Geoscience Reference
In-Depth Information
A
2
O
3
a
B
2
O
5
a
H
2
SO
4
a
H
2
SO
4
a
H
2
O
2
a
H
2
O within a wide range of concentrations
of aqueous solutions and sulfuric acid
½
C
H
2
SO
4
5
10
15 wt%
;
C
H
2
O
5
5
6 wt%
2
2
and the volume ratio V
H
2
SO
4
=
5:1
[125]
.
There are several publications on the hydrothermal growth of Bi and Sb nio-
bates, and tantalates. These compounds strongly decompose at higher temperatures
and, hence, the flux or melt techniques are not suitable to grow these compounds;
and obviously, the hydrothermal method was found to be the most suitable one
[126
V
H
2
O
5
2
:
128]
. The synthesis is carried out in two types of solvents: (i) In aqueous
solutions of alkali metals, LiOH, NaOH, and KOH and (ii) in aqueous solutions of
alkali fluorides, Li
Cs. The autoclaves used for the synthesis of these bismuth and
antimony niobates and tantalates are provided with copper, silver, or titanium lin-
ings. The growth takes place at T
30
C. The nutrient
materials are the mixtures of Sb
2
O
3
and Nb
2
O
5
(Ta
2
O
5
). The analysis of the pro-
ducts of hydrothermal reaction shows that the alkali solutions bring in the reaction
2MeOH
650
C and
450
Δ
T
15
5
5
H
2
O.
Synthesis in strong fluoride solutions KF, RbF, and CsF increases the growth
rate; however, further increase in the concentration of F leads to the formation of
fluoroniobates and fluorotantalates of alkali metals Me
2
NbF
7
and Me
2
TaF
7
.
Obviously, such concentrated solutions do not give 100% niobates and tantalates of
Bi and Sb. It has been found that in aqueous solution of KHF
2
, even the formation
of complex niobateK
2
SbNb
2
O
15
takes place.
Thus, the hydrothermal method is a most valuable tool for the synthesis of a
variety of simple and complex niobates and tantalates. However, a complete under-
standing of the growth technology for these compounds insists on systematic inves-
tigations of various aspects of the growth parameters, like temperature, pressure,
pH, type of nutrient, and solvent.
Sb
2
O
3
1
2Nb
2
O
5
!
2SbNbO
4
k
1
2Me NbO
3
k
1
1
M
BiNbO
2
(%)
3:1
3.5
Bi
2
O
3
+
Bi
2
O
3
+ BiNbO
4
BiNbO
4
BiNbO
4
Nb
2
O
5
2.5
1:1
I
II
III
1.5
1:3
450
500
550
T
(°C)
0
10
20
30
40
C
KNF
2
(wt %)
(a)
(b)
Figure 8.32 (a) Dependence of the growth temperature and crystal output of BiNbO
4
and
(b) the NC-diagram of fields of crystallization of BiNbO
4
[125,126]
.
