Geoscience Reference
In-Depth Information
9.5.2 Hydrothermal Synthesis of Antimonites and Antimonates
Oxides of trivalent antimony belong to the group of antimonates. Among hydro-
thermally synthesized rare earth penta-antimonites, R
3
Sb
5
O
12
and In
3
Sb
5
O
12
are
the important ones
[111
113]
. Following are the penta-antimonites obtained by
hydrothermal technique:
In
3
Sb
5
O
12
;
La
3
Sb
5
O
12
;
Pr
3
Sb
5
O
12
;
Tb
3
Sb
5
O
12
;
and Yb
3
Sb
5
O
12
The synthesis of these compounds is usually carried out by the hydrothermal
technique within the temperature interval 450
550
C(
1.0
C and
Δ
T
5
0.5
P
1100 atm).
Variation in the ratio of R
2
O
3
(In
2
O
3
)/Sb
2
O
3
in the nutrient limits the crystalliza-
tion of the product with the appearance of Sb
2
O
3
crystals. Higher concentration of
KF or CsF results in the formation of K
3
RF
6
, and both of these phases often reduce
the output of penta-antimonites field.
Penta-antimonite,
5
110
In
2
Sb
5
O
12
,
forms in the aqueous solutions of KHF
2
,KF
(45
55 wt%), an
d
CsF (20 wt%). The
c
rystals are 1
2 mm in
si
ze. The
b
asic f
or
ms
are h
e
x
ago
nal
ð
1120
Þ
and trigonal
ð
0110
Þ
prisms, pyramids
ð
0112
Þ; ð
1011
Þ; ð
0221
Þ;
and
This compound has three polymorphic modifications and two are revers-
ible transformations: trigonal
ð
1321
Þ:
140
C), trigonal
450
C), and
α
-form (
β
-form (140
,
450
C)
[112]
.
Figure 9.26
shows the In
3
Sb
5
O
12
crystals obtained
under hydrothermal conditions
[113]
. These crystals show interesting piezoelectric
and ferroelectric properties.
Similarly, Sc
3
Sb
5
O
12
crystallizes in the aqueous solutions of KF with a concen-
tration of up to 60 wt%. The synthesis of scandium penta-antimonite takes place
simultaneously with the formation of K
3
ScF
6
and attains 70% output in 30% KF
solution. In aqueous solutions of CsF with concentrations of 12
cubic
γ
-form (
.
60 wt% and
Figure 9.26 In
3
Sb
5
O
12
crystals obtained
under hydrothermal conditions
[113]
.