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situation of ZnS growth with the above specific complex formation in the system
ZnS
H 2 O.
The interaction of the sulfides with the aqueous solutions can be accompanied
by their hydrolysis and redox reactions. In an effort to estimate the stability of the
above sulfides under hydrothermal conditions, Kuznetsov [98] has constructed the
theoretical diagrams for Zn
a
KOH
a
a
S
a
H 2 O, Cd
a
S
a
H 2 O, Pb
a
S
a
H 2 O, Zn
a
Se
a
H 2 O,
Hg
pH coordinates at high temperatures. In
Figure 8.28 , the upper lines limiting the stability fields of CdS correspond to the
equilibria of these compounds with the solutions containing the oxidized sulfur; the
lowest lines correspond to their equilibria with the solutions enriched by the
reduced forms of the sulfur.
CdS and ZnS are stable in a wide range of Eh
a
S
a
H 2 O systems with the Eh
pH values (from acid to the
most alkaline solutions). The fields of the metallic Cd and Zn are located outside
the stability field of water; therefore, the “decomposition” reaction of CdS, ZnSe,
ZnS cannot take place during the usual hydrothermal experiments. Another situa-
tion characterizes the Pb
H 2 O system. Unlike the above sulfides, lead sulfide
is unstable at high temperatures in alkaline solutions and the fields of metallic lead
appear at both low and high redox potentials.
Among the considered compounds, mercury sulfide has the most narrow field of
stability at elevated temperatures which lies inside the water stability field.
Moreover, the field of HgS is surrounded by metallic mercury. It means that the
a
S
a
0.8
25 ° C
300 ° C
0.8
C d 2+
HSO -
SO 2-
0.4
Cd 2+
CdSO 4
0.4
0
CdO
HSO -
SO 2-
0
-0.4
CdO
Eh
Eh
CdS
-0.4
-0.8
CdS
-0.8
-1.2
Cd
Cd
-1.2
S 2
-1.6
HS -
HS -
H 2 S
HS -
H 2 S
S 2-
4
0
8
p H
0
4
8
12
p H
Figure 8.28 Stability fields of CdS [98] .
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