Geoscience Reference
In-Depth Information
hydroxyl chloride of ytterbium, Yb
3
O(OH)
5
Cl
2
, using acidic chloride solutions
at 1400 atm pressure and 550
C. Under hydrothermal conditions, the chloride
group can be replaced by carbonate ions with the formation of hydroxyl carbonate,
R (OH)CO
2
[42]
. Similarly, Christensen and Hazeil
[43]
have obtained mixed
hydroxides of type MeSn(OH)
6
, where Me
Ca, Mn, Fe, Co, and Zn, through the
interaction of aqueous solutions of NaSnO
3
with chlorides or nitrates of the respec-
tive metals. Wolski et al.
[44]
have obtained Mn
x
Fe
2
2
2x
(OH)
6
2
4x
which finds
numerous applications in technology. For certain x values, the initially amorphous
mixtures, Mn
x
Fe
2
2
2x
(OH)
6
2
4x
, are converted into ferrimagnetic species when
stored as water suspensions well below 100
C, and the higher the temperature, the
shorter the time of transformation. Aqueous solutions of titrated 0.25 M Fe (III)
nitrates, mixed together in the desired proportions to give the hydroxide mixtures
Mn
x
Fe
2
2
2x
(OH)
6
2
4x
in 0.1
5
intervals, were treated by constant stirring with 2 M
NaOH till pH 9.5 was reached to ensure full Mn
2
1
precipitation and at the same time
to avoid oxidation to Mn
3
O
4
and
3
-MgOOH at higher pH. The washed and filtered
hydroxides were placed in Teflon vessels and were stored in the form of water suspen-
sions in autoclaves at 100
C for 24 h.
Thus, the hydrothermal technique provides unique opportunities for a prepara-
tive chemist to obtain a large variety of hydroxides, hydroxyl carbonates, hydroxyl
chlorides, etc., of various metals. Also hydrothermal and solvothermal methods
help in the structural stabilization of a variety of hydroxides, which cannot be stabi-
lized through other preparative chemistry routes.
β
9.4 Hydrothermal Synthesis of Selected Oxides
Oxides form one of the largest groups of inorganic compounds both in nature and in
the laboratory. The hydrothermal technique is one of the most popular techniques
for growing these high-temperature oxides, complex oxides, and low-temperature
modifications of the oxides. The work on the hydrothermal synthesis of oxides
began during the nineteenth century with quartz in 1845
[45]
. Followed by this,
large-scale research activity began during the 1890s on corundum and other related
high-temperature oxides
[46,47]
. Today, we find a large number of oxides obtained
by the hydrothermal method: TiO
2
, ZrO
2
, HfO
2
,Cu
2
O, BeO, Bi
2
O
3
,In
2
O
3
,Al
2
O
3
,
ZnO, and Fe
2
O
3
, to mention a few, and a great variety of mixed oxides. Some of
these carry great significance as synthetic gemstones, and some as technological
materials both in nanosize and bulk size. Here, we describe the hydrothermal growth
of a few selected oxides of both gemstone and technological grades.
9.4.1 Cu
2
O (Cuprite)
The growth of cuprite monocrystals under hydrothermal conditions can be carried
out using LiOH, NaOH, and KOH in aqueous solutions of alkali ammonium halo-
genides. Kuzmina and Kaidukov (1977) have worked out a detailed technology of
