Geoscience Reference
In-Depth Information
nezoite crystals, it is first necessary to bring Nd ions into a dissociated state, fol-
lowed by creating the conditions for the formation of nezoite complexes, and only
then does the desired crystallization take place. The most suitable systems for this
purpose appear to be Nd containing ternary systems of type M
2
O
a
Nd
2
O
3
a
T
x
O
y
,
where M
H, Li, K, Rb, Cs, and Tl or their admixtures. The process of crystalliza-
tion is much more simple in the multicomponent systems. The quantitative ratio of
the component largely determines the stoichiometric nezoitic complexes in these, i.
e., the ratio of T
x
O
y
/Nd
2
O
3
should not be less than 8. The main problem of concern
is the incorporation of some components in excess than the stoichiometric ratio. As
a result, the molar ratio of the nutrient components in the growth of nezoites is not
regular and can be expressed approximately as M
2
O(M)/Nd
2
O
3
/T
x
O
y
5
5
8.
The additional amount of the components is introduced into the nutrient depending
upon the individual cases. Thus, the growth of nezoites requires a definite surplus of
T
x
O
y
components, and they may not be taken from the stoichiometric melt.
Therefore, in the growth of nezoites, either high-temperature aqueous solution or
flux agents are used. Similarly, in the synthesis of many phosphates in the form of
crystals, highly concentrated solution of phosphoric acid is used. It is considered
that at over 250
C, orthophosphoric acid converts into pyrophosphoric acid and with
a further increase in temperature it transfers to an even more condensed state due to
the expulsion of water
[257]
.
Figure 7.60
represents a temperature-composition dia-
gram for the system H
2
O
2
1
.
,
P
2
O
5
, at pressure up to 1 atm
[240]
. In region (2) of the
diagram, crystallization can be carried out at normal pressure, whereas the crystal
growth from solution corresponding to the composition of the region (1) requires the
use of a hydrothermal apparatus. Here, the authors discuss the crystallization of rare
earth phosphates as an example to explain the influence of the physicochemical con-
ditions and the primary structural units. The formation of rare earth phosphates in
acidic (pH
a
7.0) phosphoro-oxygen media takes place as follows:
.
R
n
L
M
x
P
y
O
z
!
R
n
P
y
O
z
1
M
x
L
1
ð
7
:
17
Þ
R
n
L
M
x
P
y
O
z
!
M
x
R
n
P
y
O
z
1
anion and it is usually Cl
2
,NO
3
,orSO
4
,M
where L
alkali cation and NH
4
.
5
5
Figure 7.60 Temperature-composition
diagram for the system H
2
O
a
P
2
O
5
at
1 atm pressure
[240]
.
800
600
1
400
200
2
0
20
40
60
80
100
