Page 145 - Handbook of Battery Materials

3.3 Layer Structures 113

are completely missing. Giovanoli [3] has developed a model that explains this
somewhat extraordinary behavior of different δ-MnO 2 materials. In the model
of birnessite-type materials proposed by Giovanoli, several arrangements of the
Mn–O layer and the interlayer cations or water molecules are possible. The ideal
is the perfect ordering of both types of sheets in the structure (see Figure 3.15a),
thus building up a fully commensurate crystal lattice such as is realized in chal-
cophanite. The XRD pattern of sample I in Figure 3.14 may be of a structure
type with a high degree of order. In the second case a certain kind of disorder
appears within the cation or water layer in terms of an inhomogeneous distribution
of the interlayer species, while the distance between the main layers is constant
(Figure 3.15b). A very similar situation is demonstrated in Figure 3.15c: a disorder
of the interlayer species is combined with a shift of the main Mn–O layer within
its own plane. This results in an incommensurate arrangement of the layers in the
direction perpendicular to the layers. As a result the XRD patterns show only weak

Perfectly ordered Mn-O and disorder of the foreign cation sublattice
foreign cation sublattices Mn-O layers incommensurably shifted
(constant interlayer d-spacing)
(a) (c)

disorder of the foreign cation sublattice disorder of the foreign cation sublattice
ordered arrangement of the Mn-O layers ″turbostratic″ arrangement of the Mn-O layers
(order only within the Mn-O sheets)

2+
+
layer of Foreign metal cations (e.g. K or Ba )
MnO octahedra O molecules
6 or H 2
(b) (d)
Figure 3.15 Arrangement of the Mn–O distance, (c) disorder of the interlayer atoms
layers and separating sheets according to and an incommensurate shift of the com-
Giovanoli [3]. The layer structure can be (a) plete Mn–O sheet within the layer plane,
completely ordered or (d) completely dis- resulting in an incommensurate superstruc-
ordered (turbostratic disorder). The cases ture along the c-direction (perpendicular to
(b) and (c) represent situations between the the layer) and in a diffuse distribution of the
two extremes: (b) disorder of the interlayer electron density in this layer, resulting in a
atoms or molecules but an ordered stack- lower contribution of this layer to the 0 0 l
ing of the Mn–O layers with constant layer reflections. (Adapted from Ref. [41].)