Page 134 - Handbook of Battery Materials

102 3 Structural Chemistry of Manganese Dioxide and Related Compounds

true symmetry respective to the atomic arrangement is monoclinic), or monoclinic
cell.
The presence of the foreign cations stabilizes the crystal structure of α-MnO 2
compounds. This manganese dioxide modification (more exactly it is not a real
MnO 2 modification, since the structure contains a considerable proportion of
◦
foreign atoms) can be heated to relatively high temperatures (300–400 C) without
destruction of the lattice. Although Thackeray et al. reported the synthesis of
cation-and water- free α-MnO 2 [48, 49], which is reported to be stable up to 300 C
◦
without destruction of the [2 × 2] tunnel structure, it is commonly believed that a
small, but significant, amount of water or foreign cations is necessary to prevent
the collapse of the lattice. Otherwise submicro heterogeneities are formed, in which
pyrolusite, ramsdellite, and intergrowth domain of these two structural elements
coexist with the [2 × 2] tunnel structure of α-MnO 2 within the real lattice.
An interesting property of the sieve-like structure of α-MnO 2 is that it shows
pronounced cation exchange. For example, an α-MnO 2 with a high barium content
is easily prepared by stirring a sample of (NH 4 ) 2−x Mn 8 O 16 (obtained from
oxidation of MnSO 4 with a concentrated solution of (NH 4 ) 2 S 2 O 8 in the presence
of additional NH 4 ions) in an acidified solution of barium nitrate at elevated
temperatures. Consequently, the crystal structure of α-MnO 2 must contain OH −
groups, cation vacancies, and/or a proportion of manganese with an oxidation
state below Mn 4+ in order to maintain the charge balance. This is reflected in the
significantly longer Mn–O distances in the MnO 6 ocathedra in α-MnO 2 (198 pm)
compared with those in β-MnO 2 (188 pm).
The wide variety of natural minerals with [2 × 2] tunnels already indicates that
a huge number of different compounds of the α-MnO 2 type can be obtained by
a laboratory synthesis. Most chemical synthesis can be modified by working with
concentrated solutions of the chosen foreign cations or by adding larger quantities,
for example, of potassium salts or ammonium salts during the reaction in order
to produce the α-modification as the major product (for references, see Ref. [47]).
Similarly, it is possible to produce samples with a controlled α-MnO 2 /γ -MnO 2
ratio by electrolysis in the presence of various amounts of K 2 SO 4 or CaSO 4 [50].

3.2.5
Roman` echite, Todorokite, and Related Compounds
The crystal structure of roman` echite (or psilomelane) is closely related to that
of α-MnO 2 . Whereas the α-modification of manganese dioxide consists of
corner-sharing double chains of MnO 6 octahedra connected through common
edges, the roman` echite structure is build up by crosslinking of chains of double
and triple octahedra, as shown in Figure 3.7b. The resulting [2 × 3] tunnels,
extending in the b direction of the monoclinic cell, are partially filled by potassium
cations or barium cations and by water molecules. Both an orthorhombic
setting [22] and a monoclinically distorted structure [24] have been described
in the literature (Table 3.1). Mukherjee found a doubling of the short b-axis of
an orthorhombic subcell [23] due to cation/water ordering within the [2 × 3]