(Na,□)5[MnO2]13 nanorods: a new tunnel structure for electrode materials determined ab initio and refined through a combination of electron and synchrotron diffraction data

(Na_x□_1 - x)₅[MnO₂]₁₃ has been synthesized with x = 0.80 (4), corresponding to Na_0.31[MnO₂]. This well known material is usually cited as Na_0.4[MnO₂] and is believed to have a romanèchite-like framework. Here, its true structure is determined, ab initio, by single-crystal electron diffraction tomography (EDT) and refined both by EDT data applying dynamical scattering theory and by the Rietveld method based on synchrotron powder diffraction data (χ² = 0.690, R_wp = 0.051, R_p = 0.037, R_F2 = 0.035). The unit cell is monoclinic C2/m, a = 22.5199 (6), b = 2.83987 (6), c = 14.8815 (4) Å, β = 105.0925 (16)°, V = 918.90 (4) ų, Z = 2. A hitherto unknown [MnO₂] framework is found, which is mainly based on edge- and corner-sharing octahedra and comprises three types of tunnels: per unit cell, two are defined by S-shaped 10-rings, four by egg-shaped 8-rings, and two by slightly oval 6-rings of Mn polyhedra. Na occupies all tunnels. The so-determined structure excellently explains previous reports on the electrochemistry of (Na,□)₅[MnO₂]₁₃. The trivalent Mn³⁺ ions concentrate at two of the seven Mn sites where larger Mn-O distances and Jahn-Teller distortion are observed. One of the Mn³⁺ sites is five-coordinated in a square pyramid which, on oxidation to Mn⁴⁺, may easily undergo topotactic transformation to an octahedron suggesting a possible pathway for the transition among different tunnel structures.