Page 142 - Handbook of Battery Materials

110 3 Structural Chemistry of Manganese Dioxide and Related Compounds

compounds are the major components in the manganese nodules found on the
sea floor [91–94].
Because of the low crystallinity of δ-MnO 2 and birnessite samples, Giovanoli
et al. used high-resolution diffraction techniques, XRD powder methods, and
chemical analyses in order to study a number of synthetic layer manganates
[1, 62, 63]. The crystal structures of the sodium-manganese-(II, III) manganate-(IV)
hydrate (Na 4 Mn 7 O 27 ·9H 2 O) and the manganese-(III) manganate-(IV) hydrate
Mn 7 O 13 ·5H 2 O have been modeled on the basis of the structure of chalcophanite.
It could be shown that the lattice of the sodium-manganese manganate (IV) is
built up by a stacking of alternating sheets of water and hydroxide ions and layers
of edge-sharing MnO 6 octahedra with a distance of about 713 pm between two
Mn–O layers. One of every six manganese sites in these layers is unoccupied,
and Mn 2+ and Mn 3+ are considered to lie above and below these vacancies in
a distorted octahedral arrangement formed by three oxygen atoms of the Mn–O
layer and three hydroxide ions or water molecules of the intermediate layer. The
position of the sodium atoms in this layer remained uncertain. The orthorhombic
lattice constants for Na 4 Mn 7 O 27 ·9H 2 O determined by Giovanoli et al. [62] can be
taken from Table 3.3. The sodium-free compound Mn 7 O 13 ·5H 2 Oisobtainedby
leaching of Na 4 Mn 7 O 27 ·9H 2 Owith HNO 3 . The structure of the main Mn–O layer
(including the Mn 3+ ions above and below the Mn 4+ vacancies) is the same as
in the sodium-sheets of containing sample. The sodium ions are missing in the
water/hydroxide layer separating the MnO 6 octahedra. The distance between the
main Mn–O layers is slightly larger (727 pm) than that in the sodium-containing
compound (713 pm). For Mn 7 O 13 ·5H 2 O a small hexagonal cell of a = 284 pm and
c = 727 pm was found. Recently Chen et al. [64] described a potassium-containing
birnessite, K 0.27 MnO 2 ·0.54H 2 O, with a similar rhombohedra1 symmetry (R 3m),
but a tripled c-axis with c = 2153.6 pm as compared with 727 pm in Mn 7 O 13 ·5H 2 O,
while the a lattice parameters are quite similar (see Table 3.3). The distance be-
tween two Mn–O layers in K 0.27 MnO 2 ·0.54H 2 O (718 pm) is comparable with that
in Na 4 Mn 7 O 27 ·9H 2 O (713 pm). Giovanoli found that the ‘pure’ Mn 7 O 13 ·5H 2 O
compounds are less stable than the samples containing foreign ions. Hence, one
can conclude that foreign ions (e.g., sodium or potassium) have a stabilizing effect
on the layer structure [63]. Additionally, the presence of water in layered man-
ganates might assist in stabilizing the layered arrangement. Figure 3.12a shows a
schematic drawing of the crystal structures of a birnessite-type material. Generally,
according to Stouff and Boul` egue [8], the foreign cations can occupy two different
positions: between the water and the Mn–O layer (above an Mn 4+ vacancy, position
B1) or directly between the two main Mn–O layers (at the same height as the water
molecules or hydroxide ions, position B2).
Post and Veblen [65] investigated the crystal structures of several synthetic
sodium, potassium, and magnesium birnessites by electron diffraction methods
and Rietveld refinements of XRD patterns. Starting from the crystal structure of
chalcophanite they determined the atomic arrangement in these compounds. The
materials of the composition A x MnO 2 ·yH 2 O(A = Na, K, Mg) have monoclinic
symmetry (see Table 3.3). Post and Veblen reported a number of detailed structural