Geoscience Reference
In-Depth Information
The trioxides of tungsten (VI) dissolve in aqueous alkali metal hydroxide to form
tungstates and water. These normal tungstates contain the discrete tetrahedral ions
WO
2
4
It is not certain that the WO
2
4
ions are also tetrahedral in an aqueous solution.
Tungstate ions polymerize in an acid solution to give meta- and paratungstate ions.
The degree of polymerization in the solution increases as the pH is lowered, and a
number of tungstates M
2
O
:
H
2
O, differing in the value of n, crystallize
from the solution at different pHs. It has been observed that the diffusion coefficient
of tungstate ions in aqueous solutions decreases considerably with the increasing acid-
ity of the solution, which is probably due to an increase in the polymerization of ions.
In all the experiments, pH was measured at the beginning and at the end of the experi-
mental runs and it was found that the system was moderately alkaline, which avoids
the possibility of polymerization. Lanthanide ions, in spite of high charges (
nWO
3
M
3), have
low charge densities because of their large size. Therefore, they cannot cause much
polarization and, consequently, they do not have much tendency to form complexes.
The tendency to form complexes and their stability increase with increasing atomic
number. Since ionic size decreases from La
3
1
to Lu
3
1
, the basicity of hydroxides
decreases in the same order. Thus, La(OH)
3
is the strongest and Lu(OH)
3
is the weak-
est base. La(OH)
3
precipitates during the reaction as follows:
1
Þ
ð
n
2
3
Þ
n
La
ð
OH
Þ
3
1
n
3
"
La
ð
OH
ð
8
:
15
Þ
2
and later dissolves in an NaOH solution through any one of the earlier quoted reac-
tions. Like La complexes, we can also expect NaLa complexes such as Na
x
La
(OH)
3
2
x
in the solution. Being a good base, La(OH)
3
reacts with WO
3
to form lan-
thanum tungstates, which in turn dissolves in normal sodium tungstate to form NaLa
(WO
4
)
2
crystals under controlled conditions. In all the previous works on the synthe-
sis of alkali rare earth tungstates, mineralizers such as NH
4
Cl and alkali halides were
used at very high PT conditions, but in the work of Byrappa and Jain
[53]
, they have
essentially used water and NaOH as mineralizer, with an emphasis on controlling the
release of hydration shell water in the system through moderately lower PT condi-
tions of synthesis. This permits a considerable reduction in the free energy of the sys-
tem, leading to the complexation process. Therefore, the use of NaOH in the system
and the formation of La(OH)
3
in the course of the reaction are quite useful in the
complexation process, as it is well known that hydration shell water experiences less
competition from bulk solvent water at moderate temperature. They have also carried
out a series of experiments using heavier lanthanides under the same experimental
conditions; however, it was found that the solvents used are suitable for the first group
of lanthanides (La, Ce, Nd) but not for the other lanthanides. The reason for this may
be the decrease in the basicity with an increase in the atomic number. Hence, the syn-
thesis of NaLn(WO
4
)
2
(where Ln
Lu) insists on the use of other solvents.
The most common morphological habits observed in NaLa(WO
4
)
2
crystals are pris-
matic, rod shaped, rhombohedral, needle like, and so on. The prism faces dominated
over all other faces in the crystals followed by pinacoidal and dome faces. NaLa
(WO
4
)
2
crystals belong to the tetragonal system with scheelite structure type; hence,
their morphology is supposed to be simple, at least in the case of single crystals.
Pr
5
