Page 122 - Handbook of Battery Materials

90 3 Structural Chemistry of Manganese Dioxide and Related Compounds

models. Thus other methods have to be applied to overcome this problem. The most
powerful tool is high-resolution transmission electron microscopy (HRTEM) [4–6].
Other methods, for example, IR spectroscopy [7] and extended X-ray absorption
fine structure (EXAFS) measurements [8], give additional information about the
near-neighbor environment, the connection scheme of Mn(O,OH) n polyhedra, and
the different oxidation states of the manganese atoms in the structure.
The most obvious structural feature in all oxides containing manganese in the
2−
oxidation states II, III, or IV is the more or less distorted octahedral 0 × 0- (O )
or hydroxo- (OH ) coordination. The Mn(O,OH) 6 octahedra can be connected to
−
each other by sharing common corners or edges. Face-sharing (as it is known
from Nb cluster compounds containing close Nb–Nb bonds) does not usually
occur, since in this case the central atoms of the polyhedra would come into too
close contact. So far, a metal cluster is unknown in the structural chemistry of
manganese dioxide and related compounds. The closest Mn–Mn distance in the
various modifications of MnO 2 usually occurs along the shortest crystallographic
axis. Many compounds contain a short translation period ranging approximately
from 280 to 290 pm, which represents the distance between the central atoms
of two edge-sharing octahedra. The octahedral unit is the fundamental building
element for manganese oxides. How the octahedra are connected together can
be used to classify the crystal structures. Similarly to silicate chemistry, the large
family of manganates (II, III, or IV) can be divided into subgroups which contain
characteristic building blocks of edge/corner-sharing Mn(O,OH) 6 octahedra [9].
A common structural feature is the formation of one-dimensionally infinite
strings of edge-sharing octahedra, which extend along the shortest translation
period. Two or three of these strings can be connected to one another by further
edge-sharing, thus forming double or triple chains. Four such MnO 6 strings,
connected by corner-sharing, enclose, a one-dimensionally infinite tunnel of var-
ious dimensions. This category of compounds is generally described as chain
or tunnel structure. The other frequently occurring structural element is formed
from two-dimensionally infinite layers of edge-sharing Mn(O,OH) 6 octahedra. The
stacking sequence of the octahedral layers and the kind/number of the interlayer
atoms or molecules (metal cations, water, hydroxide anions) are further criteria for
a structural classification of these layer or sheet structures (‘phyllomanganates’).

3.2
Tunnel Structures

3.2.1
β-MnO 2

The crystal structure of pyrolusite, or β-MnO 2 , is the simplest one within the family
of compounds with tunnel structures. The manganese atoms occupy half of the
octahedral voids in the hexagonal close packing of oxygen atoms in an ordered
manner, thus forming a rutile-type structure. The distorted MnO 6 units build