title: (Na,□)5[MnO2]13nanorods: a new tunnel structure for electrode materials determined ab initio and refined through a combination of electron and synchrotron diffraction data.
abstract: (Nax□1 - x)5[MnO2]13has been synthesized with x = 0.80 (4), corresponding to Na0.31[MnO2]. This well known material is usually cited as Na0.4[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, Rwp= 0.051, Rp= 0.037, RF2= 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) Å3, 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.
authors.affiliation: Department of Physical, Earth and Environmental Sciences, University of Siena, via Laterina 8, 53100 Siena, Italy.
authors.forename: Enrico
authors.initials: E
authors.lastname: Mugnaioli
authors.affiliation: Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy.
authors.forename: Mauro
authors.initials: M
authors.lastname: Gemmi
authors.affiliation: Department of Earth Sciences, University of Milan, via Botticelli 23, 20133 Milano, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Merlini
authors.affiliation: Department of Physical, Earth and Environmental Sciences, University of Siena, via Laterina 8, 53100 Siena, Italy.
authors.forename: Michele
authors.initials: M
authors.lastname: Gregorkiewitz
keywords: Rietveld refinement; cation intercalation electrode material; dynamical refinement; electron diffraction tomography; octahedral molecular sieves