Geoscience Reference
In-Depth Information
RbF, CsF). The crystallization depends upon the growth temperature, temperature
gradient, type and concentration of alkali solutions, ratio of the nutrient compo-
nents, and a series of other parameters.
According to the authors, Nb
2
O
5
or Ta
2
O
5
exists in the alkaline solutions as
hydroxyl complexes like [Nb
6
(OH)
38
]
8
2
or [Ta
6
(OH)
38
]
8
2
[123]
.
The bonding between the individual atoms of metals in such a structure is obtained
through OH-bridging which is necessary for the formation of the polymerized solu-
tions. The entire Nb or Ta is bonded in the above form to develop SbNbO
4
or SbTaO
4
.
The composition of these single crystals shows that under distinct concentrations
of aqueous solutions KF, RbF, CsF, there occurs the breaking of valent bonds Nb
(Ta)F
7
. These complexes, in turn, combine with the alkali cations to form Me
2
Nb
(Ta)F
7
. The thermal hydration is different for different salts. MeF and the aqueous
solutions of KF, RbF, and CsF, consisting of larger cations, are less hydrated.
Under higher concentration of aqueous solutions of (K, Rb, Cs)F, the activity of
the free ions of F is so high that it forms valent bond with Nb(Ta)
O, resulting in
the formation of Me
2
Nb(Ta)F
7
. These studies show that the ability to dissolve the
Nb
2
O
5
or Ta
2
O
5
and Sb
2
O
3
increases as follows: LiF
!
NaF
!
CsF
!
RbF
!
KF;
therefore, the crystallization of ABO
4
crystals depends upon the solvent
solute
interaction under hydrothermal conditions.
When CO
3
or C
2
O
4
is introduced, in the form of potassium carbonate and potas-
sium oxalate, into the mineralizer solutions (aqueous solutions), it was found that
the formation of Nb(Ta)F
7
becomes very simple and effective because the anions
CO
3
and C
2
O
4
, with their larger size, strongly polarize the Nb(Ta) in the solution.
It can be explained through the following reactions:
2
2
15K
1
1
12OH
2
1
2H
1
1
F
2
Nb
2
O
5
1
15KF
7H
2
O
!
2
ð
NbF
7
Þ
ð
8
:
45
Þ
1
1
2
2
2
2
½
NbF
7
½
CO
3
!
Nb
2
ð
CO
3
Þ
5
1
OH
2
1
H
1
1
14F
2
ð
:
Þ
2
1
5
1
H
2
O
8
46
Nb
2
ð
CO
3
Þ
5
!
Nb
2
O
5
ð
solution
Þ
1
5CO
2
ð
8
:
47
Þ
40
C, good
single crystals of ABO
4
could be obtained. An increase in the concentration of
KHF
2
in the system and change in the molar ratio of Sb
2
O
3
/Nb
2
O
5
lead to the
crystallization of intermediatory phases like K
2
SbNb
5
O
15
. The hydrogen peroxide
fulfills the following function: It increases the oxidation potential in the given
system, stabilizes the soluble form of Nb
5
1
, and leads to the transition of less
soluble Nb
2
O
5
in the solution, consequently, the formation of peroxide complexes
takes place
[124]
.
The growth of BiNbO
4
has been carried out in the system Bi
2
O
3
a
530
C and
When KHF
2
mineralizer (5 wt%) was used at T
5
Δ
T
5
H
2
O
using solvents like KHF
2
and H
2
O
2
.
Figure 8.32a
shows the growth temperature
dependence of the crystal output of BiNbO
4
and
Figure 8.32b
shows the NC-
diagram of fields of crystallization. The hydrothermal crystallization of the ABO
4
crystals can be carried out
Nb
2
O
5
a
in an acidic media, for example,
in the system
