Page 133 - Handbook of Battery Materials

3.2 Tunnel Structures 101

discussed above, the larger [2 × 2] tunnel allow various cations to be located in the
middle of the cavity.
Hence, it is not surprising that a large number of different minerals with more
or less ideal α-MnO 2 types of structure are known. Minerals containing sodium
(manjiroite), potassium (cryptomelane), barium (hollandite), and lead (coronadite)
have been structurally characterized in detail. The crystallographic data for some
α-MnO 2 compounds are summarized in Table 3.1, from which it can be seen that
the members of the structural family with [2 × 2] tunnels can be described either
by an ideal tetragonal or by a monoclinic cell with very similar dimensions, but an
◦
angle slightly differing from β = 90 . Generally, α-MnO 2 type compounds have a
stoichiometry of A 2−y B 8−x O 16 (A = large cations, e.g., K ,NH 4 ,Ba ,orwater;
+
+
2+
3+
4+
3+
B = small cations, Mn ,Mn ,V ,Fe ,A1 ). Each large cation is surrounded
4+
3+
by eight oxygen atoms forming a slightly cubic environment and additionally by
four oxygen atoms outside the lateral faces of the cube. The octahedrally coordinated
manganese atoms can be replaced by other small transition-metal cations with a
similar ionic radius. Natural α-MnO 2 samples usually contain one large cationic
species as the major component (e.g., Ba 2+ in hollandite) and the other possible
elements (e.g., K ) in minor amounts. Water molecules have similar dimensions
+
to the large ions mentioned above and therefore they can replace these cations in
the tunnels. The crystallographic c-axis of the tetragonal description of α-MnO 2 or
the b-axis of the monoclinic setting, respectively, has a dimension of about 280–290
pm. Hence, the shortest distances between the large cations A would be in the same
range if their respective sites were completely filled. Since such a short distance
does not usually occur in oxides (in contrast, in intermetallic compounds or the
elemental structures such distances can be observed), the A site has an occupancy
factor of about 50% or lower. This does not mean that a superstructure due to
cation ordering will necessarily be observed, although some examples are known
in the literature (see Table 3.2, Ba 2−x (Mn,Fe) 8 O 16 , with a doubled monoclinic
b-axis). Usually the ordered domains of the cations in the α-MnO 2 are too small
to be detected by the occurrence of superstructure reflections. Thus, the α-MnO 2
◦
structure is mostly described by the tetragonal, pseudotetragonal (all angles 90 ,
Table 3.2 T(m,n) nomenclature scheme for manganese ox-
ides, according to Turner and Buseck [4].

Common Variable dimension
dimension

n = 1 n = 2 n = 3 n = 4 n = 5 n =∞
m = 1 T(1,1) T(1,2) T(1,3) T(1,4) T(1,5) T(1, ∞)
Examples β-MnO 2 Ramsdellite Birnessite
m = 2 T(2,1) T(2,2) T(2,3) T(2,4) T(2,5) T(2, ∞)
Examples α-MnO 2 Roman` echite Rb 16.64 Rb 0.27 MnO 2 Buserite
Mn 24 O 48
m = 3 T(3,1) T(3,2) T(3,3) T(3,4) T(3,5) T(3, ∞)
Examples Todorokite