Geoscience Reference
In-Depth Information
8.2.2 Spectroscopic Properties of Rare Earth Fluorides
There are over 250 Nd
3
1
-doped insulating laser crystals and anisotropic single-
centered fluorides with ordered structures which are unique owing to their excel-
lent spectroscopic properties
[5,6]
. The most important ones are tetragonal
LiLnF
4
crystals with a space group I4
1/a
, as well as monoclinic BaY
2
F
8
(space
group C2/m) crystals. Similarly multicentered disordered cubic 5NaF
9LnF
3
:
Nd
3
1
crystals with space group Fm3m, may be considered as potential media for
the excitation of a stimulated emission in the range 1.06
m
[7]
. Being
transparent in a wide spectral region, these fluorides reveal a negative dn/dT
coefficient, possess a high stability with reference to short wavelength radiation
of pumping Xe-flashlamps, and have satisfactory thermophysical characteristics.
This has led to the fact that these materials are rather widely used in quantum
electronics. However, low distribution coefficients of Nd
3
1
ions prevent obtaining
single crystals of a majority of these compounds with characteristics that are opti-
mal for solid-state lasers of the new generation, i.e., lasers using semiconductor
laser-diode excitation. That is why the search for new anisotropic single-centered
generating fluorides is an extremely urgent problem. These new crystals are able
to generate stimulated emission at 300 K in free-running pulse operating mode on
inter-stark transitions of two
1.35
μ
4
I
13/2
laser channels of
Nd
3
1
ions with Xe-flashlamp excitation as well as in CW mode (
4
F
3/2
!
4
F
3/2
!
4
I
11/2
and
4
F
3/2
!
4
I
11/2
)
under laser-diode pumping
[1,8
10]
. The radiation from these transitions pro-
motes the formation of mainly one line, which in turn provides the conditions
favorable for the excitation of the stimulated emission at frequencies of the main
intense lines. The spectroscopic data obtained have been used in the synthesis of
these rare earth fluorides by the hydrothermal technique
[4]
.
Figure 8.4
shows
the luminescence spectra (
4
F
3/2
!
4
I
13/2
) at 77 K (a) and 300 K (b) and the scheme
4
F
3/2
and
4
I
13/2
transitions (c) of Nd
3
1
ions in
of crystal
field splitting of
Distance
(10
-1
nm)
Bond
b
a
c
Y
Y polyhedron
Y-F
2.202(3)
K
+
position
K
2.215(2)
Li
Li
2.244(3)
2.304(3)
×
3
2.313(3)
2.323(3)
K
Y
Y
3+
polyhedron
Y
K
Li tetrahedron
Li
+
distored
tetrahedron
Li-F
1.842(10)
1.870(9)
1.948(9)
1.986(10)
Li
Li
K
Y
K polyhedron
2.647 to 3.112
(9 contacts)
K-F
Figure 8.3 Fragments of the LiKYF
5
crystal structure
[4]
.
