Geoscience Reference
In-Depth Information
Figure 7.61 Cross section of the autoclave
used in the synthesis of superionic
phosphates
[261]
.
Cap
Plunger
Teflon gasket
Nutrient
Steel casing
85% H
3
PO
4
Teflon liner
Crystal
Autoclave
and Li
3
Sc
2
(PO
4
)
3
a
H
2
O, besides Li
3
Sc
2
(PO
4
)
3
, the other stable phases
like ScPO
4
, and scandium hydroxides have obtained.
Thus, the hydrothermal technique is becoming a popular technique to synthesize
rare earth phosphates. What is most important in the case of hydrothermal synthesis
of phosphates in comparison with their analogous compounds—silicates and ger-
manates—is that it is still necessary to clarify why a wide range of structural varia-
tions have not been achieved through hydrothermal technique. The other synthesis
techniques like aqueous solutions, flux, and phosphoric acid solutions have resulted
in very complex structures, whereas the hydrothermal technique has resulted in
only a few anhydrous phosphates types like ortho-, diortho-, triorthogroups, pyro-
phosphates, simple tetra- and ultraphosphates. Therefore, let us discuss the crystal
chemistry of rare earth phosphates.
LiH
2
PO
4
a
7.13.3 Structure Types of Rare Earth Phosphates
The silicates and germanates have been sufficiently well studied. However, it is
only in the recent years that the rare earth phosphates are being studied extensively.
As mentioned earlier, the structures of condensed rare earth phosphates in particu-
lar do not resemble the structures of silicates and germanates, but the popular
representatives such as ortho-, diortho-, ring, layered, chain, ribbon structures, exist
in phosphates also, rare earth silicates have 40 structure types, the rare earth germa-
nates have 32 structure types, and rare earth phosphates have over 30 structure
types
[52]
. From the studies on the crystallization of rare earth silicates, germa-
nates, and phosphates, it is clear that a decrease in the R
2
O
3
/X
x
O
y
(X
Si, Ge, P)
ratio leads to the formation of condensed anions.
Tables 7.22 and 7.23
give the
structure types of alkali-free rare earth phosphates and alkali rare earth phosphates
reported so far in the literature.
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