Page 141 - Handbook of Battery Materials

3.3 Layer Structures 109

Associated with each Mn–O layer and directly above and below the unoccupied
Mn position, the Zn 2+ ions are located in a somewhat distorted octahedral void,
formed by three oxygen atoms of the Mn–O layer and three oxygen atoms from
the water layer. The two kinds of sheets in the structure are held together by H–O
bridging bonds between the water molecules and the oxygen atoms of the Mn–O
layer. Natural chalcophanites differ quite significantly from the ideal composition.
Not only can the water content be variable, but there are some Mn 4+ defects
in the lattice and the sum of the cations usually exceeds four per formula unit.
This indicates that some additional interlayer atoms are present and that some
Mn 4+ ions must be replaced by manganese atoms with a lower oxidation state, for
example, Mn 2+ and Mn .
3+
Similarly to Mn 5 O 8 and related compounds, the chalcophanite structure can be
interpreted as a ‘filled’ CdI 2 -type structure. The space in the octahedral layer is filled
by an additional layer of water molecules and some foreign cations. A comparable
situation is found in several hydroxozincates, for example, Zn 5 (OH) 8 Cl 2 ·H 2 Oor
Zn 5 (OH) 6 (CO) 3 . In these compounds the layers are formed by edge-sharing zinc
hydroxide octahedra, Zn(OH) 6 , and the space between the layers is filled with
chloride and carbonate anions and some Zn 2+ cations, which are located above
and below vacancies in the Zn–OH layers.

3.3.4
δ-MnO 2 Materials

A large number of natural mineral and synthetic materials with a layered structure
and strongly varying water and foreign-cation content have been collected in
the δ-MnO 2 group. Most of these materials have a very poor crystallinity and
a wide range of existence. Therefore many authors have described ‘subgroups’
of the layered (III, IV) manganates and named them in a mostly confusing
way [79–85], for example, manganous manganates, δ-manganese dioxide, 7 ˚ A
phyllomanganates. Giovanoli et al. [86] suggested that all these compounds belong
to only one group, so only the name δ-MnO 2 should be used to describe these layered
manganates. The differences in the XRD patterns arise from the strongly differing
composition and crystallinity, but the general arrangements of the structural units
for the δ-MnO 2 compounds are the same. The crystal structure is built up from
layers of edge-sharing MnO 6 octahedra with a certain number of water molecules
and foreign cations between the layers. Hence, the chalcophanite type structure
might be considered as a well-crystallized prototype for the structural chemistry of
δ-MnO 2 materials. In 1956 Jones and Milne [87] described a mineral of composition
(Na 0.7 Ca 0.3 )Mn 7 O 14 ·2.8H 2 O that was found in a deposit near Birness in Scotland.
Since no mineralogical name had been given to the ore, they called this mineral
‘birnessite’; this name is now used synonymously for the designation δ-MnO 2 .
A number of additional studies have revealed that a relatively large number of
natural manganese oxide deposits contain materials of the birnessite type [88–91].
Additionally, it has been shown that layered manganese oxides of birnessite-type