The tunnel manganese oxide Na4.32Mn9O18: a new Na+ site discovered by single-crystal X-ray diffraction

The title compound, tetrasodium nonamanganese octadecaoxide, Na4.32Mn9O18, was synthesized by reacting Mn2O3 with NaCl. One Mn atom occupies a site of 2/m symmetry, while all other atoms sit on mirror planes. The compound is isostructural with Na4Ti4Mn5O18 and suggestive of Mn3+/Mn4+ charge ordering. It has a double-tunnel structure built up from double and triple chains of MnO6 octahedra and single chains of MnO5 square pyramids by corner sharing. Disordered Na+ cations occupy four crystallographic sites within the tunnels, including an unexpected new Na+ site discovered inside the large S-shaped tunnel. A local-ordering model is used to show the possible Na+ distribution, and the unit-cell evolution during charging/discharging is explained on the basis of this local-ordering model.