Visualization of air-induced oxidation in single crystalline LiFe0.6Mn0.4PO4 nanowires with carbon sheath using soft X-ray spectromicroscopy

Single crystalline LiFe_0.6Mn_0.4PO₄ (LFMP) nanowires with carbon sheath were analyzed by scanning transmission X-ray microscopy (STXM) with a spatial resolution of around 130 nm. The pinpoint Fe L-edge X-ray absorption spectroscopy (XAS) spectra in the STXM images revealed the natural oxidation of Fe²⁺ to Fe³⁺ near the tip of the nanowire by air exposure. The pinpoint O K-edge XAS spectra simultaneously changed with the Fe L-edge XAS in the STXM mapping because of the hybridization between the O 2p and Fe 3d orbitals. On the other hand, the Mn L-edge XAS spectra showed an almost uniform Mn²⁺ state all over the nanowires, suggesting that the Mn²⁺ state is stable against the natural oxidation in contrast to the Fe²⁺ state. We have thus demonstrated that STXM is a useful technique to visualize oxidized areas of materials and to reveal detailed information about their electronic structure.