Geoscience Reference
In-Depth Information
The experiments on the hydrothermal synthesis of Li
2
B
4
O
7
were carried out using
Morey-type autoclaves provided with Teflon liners of capacity 25 ml. The use of
Teflon liners has helped in overcoming the corrosive nature of the melt (Li
2
O
B
2
O
3
).
The nutrient materials such as LiBO
2
or LiOH and Boric acid were taken in Teflon
liners. The molar ratio of the starting materials was selected through theoretical calcu-
lations. The % fill was varied from 50% to-
a
60%. Several mineralizers like HCl,
HCOOH, H
2
SO
4
, and HNO
3
were tried in the synthesis of Li
2
B
4
O
7
crystals. The
experimental conditions are given in
Table 7.28
. It was found that only HCOOH acts
as a suitable mineralizer, whereas the other solvents like HCl and HNO
3
did not yield
good crystals. Further, the molarity of HCOOH was varied from 1 to 3.5 M, and it
was found that good results were obtained in 1.5 M HCOOH solution.
The formation of Li
2
B
4
O
7
takes place according to the following reactions:
2 LiBO
2
1
2 HCOOH
!
2 LiHCOO
B
2
O
3
1
H
2
O
1
LiHCOO
1
H
3
BO
3
!
LiB
ð
OH
Þ
4
1
CO
2 LiB
ð
OH
Þ
4
1
2H
3
BO
3
!
Li
2
B
4
O
7
1
7H
2
O
The formation of the hydroxy borate ion on an ionization of boric acid is sponta-
neous. It is possible that the intermediate solvated complexes like boric acid (it is a
weak electrolyte and the complexes formed are rather stable), LiHCOO, and LiBO
2
interact with each other because of their ionic bonding or very low charge in order
to give a stable coordinated compound having a high lattice energy and relatively
stronger ionic bonding, Li
2
B
4
O
7
.
The crystallization was carried out in all the experiments through spontaneous
nucleation. As such, the crystals obtained under the above said conditions are colorless,
highly transparent, and vary in size from 5 to 6 mm. The crystals are transparent and
the faces are also well developed (
Figure 7.84a and b
). In general, the morphology of
the hydrothermally grown Li
2
B
4
O
7
is very interesting and it varies widely depending
upon the experimental conditions, particularly the starting materials and the % fill.
Even the size of the crystals varies depending upon the above parameters from 1 up to
12 mm. As the pressure in the system was decreased slowly (up to 40 atm), the size of
the crystals increased. In some experiments, the authors have obtained well-developed
and perfect tetragonal crystals (
Figure 7.85
)ofsmallsizeandinsomeotherexperi-
ments wedge shaped, massive, and irregular crystals
[363]
. When the size of the crys-
tals is large, the crystals become less transparent. Li
2
B
4
O
7
crystals can be obtained
within the following molar ratio: Li
2
O:B
2
O
3
5
9. The chemical analysis of the
Li
2
B
4
O
7
obtained shows that the ratio Li
2
O:B
2
O
3
is higher than the theoretical ratio
(69.2 molar% B
2
O
3
experimental ratio as against 66.6 M% of B
2
O
3
theoretical) as in
the case of Ref.
360
.
Figure 7.86
shows the schematic diagram of morphological varia-
tions in Li
2
B
4
O
7
crystals with the % fill in the autoclaves
[360]
.
In order to reduce the number of nucleation centers, the temperature of the
crystallization reactor was increased very slowly at the rate of 10
C/h up to 100
C
and beyond which at the rate of 5
C/h. The nutrient materials were held at 250
C
1:7
