Geoscience Reference
In-Depth Information
Figure 8.1 Composition
diagram of the system
KF
a
GdF
3
a
H
2
O at 450
C and
10 kpsi
[3]
.
H
2
O
α
phase
K
2
GdF
5
KF-GDF
3
-H
2
O
450
°C 10
kpsi
A
KGdF
4
KGd
2
F
7
50
K
2
GdF
5
KGdF
4
KGdF
4
+
KGd
2
F
4
α
phase
+
K
2
GdF
5
K
2
GdF
5
+
KGdF
4
GdF
3
KF
K
2
GdF
5
KGdF
4
KGd
2
F
7
field is shown along the left side of the diagram. A general point of particular inter-
est from
Figure 8.1
is that in this hydrothermal system, the phases K
2
GdF
5
, KGdF
4
,
and KGd
2
F
7
are formed and are stable only where KF is in excess in the stoichio-
metric solid, thus, they are incongruently saturating. KF must be between 17.5 and
27 total mol% for KGdF
4
stability where water is at 65 total mol%. A 17.5 total
mol% corresponds to the stoichiometry of KGdF
4
, indicating that the phase is prob-
ably marginally congruently soluble. However, the other boundary of the KGdF
4
phase field, positioned at 27 mol% total KF, indicates that 17 mol% KF is required
to stabilize 8 mol% KGdF
4
. Thus, it can be concluded that a large excess of KF is
required to stabilize the KGdF
4
phase. Analogous reasoning can be applied to
choose conditions for the growth of other phases in
Figure 8.1
.
By considering the nature of the rare earth oxides' interaction with KF solutions,
one can single out four rare earth subgroups. The first one (La
Pr), which showed
no formation of K
2
LnF
5
, begins at q
0.56. In the third subgroup
(Sm, Gd), the values of the q-ratio necessary for the formation of such compounds
were 0.36 and 0.26, respectively; and in the fourth subgroup (Tb
M
KF
/MH
2
O
5
5
Lu), the crystalli-
zation of the K
2
LnF
5
phases begins for q-values smoothly, varying from 0.16 to
0.10. Pentafluorides are crystallized as transparent, faceted, prismatic, crystals up
to 15 mm in length, of colors typical for the corresponding rare earth elements.
The conditions for the investigation of the phase relations in the fluoride systems
are KF/Ln
2
O
3
(molar) ratio up to 100, the M(H
2
O)/M(Ln
2
O
3
) ratio 200, tempera-
ture 450
500
C, and pressure 100
150 MPa. Under these conditions, fluoride
crystals were grown for spectroscopic studies. It has been shown that the K
2
GdF
5
:
Nd
3
1
compounds may be considered as potential media for the excitation of a stim-
ulated emission in the range 1.06
m, whereas the terbium-activated yttrium
and gadolinium compounds may serve as luminophores with saturated green
luminescence.
1.35
μ
