Page 135 - Handbook of Battery Materials

3.2 Tunnel Structures 103

tunnels. The most reliable and realistic structure determination seems to be the
monoclinic structure refinement from single crystal X-ray data collected from a
natural roman` echite crystal [24]. The presence of water, Ba ,and K ions in the
2+
+
structure was found to be essential for the stability of roman` echite. After being
heated to higher temperatures the crystal structure collapses and forms the much
more stable hollandite-type compound (Ba, K) 2−x Mn 8 O 16 . Furthermore, in order
to account for the charge balance of a roman` echite of the typical composition
Ba 0.66 Mn 5 O 10 ·1.5 H 2 O, a certain fraction of the manganese atoms in the structure
must have a lower oxidation state than Mn(IV), or manganese vacancies must
occur. In contrast to the other MnO 2 modification described above, roman` echite
contains three nonequivalent manganese sites. Two of these sites have average
Mn–O distances of about 191 pm, where as the third Mn octahedral position has
a significantly larger Mn–O distance of 199 pm. This indicates that manganese
species with a lower valence accumulate at this crystallographic site.
Investigation of the rubidium–manganese–oxygen ternary systems revealed the
existence of two manganates (III, IV) with tunnel structures comparable with
the mineral roman` echite. Rb 16.64 Mn 24 O 48 [25] contains [2 × 4] tunnels formed by
cross linking (through common corners) of double chains with a building element
consisting of four edge-sharing MnO 6 octahedra chains. A similar compound,
Rb 0.27 MnO 2 [26, 27], consists of MnO 6 octahedra chains, which are connected in a
way that [2 × 5] tunnels are formed. The rubidium atoms occupy ordered positions
within the tunnels. Although it may be considered speculative, it seems likely that
compounds with larger tunnels (e.g., [2 × 6] or [2 × 7]) might exist.
For a long time the structural classification of the mineral todorokite was
uncertain, until Turner and Buseck [4] could demonstrate by HRTEM investigations
that the crystal structure of that mineral consists of triple chains of edge-sharing
octahedra, which form [3 × 3] tunnels by further corner-sharing. These tunnels
are partially filled by Mg ,Ca ,Na ,K , and water (according to the chemical
+
+
2+
2+
analysis of natural todorokites). In 1988 Post and Bish could perform a Rietveld
structure determination from XRD data taken for a sample of natural todorokite
[28]. This diffraction study confirmed the results of Turner and Buseck. The cations
and water molecules in the [3 × 3] tunnels show a high degree of disorder, as
could be expected. The mineral itself is rarely found in natural deposits [51]. A
reliable laboratory synthesis was not reported until Shen et al. [6] demonstrated
by HRTEM and XRD investigations that synthetic todorokite can be obtained by
mild hydrothermal synthesis in alkaline solutions in the presence of Mg 2+ ions.
These samples show pronounced ion exchange and have been tested as absorbing
agents for organic molecules (i.e., cyclo-C 6 H 12 , CCl 4 ). The authors found that
relatively large amounts (18–20 g/100 g todorokite) of the organic compounds can
be absorbed. The tunnels in todorokite have a typical diameter of ∼690 pm, which
seems – similarly to zeolites – to be well suited for the incorporation of small
organic molecules.
The intensive investigation of manganese oxides during recent decades led to the
discovery of a large number of closely related tunnel structures. The various ways
in which the one dimensionally infinite Mn(O,OH) 6 octahedra strings in these