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Pr( 1 G 4 !
3 H 5 ) at 1.3
m; and (iii) pump Pr( 3 H 4 !
l G 4 ) at 1.02
μ
μ
m and observed
emission from Pr( 1 G 4 !
3 H 5 ) at 1.3
m. The results are given in Table 8.1 .
From the data in Table 8.1 , the following observations can be made. First, the
lifetime of the Yb fluorescence is sufficiently long to permit energy transfer from
the Yb 2 F 5/2 to the Pr l G 4 levels. At both doping levels, the lifetime of the Yb 2 F 5/2
state is longer than the lifetime of the Pr
μ
l G 4 level. Second, both the intensity and
the lifetime of the Pr( 1 G 4 !
3 H 5 ) transition increase when Yb is pumped and com-
pared to the Pr pumped.
Thus, the rare earth fluorides have found unique technological applications.
Their synthesis is highly challenging.
8.3 Hydrothermal Synthesis of Transition Metal Fluorides
The hydrothermal method permits us to synthesize single crystals of some transi-
tional metals and other ternary fluorides. The majority of these compounds show
superionic, magnetic, and other important physical properties. Besides, these com-
pounds show very interesting structural characteristics which contribute to their
superior physical properties. Thus some researchers focus mainly on their structural
variations than their bulk single crystal growth. Aldous [11] has synthesized about
57 novel crystal structures belonging to this family using mild solvothermal route.
Ti, Zr, Hf, V, Nb, Ta, and Mo have been used to make fluoride or oxyfluoride
materials at temperatures ranging from 60 C to 200 C. Both inorganic and organic
moieties (“templates”) have been used to direct the formation of the (oxy)fluoride
materials, usually as an anionic species. These structures have a tendency to form
isolated polyhedral units under the mild solvothermal conditions. Several examples
of isolated [MF 6 ] n 2 and [MF 7 ] n 2 monomers are reported and also one example of
a transconnected [TiOF 4 ] 2 infinite chain structure.
Among alkali zirconium fluorides, compounds of special
interest are those
belonging to the A 3 ZrF 7 family (A
Na, R, Rb, Cs, NH 4 ) with the characteristic
5
ordered
disordered (OD) structures. Except
for Na 3 ZrF 7
(its structure is a
Table 8.1 Fluorescent Intensity and Lifetime of KGdF 4 Crystals [4]
Crystal Fluorescent Rare Earth
Yb a
Pr b
Pr c
Intensity
(a.u.)
Lifetime
( μ s)
Intensity
(a.u.)
Lifetime
( μ s)
Intensity
(a.u.)
Lifetime
( μ s)
KGd 0.98 Yb 0.01 Pr 0.01 F 4 240
1120
40
150
90
600
KGd 0.94 Yb 0.05 Pr 0.01 F 4 250
400
90
115
280
500
a Test conditions: pump Yb( 2 F 5/2 ) at 0.94 μ m; emission from ( 2 F 5/2 !
2 F 7/2 ) Yb at 0.97 μ m.
b Test conditions: pump Pr( 1 G 4 ) at 1.02 μ m; emission from ( 1 G 4 !
3 H 5 ) Pr at 1.3 μ m.
c Test conditions: pump Yb( 2 F 5/2 ) at 0.97 μ m; emission from ( 1 G 4 !
3 H 5 ) Pr at 1.3 μ m.
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