Geoscience Reference
In-Depth Information
The solubility of KGdF
4
is a function of T in 12 mol% KF (defined as mol KF/
(mol of KF
H
2
O)) at a pressure of 10 kpsi.
Figure 8.2
shows the solubility of
KGdF
4
as a function of temperature and concentration of KF. Assuming a linear
relationship between solubility and mol% KF, a slope of 0.02 mol solute dissolved/
mol KF suggests the following generalized multistaged reaction:
1
nF
2
"
KGdF
4
1
KGdF
4
1
n
n
ð
8
:
1
Þ
Since the reaction does not proceed easily to the right, 50 mol of KF are
required to dissolve 1 mol of KGdF
4
[1]
. Past experience in growing crystals
hydrothermally indicates that the magnitudes of solubility and temperature coeffi-
cients of solubility are appropriate for growth with a KF mineralizer. The log of
solubility versus the reciprocal of absolute temperature is approximately linear.
From the slope of a log(s) versus l/T plot, the heat of solution (or the heat of reac-
tion of
Eq. (8.1)
) is approximately 12 kcal/mol
[3]
. In order to obtain reasonably
sized crystals, controlled transport in a temperature gradient of nutrient that is
thermodynamically stable is essential. Mineralizer concentration is chosen to be
within the KGdF
4
phase field, high enough to provide reasonable solubility. The
optimum conditions are as follows: KF solvent: 16 mol%, 80% initial fill, dissolu-
tion temperature: 450
C, growth temperature: 425
C. The previously synthesized
seeds of size 4
0.5 mm
3
were placed on a point gauze basket mounted at
the top of the growth zone.
The other alkali rare earth fluorides of interest are LiKYF
5
,CsY
2
F
7
, KYF
4
, and
K
2
YF
5
doped with Nd
3
1
ions up to 10 at%. These crystals were obtained at
T
5
0.5
3
3
700
C and P
5
20 days, and the
crystal size was 5 mm
[1]
. Some of these single-centered laser fluorides such as
LiKYF
5
:Nd
3
1
have better physical characteristics in comparison with oxygen-
containing substances. Most of these compounds crystallize in the lower symmetry.
Figure 8.3
shows the fragments of the LiKYF
5
crystal structure
[4]
.
450
100
150 MPa, over a period of 15
Figure 8.2 Solubility of KGdF
4
as a function
of temperature and concentration of KF
[3]
.
KF (mol%)
10
14
18
22
6
5
4
3
2
1
0
375
425 475
Temperature (°C)
525
