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(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
Author(s) -
Mugnaioli Enrico,
Gemmi Mauro,
Merlini Marco,
Gregorkiewitz Michele
Publication year - 2016
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520616015651
Subject(s) - crystallography , octahedron , ab initio , monoclinic crystal system , crystal structure , jahn–teller effect , square pyramid , ab initio quantum chemistry methods , rietveld refinement , materials science , chemistry , ion , molecule , organic chemistry
(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 F 2 = 0.035). The unit cell is monoclinic C 2/ 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.