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spread over different positions of the sublattices. Wolski et al.
[44]
have studied the
hydrothermal treatment of such phases to eliminate the nonmagnetic traces from the
product and also suggested the probable stages of conversion of Mn
0.5
Fe
1.0
(OH)
4
,
for example, up to the product MnFe
2
O
4
, which is a ferrimagnetic spinel.
9.5.5 Hydrothermal Synthesis of Complex Oxides
The mixed/complex oxides represent only a small but important and diversified
part of the entire family of inorganic compounds. Mixed oxides are widely used as
superionics, fillers, or pigments in pulp and paper, paint, and ceramic electronic
industries, and so on. Most of these mixed or complex oxides have highly complex
structures, and are highly heat-resistant and chemically inert. In recent years, there
has been a growing demand for these complex oxides, especially from the manga-
nese family and other related transitional metal families owing to their unique lay-
ered structures
[137,138]
. These advanced mixed oxides are those which present
specific characteristics in their composition, such as pigments for electronics, or in
their morphology, such as nanometer-size particles. The hydrothermal method is
potentially superior for low-cost production of advanced mixed oxides because
complex oxide powders are formed directly. The temperature of synthesis lies
between the boiling point of water and its critical temperature (374
C), whereas the
pressure can go as high as 15 MPa. The major advantages are the use of inexpen-
sive raw materials such as oxides, hydroxides, chlorides and nitrates, control of
growth rate, particle size, stoichiometry, particle shape, elimination of impurities,
and so on.
Among these mixed oxides, manganese-bearing mixed oxides are very important as
the majority of them show layered structures. Feng et al.
[139]
have synthesized
birnessite-type lithium manganese oxide, by reacting a Mn(NO
3
)
2
solution with a mixed
solution of H
2
O
2
and LiOH at room temperature. This manganese oxide has a layered
structure with a single sheet of crystal water and lithium ions between 2D edge-
shared MnO
6
octahedral sheets. The interlayer spaces can have Na
1
,K
1
,andCs
1
.
Such manganese oxides can be used as metal ion adsorbents and cathodes for lithium
rechargeable batteries
[140,141]
.
Figure 9.31
shows the structure of birnessite-type lith-
ium manganese oxides. The lithium ions may locate between the MnO
6
octahedral
sheets, and they can be topotactically exchanged with metal ions.
Manganese oxides like LiMn
2
O
4
are of particular interest because they readily
intercalate lithium into their structures and are therefore potentially useful as the
cathode of lithium batteries. For example, Sony's lithium ion cell uses the very
expensive LiCoO
2
cathode. Hence, extensive research is currently underway to find
Figure 9.31 Structure of birnessite-type
lithium manganese oxides
[139]
.
H
2
O
H
2
O
H
2
O
Li
+
Li
+
Li
+
