Geoscience Reference
In-Depth Information
highly complicated, scientists continue to strive to develop new superionic conduc-
tors with simple structures. Phosphates are found to be the most suitable ones. The
structure of these phosphates consists of a three-dimensional framework. A series
of new NASICON analogs like Na
3
M
2
1
(PO
4
)
3
,(M
Sr, Mg, Fe, Mn)
[278]
.
5
Na
2
(R, M
3
1
)M
4
1
(PO
4
)
3
[263]
.A
3
M
3
1
(PO
4
)
3
,(A
Li, Na, Ag, K; M
Cr, Fe)
5
5
[279]
, ABC(PO
4
)
3
(A
Sc, Cr,
Fe)
[281]
were synthesized with an intention to develop compounds having stoi-
chiometric composition and relatively simple structures. However, all these com-
pounds contained only the orthogroup of radicals, irrespective of the wide variety.
Byrappa and coworkers
[282
K, Rb, Cs; B
Ca, Sr; C
Ln and Bi)
[280]
(M
5
5
5
5
284]
have reported, for the first time, pyropho-
sphates showing high ionic conductivity grown by a hydrothermal method.
It has not been attempted, using the hydrothermal method, to grow phosphates
widely, except for rare earth phosphates and aluminum orthophosphates during the
1950s
[246,251,252,280
289]
. Only a few reports on the hydrothermal growth of
superionics are seen in literature. Clearfield et al.
[276]
and Genet and Barj
[290]
adopted this technique to process NASICON compounds. According to them, an
insufficient amount of Si
1
4
enters the composition, but it is possible to develop
stoichiometric NASICON. The problem arises when Si
1
4
is added to the system.
By adopting the hydrothermal method, almost all the problems quoted earlier have
been overcome by Byrappa and Gopalakrishna
[248]
.
In the case of hydrothermal growth, the selection of the nutrient materials,
including solvents, is the most important aspect. Such studies are not readily avail-
able in the literature as the hydrothermal crystal growers touch upon the solvent
effect, solvent
solute interaction, complex formation, etc. only in the recent years.
Keeping this in mind, the authors have made an attempt for solvent selection with
reference to the superionic pyrophosphates containing different divalent metals. A
full understanding of the complex system formed in solutions is inconceivable
without a study of the effects of the different solvents on dissolved ionic species.
The properties of a solvent are determined jointly by its general chemical nature
and physical properties. These properties are involved in such a complex interac-
tion that it is difficult to establish unambiguously how they contribute individually
to the general behavior of the solvents. Complex formation reactions occurring in
solutions are known to be, in effect, substitution reactions in which the legend
replaces solvent molecules bound in the solvate sphere of the metal ion. Thus, it is
understandable that the rates, kinetics, and mechanism of such reactions are depen-
dent on the solvents. The magnitude of the interaction between the coordinated sol-
vent molecules and the cation (the strength of the coordinate bond) influences the
rate of exchange of the coordinated solvent with some other legend.
In the most complex-equilibrium investigations in solvent mixtures, the choice
of the solvent, was determined by the solubilities of the components of the system.
The aim of the investigations, therefore, was not the detection of the solvent effect,
but the determination of the compositions and stabilities of the complexes formed
in solution. The linkage between the solvent and solute is essentially a coordination
type and hence varied acid
base complexes have been observed in the solution.
Byrappa and Umesh Dutt
[291]
and Byrappa and Nirmala
[292]
have studied in
