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] .
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