Page 137 - Handbook of Battery Materials

3.3 Layer Structures 105

space between the MnO 6 octahedra sheets, from the various ways the layers may
be stacked, and from a large number of possible defects and superstructures in
this family of crystal structures. Table 3.3 gives an overview of the crystallographic
properties of some manganese oxides with a layer structure.

3.3.1
Mn 5 O 8 and Similar Compounds

The compound Mn 5 O 8 was first described in 1934 by Le Blanc and Wehner [67].
At that time the compound was believed to be a modification of Mn 2 O 3 . About
30 years later Oswald and Wampetich correctly determined the crystal structure of
Mn 5 O 8 and the isotypic compound Cd 2 Mn 3 O 8 [68] from single-crystal data. These
two manganese oxides, as well as the isotypic copper- and zinc- containing phases
Cu 2 Mn 3 O 8 [53] and Zn 2 Mn 3 O 8 [69], crystallize monoclinically. Mn 5 O 8 represents
a mixed-valence compound containing manganese in the oxidation states Mn 2+
2+
4+
4+
and Mn . Hence, the formula can written as (Mn ) 2 (Mn ) 3 O 8 , suggesting that
2+
2+
in the isotypic compounds Zn ,Cu , and Cd 2+ replace the Mn(II) atoms at their
respective sites. The crystal structure (see Figure 3.9) is best described as a strongly
distorted pseudohexagonal layer structure, derived from the CdI 2 -type structure.
The lattice is build up of wave-like sheets of heavily distorted MnO 6 octahedra,
in which every fourth manganese atom is missing. The Mn–O distances in the
octahedral units range from 185 to 192 pm. The Mn 2+ cations are placed below
and above the Mn 4+ vacancies, occupying distorted trigonal-prismatic voids. Riou
and Lecerf [54] found that the cobalt compound Co 2 Mn 3 O 8 crystallizes with the
higher-symmetry space group Pmn2, compared with Mn 5 O 8 (space group C2/m).
The authors suggested that this might be due to the finding that the cobalt
atoms occupy two kinds of sites with differing coordination polyhedra, where as
in structures of the Mn 5 O 8 type only one coordination occurs for the Mn 2+ site.
The relatively short interlayer distances of 472 pm (Cu 2 Mn 3 O 8 ), 488 pm (Mn 5 O 8 ),
and 510 pm (Cd 2 Mn 3 O 8 ) indicate a strong interaction between the layers and the
interlayer atoms. None of the compounds occurs in natural deposits; they can
only be prepared in the laboratory, either by classical solid state chemical methods
(e.g., as in Refs [53, 54]) or by soft-chemical reaction paths under mild conditions.
Mn 5 O 8 can be obtained by hydrothermal oxidation of MnO in the temperature
◦
range 120–910 C at a water pressure of up to 1 kbar and at oxygen partial pressures
ranging from 1 to 100 bar [70]. Another method is the oxidation of manganese
oxide hydroxides in air or in oxygen at moderate temperatures [71–73].
A manganate (III, IV) similar to Mn 5 O 8 is known in the literature: ternary lead
manganese oxide Pb 3 Mn 7 O 15 . In this compound the sheets of MnO 6 octahedra
contain defects at the manganese sites. Four out of 14 manganese atoms are
missing within the layer and are positioned in an octahedral environment above
and below the vacancy in the sheets. Additionally, the structure contains Pb–O
layers that separate the Mn–O sheets. Thus the stacking sequence in Pb 3 Mn 7 O 15
is: MnO 6 (main layer) – MnO 6 (interlayer) – PbO–MnO 6 (interlayer) – MnO 6
(main layer). The distance between the main MnO 6 sheets is larger (678 pm) than