Page 130 - Handbook of Battery Materials

98 3 Structural Chemistry of Manganese Dioxide and Related Compounds

Subsequently, the difference d (110) (in angstrom) is calculated from the
theoretical value and the measured d value of the (1 1 0) reflection. Using
the empirical calibration curve (a second-order polynomial, Equation 3.5), the
pyrolusite concentration can be calculated or it may be taken from a diagram,
as shown in Figure 3.5.
2) The model of De Wolff disorder gives no explanation for the line broadening
of reflections which are not affected by this type of lattice disorder. Chabre
and Pannetier ascribed this effect to a micro twinning of the ramsdellite/rutile
lattice on the planes [0 2 1] and [0 6 1]. These faces are believed to be growth
planes of EMD [45, 46]
It is well known that in rutile-like structures the planes [0 1 1] and [0 3 1] are
twinning planes. Hence, Chabre and Pannetier concluded that twinning faults
in the planes [0 2 1] and [0 6 1] (the equivalent planes in the ramsdellite doubled
unit cell) are the explanation for some features in the diffraction patterns of
γ -MnO 2 : for example, the lineshift of the (1 1 0) reflection toward lower angles
or the merging of the reflection groups (h 2l)/(h 4 0) and (h 6l)/(h 02).
In Figure 3.6 the arrangement of the manganese atoms is shown in a projection
along the a-axis. The unit cells are marked by the shaded regions. It can easily be
seen, that no lattice distortion is necessary to form the [0 2 1] and [0 6 1] twins. The
very low activation energy for the twinning usually results in a very high number
of micro twin boundaries in the crystal structure. The exact number of micro twin
domains is difficult to estimate from the diffraction patterns and according to
Chabre and Pannetier it is difficult to distinguish between the effects of the two

50
De wolff disorder
Pyrolusite concentration Pr [%] 30
40

20

10

0,05 0,10 0,15 0,20 0,25 0,30
∆d(110) / Å
Figure 3.5 Calibration curve for the determination or
the pyrolusite concentration of orthorhombically indexed
γ -MnO 2 samples by comparison of the calculated with
the observed d (110) value.