Page 125 - Handbook of Battery Materials

3.2 Tunnel Structures 93

Ramsdellite
β-MnO 2

(a) (b)
intergrowth

γ-MnO 2

(c)
Figure 3.1 Crystal structure of (a) c-axis, respectively. (Small circles: man-
β-MnO 2 , (b) ramsdellite, and (c) the in- ganese atoms; large circles: oxygen
tergrowth structure of these two com- atoms; open circles: height z = 0;
pounds, γ -MnO 2 . The structures are filled circles: height z = 1/2.) The shaded oc-
shown as three-dimensional arrangements tahedra in (c) represent the β-MnO 2 parts of
of the MnO 6 octahedra and as projec- the intergrowth structure of γ -MnO 2 .
tions along the short crystallographic
pyramid formed by one oxygen and three manganese atoms. The crystal structure
is shown in Figure 3.1b.
Ramsdellite is thermodynamically unstable toward a transformation into the
stable β-modification. Hence, it is rarely found in natural deposits. Natural rams-
dellite has a stoichiometry close to the composition of MnO 2 and can be considered
another true modification of manganese dioxide. Attempts to synthesize ramsdel-
lite in the laboratory usually lead to materials of questionable composition and
structural classification. It is very likely that synthetic ‘ramsdellite’ materials are
more or less well-crystallized samples of the γ -modification that will be described
in more detail below.

3.2.3
γ -MnO 2 and ε-MnO 2

For a long time there was uncertainty about the crystal structure of γ -MnO 2
or the naturally occurring species nsutite. Single-crystal material could be taken
neither from natural deposits nor from synthetic manganese oxides prepared
by various methods in the laboratory. Powder diffraction patterns of only a
very poor quality with diffuse peaks, a high background and a selective peak