(Na,□)(5)[MnO(2)](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)[MnO(2)](13) has been synthesized with x = 0.80 (4), corresponding to Na(0.31)[MnO(2)]. This well known material is usually cited as Na(0.4)[MnO(2)] 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 (χ(2) = 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) Å(3), Z = 2. A hitherto unknown [MnO(2)] 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)[MnO(2)](13). The trivalent Mn(3+) ions concentrate at two of the seven Mn sites where larger Mn—O distances and Jahn–Teller distortion are observed. One of the Mn(3+) sites is five-coordinated in a square pyramid which, on oxidation to Mn(4+), may easily undergo topotactic transformation to an octahedron suggesting a possible pathway for the transition among different tunnel structures.