Geoscience Reference
In-Depth Information
only on the rate of reaction but also on the stability of various phosphate anions. For
the benefit of a reader, we have described the hydrothermal growth of phosphates
containing rare earth elements followed by transitional elements.
7.13.2 Hydrothermal Growth of Rare Earth Phosphates
The first work on the hydrothermal synthesis of rare earth phosphates was by
Anthony
[222]
, who synthesized monocrystals of monazite, CePO
4
, from an aque-
ous solutions of 85% H
3
PO
4
at 300
C using Teflon liners. Carron et al.
[246]
have
described the formation of rare earth orthophosphates from HCl solutions at
105
300
C and at pressure of 90 atm. A more systematic work on the hydrother-
mal synthesis of rare earth orthophosphates was carried out by Carron et al.
[247]
.
However, there was a break in the activity of the hydrothermal synthesis of rare
earth phosphates until the 1980s. This was mainly connected with the lack of
growth technology, high volatility of phosphorus at higher temperatures, high reac-
tion susceptibility of phosphorus, corrosive nature of phosphorus at higher tempera-
tures, amongst other factors. The experiments on the hydrothermal synthesis of
phosphates cannot be carried out at temperatures greater than 500
C, because phos-
phorus corrodes even the metals like Pt, Ir, Mo, Au, and Ni. Some of these pro-
blems could be overcome using Teflon or vitreous carbon glass liners. However,
the main drawback in using these liners is sealing. If one can seal the glassy carbon
liners by some means, the growth of pure phosphates can be carried out even at
higher temperatures by hydrothermal methods. Similarly, the use of Teflon liners is
again a problem, because it cannot be used beyond 300
C and 200 atm. Of course,
pressure can be slightly increased by means of a counterbalance technique, i.e., by
creating the same pressure between the autoclave walls and inside the liner. These
problems have been discussed in great detail by Byrappa and Gopalakrishna
[248]
,
Byrappa
[249]
, and Rabenau
[250]
. Byrappa and Gopalakrishna
[248]
have studied
the rare earth phosphate system Na
2
O
P
2
O
5
.
The first ever report on the study of phase equilibria in the rare earth phosphate
system on the whole appeared in 1981
[251]
, where the phase equilibria in the sys-
tem Nd
2
O
3
a
a
R
2
O
3
a
ZrO
2
a
H
2
O at 500
C under 100 MPa and the synthesis of NdP
5
O
14
crystals were studied. This is also the first phosphate system studied hydrother-
mally. All the earlier reports deal with only a part of the phase diagram. The start-
ing materials—Nd
2
O
3
,P
2
O
2
, and 85% H
3
PO
4
solution—were taken in definite
amounts within a sealed golden capsule. Although this method has been included
in the hydrothermal method by the Japanese workers, the partial pressure of water
under these conditions was always low and often less than 1 atm. Therefore, it can
be considered as a special case of the hydrothermal method. This report was fol-
lowed by another publication
[252]
, which deals with the hydrothermal synthesis of
mixed phosphates of neodymium and alkaline metals (M
2
O
P
2
O
5
a
4P
2
O
5
). Here,
the authors have studied the phase formation in the alkaline rare earth phosphate
system (M
2
O
Nd
2
O3
a
Nd
2
O
3
a
P
2
O
5
a
H
2
O, where M
Li, Na, K, Rb, and Cs) under hydro-
700
C and
thermal conditions
in the temperature and pressure range 300
0.5
600 atm, respectively, using vitreous carbon glass, Teflon, and platinum liners.
