Page 123 - Handbook of Battery Materials

3.2 Tunnel Structures 91

up strings of edge-sharing octahedra extending along the crystallographic c-axis.
These chains are crosslinked with neighboring chains by sharing common corners,
resulting in the formation of narrow [1 × 1] channels in the structure. The voids
within the channels are too small for larger cations, but there is enough space for
an intercalation of hydrogen or lithium ions. The crystallographic data for β-MnO 2
and other tunnel structures are summarized in Table 3.1. The crystal structure of
β-MnO 2 is shown in Figure 3.1a.
The β-modification is the thermodynamically stable form of MnO 2 . Hence,
pyrolusite frequently occurs in natural ores and it is easily prepared in a high-purity
form by thermal decomposition of manganese nitrate, Mn(NO 3 ) 2 ·nH 2 O [29, 30].
Another method for the synthesis of β-MnO 2 is by heating γ -MnO 2 in closed
◦
reaction vessels in the presence of strong acids (H 2 SO 4 or HNO 3 ) at 130–150 C
[11, 31] or under hydrothermal conditions within a wide range of temperature and
pressure [11, 32]. Naturally occurring β-MnO 2 as well as the synthetic materials
with a rutile-type structure usually have a stoichiometry very close to the ideal
ratio of Mn:O = 1 : 2. The stability region of MnO 2−x is assumed to be in the
range of x = 0–0.1 [33–35]. Pyrolusite is the only modification in which the ideal
composition MnO 2 can be reached. Hence, the β-modification can be regarded as
a true MnO 2 compound.

3.2.2
Ramsdellite

The manganese and oxygen atoms in the ramsdellite modification of MnO 2 occupy
the same crystallographic sites as the aluminum and oxygen atoms in the diaspore
(AIOOH) structure and may thus be considered to be isopointal (according to
Parth´ e and Gelato the term ‘isotypic’ should be reserved for compounds in which
the occupancy of the same sites results in identical coordination polyhedra and
the same stoichiometry [36]) with this aluminum oxide hydroxide. The crystal
structure of ramsdellite is very similar to that of pyrolusite except that the single
chains of octahedra in β-MnO 2 are replaced by double chains in ramsdellite.
Consequently, the tunnels extending along the short c-axis of the orthorhombic
structure (a = 446 pm, b = 932 pm, c = 285 pm; see also Table 3.1) have a larger
dimension [1 × 21] compared with those of β-MnO 2 . The cell volume of ramsdellite
is approximately double the cell volume of β-MnO 2 . Similarly to pyrolusite, the
[1 × 2] channels are too small to allow the presence of cations other than protons or
+
Li . However, the transport properties of the ramsdellite crystal structure for pro-
tons or lithium ions as well as the structural integrity of the protonated or lithiated
compounds are extremely important for the performance of electrochemical cells.
As has already been pointed out for pyrolusite, the oxygen atoms in ramsdellite
occupy the positions of a hexagonally close-packed lattice. The manganese atoms
are located in every second pair of neighboring octahedral voids, which share a
common edge. One-half of the oxygen atoms has more or less ideal trigonal planar
coordination of manganese; the other half is located at the apex of a flat trigonal