Page 146 - Handbook of Battery Materials

114 3 Structural Chemistry of Manganese Dioxide and Related Compounds

and broadened (0 0 l) peaks because of the imperfection of the crystal lattice or (in
other words) because of the very small homogeneously scattering domains in the
crystallites. Sample II in Figure 3.14 may be of this kind. The highest degree of
disorder is described by the model in Figure 3.15d.
In this case the foreign cations and water molecules are inhomogeneously
distributed and the Mn–O layers are stacked with no periodicity. In some regions
of the lattice, where no foreign cations or water molecules are present, the distance
between the layers becomes quite small (comparable with Mn 5 O 8 ), while in regions
with separating cations or water molecules the Mn–O sheets are at a regular
distance from one another. The XRD spectra of such compounds show only the
(1 0 0) and (1 1 0) peaks because of a relatively high degree of order within the Mn–O
layers, as is also the case for Figure 3.15a–c. The situation for birnessites is similar
to that found in another layer compound with different degrees of crystallinity,
namely graphite in comparison with its disordered species carbon black. Graphite
has a high crystallinity and a more or less perfect and commensurate order of the
carbon layers, while the hexagonal nets of carbon atoms in the structure of carbon
black exhibit a large number of different interlayer distances and stacking faults,
also called a turbostratic disorder.
3.3.5
10 ˚ A Phyllomanganates of the Buserite Type

Buserite is structurally closely related to the 7 ˚ A manganates discussed above. The
crystal structure is built up by slabs of edge-sharing MnO 6 octahedra, which are
separated by two layers of water molecules or hydroxide anions. The latter layer
+
3+
2+
contains various amounts of foreign cations (e.g., Na ,or Mn ,Mn ). The main
Mn–O layers are at a distance of about 1000 pm. Buserite-type materials were
found to be one of the major components of marine manganese deposits. Synthetic
buserites of the composition (Na,Mn) Mn 3 O 7 ·xH 2 O have been studied by Wadsley
[66]. The symmetry of this hygroscopic material was found to be hexagonal with
the lattice constants a = 841 pm (see Table 3.3). Additionally, Wadsley found that
this 10 ˚ A manganate contains not only various amounts of water, part of which can
be reversibly extracted and re-introduced into the crystal lattice without destruction
of the structure, but also of a significant amount of sodium atoms, which can
be easily exchanged within the water and hydroxide layer. In further experiments
2+
Giovanoli et al. [95] observed that even bivalent metals (Ca 2+ or Mg )can be
incorporated into the buserite structure. In acidic solution the stability of the 10
˚ A manganates decreases with increasing valence of the interlayer ions and with
decreasing number of foreign cations in the structure. On complete dehydration
the 10 ˚ A manganates decompose irreversibly to the 7 ˚ A manganates. Hence, the
10 ˚ A phase can be interpreted as a hydrated form of the 7 ˚ A phase, as shown
in the schematic drawing of Figure 3.12b. Due to the ion-exchange properties of
sodium-rich buserites it is possible to replace sodium by large organic cations (e.g.,
n-dodecyl ammonium cations) as Paterson did [96]. This ion exchange increases
the layer distance from 1000 pm to about 2600 pm.