Figures 1A and 1B show the X-ray diffraction patterns of the Co3
O4
and
MnCo2
O4
phases respectively. Even if the crystalline state is not better defned
due to the low temperature of synthesis
41
, Rietveld analysis shows single
phases corresponding to the expected compounds which crystallize in the
face centered cubic system spinel type, spatial group
7
h
O (Fd3m). All the main
peaks (indicated by their hkl indices) could be indexed by comparison with the
diffraction patterns of Co3
O4
and MnCo2
O4
. Both the position and the relative
intensities of the diffraction lines were found to be in good agreement with the
ASTM X-ray powder data fles JCPDS-ICDD for Co3
O4
and MnCo2
O4
, thus
showing that a single phase of each component was present. A remarkable
common feature of the X-ray spectra of both spinel oxides prepared at such
a low temperature was the good defnition of the diffraction lines. The mayor
peak in both XRD profles was found to correspond to the (311) crystallographic
plane. The Co3
O4
lattice constant (ao
= 0.8079
nm) shows an acceptable good
agreement with the 8-418 ASTM fle value of 0.8084 nm for a product prepared
at 850 ºC. For MnCo2
O4
a more pronounced discrepancy was observed (ao
=
0.8089 nm) with the 23-1237 ASTM fle value of 0.8269 nm, not only due
to differences between the preparation method (ceramic procedure), but also
due to the temperature of the preparation (720ºC). This higher variation for
MnCo2
O4
suggests the presence of cations with a high oxidation state that must
be distributed in a smaller size cell and a cationic stoichiometric defciency
should not be ruled out.
The diffractograms in Fig. 1 show a loss of crystallinity going from Co3
O4
to MnCo2
O4
. Broader diffraction peaks at the same positions were observed
indicating that MnCo2
O4
(13 nm) have a smaller crystallite size than Co3
O4
(45nm) (see Table I).