Geoscience Reference
In-Depth Information
As is evident, the type of rare earth phosphate determines the degree of conden-
sation of phosphoro-oxygen anions, which depends upon the pH of the solution and
concentration of P
2
O
5
. And during the interaction of NdCl
3
and YCl
3
with sodium
tripolyphosphate at pH
4.5, formation of Nd
5
(P
3
O
10
)H
2
O was established
[258]
.
The actual process of synthesis from aqueous solution takes place in many stages.
The soluble pyrophosphates M
4
P
2
O
7
(where M
.
NH
4
, Na, K) serves as the source
of phosphate anions. In the crystallization of Na
5
R(P
4
O
12
)
2
, triphosphate anions
(P
3
O
10
)
2
5
are initially stabilized in the form of solution-(NH
4
)
5
P
3
O
10
or Ca
5
P
3
O
10
,
in which ErCl
3
or NdCl
3
dissolves followed by the crystallization of monocationic
and mixed triphosphates Er
5
P
3
O
10
,MErP
3
O
10
3
5
Cs and NH
4
[259]
. By changing the partial pressure of water and the temperature of the system,
the existence of phosphoro-oxygen anions (starting from orthophosphate up to
ultraphosphate) can be stabilized. A required [P
2
O
7
], [P
4
O
12
], or [P
5
O
14
] anion is
created first in the solution, and subsequently it was allowed to react with metals to
obtain respective metal phosphate complexes. By such a direct method ortho-,
pyro-, meta-, and ultraphosphates of rare earth elements have been synthesized.
A series of mixed and simple anhydrous and hydrous rare earth phosphates have
been obtained by the Russian and Chinese groups
[46,257
H
2
O, where M
5
260]
. Under hydrother-
mal conditions, the crystal chemistry of these rare earth phosphates have been stud-
ied in great detail and correlated with growth conditions. Xu et al.
[258
260]
and
Pang (Personal communication, 1999) have studied M
2
O
H
2
O
(M
5
NH
4
, Na, K) system under hydrothermal conditions and obtained M
2
Ce
(PO
4
)
2
a
CeO
2
a
P
2
O
5
a
4
1
) and M
3
Ce(PO
4
)
2
,M
5
Ce(PO
4
)
3
.
Byrappa et al.
[261,262]
synthesized Na
2
(R, Co)Zr(PO
4
)
3
(R
H
2
O, (Ce
5
rare earths)
5
200
C and pressure 50
crystals under hydrothermal conditions at T
5
60 atm.
The experiments were carried out in Morey-type autoclaves provided with Teflon
liners. The cross section of the autoclave used in the synthesis is shown in
Figure 7.61
.
The crystallization was carried out by spontaneous nucleation that was con-
trolled through a systematic rate of heating the alkaline component (Na
2
O). It was
taken in the form of a solution (NaOH) with a known molarity and this solution
acts as a mineralizer. In the growth of double pyrophosphates, the addition of sur-
plus P
2
O
5
did not change the resultant product, which can be explained from the
influence of H
2
O vapor pressure in the system. Surplus P
2
O
5
should actually result
in the formation of simple or compound orthophosphates.
These rare earth orthophosphates have been obtained using the starting compo-
nents in the following molar ratio:
Na
2
O:M
2
1
O
M
3
2
O
3
:M
1
4
O
2
:P
2
O
5
5
:
4
:
5:1:0
:
8:10
ð
7
:
18
Þ
This has been reviewed by Byrappa
[263]
.
Bykov et al.
[264]
have obtained Li
3
M
2
(PO
4
)
3
(M
Fe, Sc, Cr) using different
techniques including hydrothermal technique. However, the resultant product was
polycrystalline powder. The crystallization of Li
3
Sc
2
(PO
4
)
3
was carried out from
the systems: LiH
2
PO
4
a
5
Sc
2
O
3
a
H
3
PO
4
a
H
2
O; Li
3
Sc
2
(PO
4
)
3
a
NH
3
H
2
PO
4
a
H
2
O;
