Geoscience Reference
In-Depth Information
evident that sulfides are next only to silicates and oxides. Similarly, in nature also,
sulfides are one of the most abundant minerals whose origin is closely related to
hydrothermal—high pressure and temperature. The experimental studies on the
growth of sulfides have shown that their crystallization can occur within a large
variety of solutions and also strong acid. Thus, the study on the synthesis of sulfides
began in the nineteenth century; however, a systematic study began after the
discovery of their technological potential, especially as photosemiconductors and con-
sidering their piezoelectric properties. However, in recent years, the interest in the
growth of bulk single crystals has declined significantly, but in contrary, the interest in
the synthesis of nanosize sulfide crystals has increased exponentially owing to their
unique properties in the nanosize regime. This is discussed in detail in Chapter 11.
Sulfides can be divided as simple or complex, containing a wide range of
elements (uni-, di-, tri-, and tetravalent metals).
The easiest way of obtaining sulfides under hydrothermal conditions is through
the following reaction:
XS
2
2
2OH
2
1
Me
2H
2
O
ð
MeSx
H
2
Þ
ð
8
:
21
Þ
1
1
1
However, the presence of hydrogen hinders the sulfides formation reaction and
hence, the following reaction would be more practical:
MeSO
4
1
Na
2
Sr
!
MeS
Na
2
SO
4
ð
8
:
22
Þ
1
By this exchange reaction, one can obtain a variety of sulfides with 100% output
[93
96]
. Latimer
[97]
has observed that the monophase synthesis usually occurs
through hydrolysis and oxidation of the nutrient with the formation of sulfates.
S
2
2
SO
2
4
4
4H
2
O
2
4H
2
m
ð
8
:
23
Þ
1
1
i.e., with a raise in partial pressure of hydrogen,
the sulfate ion becomes
unstable leading to the process of hydrolysis:
SO
2
4
1
MeS
5H
2
O
2
MeO
5H
2
O
ð
8
:
24
Þ
1
1
SO
2
4
4
2
ð
:
Þ
Me
2
S
3
1
7H
2
O
Me
2
O
3
1
1
7H
2
1
2S
8
25
The hydrolysis reaction usually takes place at pH
7
[97]
.
.
8.11.1 Hydrothermal Synthesis of Sulfides of Univalent Metals
Simple sulfides of univalent metals, like silver and copper, with 100% output,
could be obtained in chloride solutions (1
7 wt%) and the lower partial pressure of
500
C and P
hydrogen at T
1500 atm. The growth is usually car-
ried out in titanium-lined autoclaves with a temperature gradient (40
400
1000
5
5
60
C). For
