x-ray scattering, optical absorption, and vibrational spectroscopy from ambient to cryogenic temperatures. Finally, we discuss the implications of this bond-centered polaronic state for future design of structurally robust energy materials.