Journal of Alloys and Compounds 274 (1998) 164–168
L
EPR and optical investigation of europium doped Ba 2 Mg 3 F 10
J.M. Rey, H. Bill*, D. Lovy, H. Hagemann
´
` 4, Switzerland
Departement
de chimie physique, Sciences II, 30 quai E. Ansermet, 1211 Geneve
Received 3 March 1998; received in revised form 25 March 1998
Abstract
A recent investigation of the (BaF 2 –MgF 2 ) phase diagram produced several new compounds which are suitable hosts for Rare Earth
impurities. We present results on single crystals of Ba 2 Mg 3 F 10 doped with Eu 2 1 . The local structure and optical properties of this system
were investigated by luminescence emission and by EPR. We observed two different Eu 2 1 sites. Both show C s point symmetry and an
important ground state splitting. Correlating our EPR and optical results with the new Ba 2 Mg 3 F 10 structure data allowed the assignment
of each of them to a specific barium lattice site. The luminescence emission of both the 4f 7 –4f 6 5d and the 4f 7 –4f 7 transitions is observed.
The relative importance of the two emissions is strongly temperature dependent. The emission intensities of the intra f-shell 6 P7 / 2 → 8 S 7 / 2
transitions increase strongly on going from 295 K to 77 K. Thus, the lowest levels of the 4f 6 5d configuration are approximately
degenerate with the 6 P7 / 2 manifold.  1998 Elsevier Science S.A.
Keywords: EPR; Luminescence; Ba 2 Mg 3 F 10
1. Introduction
The luminescence of divalent europium is employed for
fluorescent illumination purposes, in X-ray intensifying
panels and as X-ray storage phosphors, for e.g. [1–4]. As
this impurity presents strong broad band emission in many
host crystals it is further a possible candidate for solid state
lasers in the blue / green, tunable within an extended
wavelength range [5]. The energy transfer processes
occurring between the Eu 2 1 and the host crystal are,
however, often not well understood. If their optimization is
looked for, details of the structure of the local complex
have to be known for the specific hosts of interest. One
aspect is the knowledge of the location of the Eu 2 1 ion,
another one the influence of the ligand field on the
electronic structure. Within the phase diagram (BaF 2 –
MgF 2 ) there exist several stable compounds which have
promising properties as hosts for Rare Earth (RE) impurities. For the reasons mentioned above the Eu 2 1 ion is
of particular interest which motivated us to study the
luminescence and further Electron Paramagnetic Resonance (EPR) of this ion state in BaMgF 4 single crystals
[6], as this latter method is very useful for the identification of the site(s) occupied by the europium ions in the
single-crystalline host matrix. Other promising compounds
*Corresponding author. Fax: 0041 22706518.
0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved.
PII: S0925-8388( 98 )00557-X
within this phase diagram were only recently synthesized
and characterized. These include, for instance, the hosts
Ba 2 Mg 3 F 1 0 [7,8] and Ba 6 Mg 7 F 2 6 [9] which were investigated with the aid of single crystal X-ray diffraction
studies. Common to all of these fluorides is their rather low
crystal symmetry, their excellent optical properties and the
good solubility of RE ions. The structure of these hosts
may be described as consisting of building blocks of
deformed MgF 42
octahedra. The barium sites are of low
6
symmetry and a strong crystal field at these sites was
expected. The present paper presents EPR and optical
excitation and luminescence results of Eu 2 1 in the
Ba 2 Mg 3 F 10 host. This compound crystallizes in the monoclinic space group C2 /m (No. 12). We chose b as the
unique axis. Two different barium sites are present. They
are located in the mirror plane (both on Wyckoff site 4i),
and situated in the interstices of the deformed MgF 46 2
octahedra. Crystals were grown in our laboratory.
2. Experimental
The compound Ba 2 Mg 3 F 1 0 doped with 2 mol‰ of EuF 2
was synthesized from a stoichiometric melt of the fluorides
BaF 2 (Merck, suprapure), MgF 2 (Balzers, oxygen-free
vacuum deposition grade) and EuF 2 (Cerac, 99,9% pure).
Ba 22x Mg 3 F 10 : Eu x21 crystals with nominally x50.002
J.M. Rey et al. / Journal of Alloys and Compounds 274 (1998) 164 – 168
were studied. Single crystals were grown (using a pyrolitic
graphite crucible) by slow cooling of the melt under 0.2
atm of argon. Great care was taken to avoid oxygen
contamination during growth. In particular BaF 2 was
precrystallized beforehand and the absence of oxygen was
verified by UV absorption and luminescence spectroscopy.
A typical size of the single crystals was 53533 mm 3 . The
samples used for the EPR experiments were oriented with
the aid of a polarizing microscope or by the Laue X-ray
technique and then ground to shape on no. 1200 emery
paper.
EPR spectra were obtained at 36.4 GHz on a home-built
Ka-band spectrometer provided with a low temperature
microwave cavity / sample holder system of our design.
Data analysis and computer simulations were performed
with our SPINHAM program [6].
Low resolution optical measurements were carried out
with a home-built spectrophotometer already described
elsewhere [6]. High resolution luminescence measurements
where performed on a SPEX 1403 monochromator with
excitation light provided by a deuterium lamp and spectrally decomposed with a DURRUM 13000 monochromator.
3. Results
Most of the Ka-band EPR spectra were recorded at 295
K and 77 K. A few experiments were performed at 4.2 K
to determine the absolute sign of each of the b ml . Two EPR
centers are observed, denoted site no. 1 and no. 2,
respectively in the following. The spectrum recorded with
B oriented such that site no. 1 shows the largest splitting
(Fig. 1) consists of two groups of seven fine-structure
packets, each one showing hyperfine structure due to the
isotopes 151 Eu and 153 Eu. Although the detailed treatment
of hyperfine and super-hyperfine interaction terms is more
conventional, it is not important in the following and this
detailed analysis is not given at present. The angular
variation in the (010) plane at 77 K (Fig. 2) shows extreme
line positions at 318 from the a-axis for the first group of
165
Fig. 1. Ka-band EPR spectra of Ba 2 Mg 3 F 1 0 :Eu (2‰) with B parallel to
the maximum splitting of the center no. 1. T577 K and y 536.4 GHz.
lines (site no. 1) and at 52.88 for the other one (site no. 2).
The angular variation obtained when B was rotated parallel
to a (001) plane was further studied. We observed that the
spectra are symmetrical with respect to (010). Thus, (010)
is a symmetry plane for the two sites. Thus two different
Eu 2 1 paramagnetic centers were observed, both having Cs
point symmetry.
The angular variation of both, the 295 K and the 77 K
Eu 2 1 spectrum, were successfully parametrized with the
following spin Hamiltonian:
H 5 gb0 BS 1 (b 20 O 20 1 b 21 O 21 1 b 22 O 22 )
1 (b 04 O 04 1 b 14 O 14 1 b 24 O 24 1 b 34 O 34 1 b 44 O 44 )
(1)
The symbols have their usual meaning with the O ml
representing Steven’s operator equivalents [10,11]. Formally, the b m6 terms have to be included. But the spectral
components have typical line widths of 60 MHz and a
residual error of approximately 50 MHz / point was already
attained by fitting Eq. (1) to the angular variations. The b m6
were indeed neglected because their inclusion resulted in
only a marginal improvement with numerical values of the
Table 1
Spin-Hamiltonian parameters (in 10 2 4 (cm 2 1 )) for Eu 2 1 in Ba 2 Mg 3 F 1 0
Site
Temperature
No. 1
R.T.
gisotropic
b 02
b 12
b 22
b 04
b 14
b 24
b 34
b 44
Angle a
1.995
2469.6
21.971
144.1
20.042
20.087
20.076
0.610
20.285
30.3
a
No. 1
77 K
(3)
(62.3)
(612)
(64.5)
(60.037)
(60.27)
(60.27)
(61.1)
(60.29)
(5)8
1.992
2524.6
20.966
142.1
20.044
20.066
20.036
0.858
20.211
31.0
No. 2
R.T.
(4)
(62.6)
(616)
(65.5)
(60.045)
(60.40)
(60.37)
(61.3)
(60.38)
(5)8
1.997
255.6
0.109
2144.9
0.013
20.011
0.014
20.742
0.037
55.5
No. 2
77 K
(3)
(61.5)
(68.1)
(63.9)
(60.033)
(60.20)
(61.21)
(60.85)
(60.26)
(5)8
Angle between the a crystal axis and the z axis of the local EPR referential with the EPR y axis parallel to the b crystal axis.
1.993
290.9
20.200
2205.5
0.018
0.036
0.073
20.869
0.031
52.8
(3)
(62.3)
(611)
(64.7)
(60.047)
(60.26)
(60.30)
(61.1)
(60.32)
(5)8
J.M. Rey et al. / Journal of Alloys and Compounds 274 (1998) 164 – 168
166
Table 2
Transformation matrix between crystal axes (a, b, c*) and local EPR axes
(x, y, z) with ‘Angle’ to be taken from Table 1
x EPR-ref.
y EPR-ref.
z EPR-ref.
a crystal-axis
b crystal-axis
c* crystal-axis
sin (Angle)
0
cos (Angle)
0
1
0
cos (Angle)
0
2sin (Angle)
corresponding constants being zero within the error limits.
The optimized parameters for the two sites obtained at 295
K and 77 K are given in Tables 1 and 2. The indicated
errors correspond to the variation needed to increase the
residual error of the fits by a factor of 1.5. The absolute
sign of the crystal field parameters was determined with
the aid of a Ka-band experiment at 4.2 K. As often
observed in low symmetry complexes, the b 02 and b 22
parameters are the most important ones for both Eu 2 1
centers, but the inclusion of the other terms (especially b 04
and b 34 ) is essential as it decreases the residual error by as
much as 50%. The theoretical angular variation at 77 K
calculated with the aid of the constants of Table 1 is shown
as a solid line in Fig. 2.
Optical emission and excitation spectra were obtained at
295 K and 77 K from the same Ba 2 Mg 3 F10:Eu (2‰)
sample previously studied by EPR. Typical results are
shown in Fig. 3.
At 295 K, the emission spectrum (excited at 33 300
cm 2 1 ) exhibits two broad f–d bands at 27 730 and 24 800
cm 2 1 . Weak and sharp f–f lines centered at 27 820 cm 2 1
are also present. The excitation spectrum (detected at
27 800 cm 2 1 ) consists of two large features formed by
several unresolved bands.
When the temperature is lowered to 77 K, the broad
emission bands decrease in intensity at the expense of the
f–f transitions. The 77 K emission spectrum (excited at
33 300 cm 2 1 ) is formed by eight f–f lines (see insert in
Fig. 3). As their relative intensities depend on the wavelength of the excitation light, their assignment into two
Fig. 3. 295 K and 77 K emission (excitation at 33 300 cm 2 1 ) and
excitation (detection at 27 800 cm 2 1 ) spectra of a Ba 2 Mg 3 F 1 0 :Eu (2‰)
single crystal. Insert: high resolution emission spectrum (excited at
33 300 cm 2 1 ).
groups of four lines was made possible. Their positions are
given in Table 3. The remaining two rather weak f–d bands
of the emission spectrum have their maxima at 24 400 and
27 500 cm 2 1 . The excitation spectrum on the other hand
(detected at 27 800 cm 2 1 ) exhibits two large bands with
maxima at approximately 43 400 and 36 500 cm 2 1 .
4. Discussion
On the basis of ionic radius and isocharge arguments the
Eu 2 1 ions are expected to occupy preferentially the Ba 2 1
sites in the crystal lattice, similar to the situation in
BaMgF 4 [6]. The EPR results demonstrate that both Eu 2 1
ions are located in a mirror plane parallel to (010). The C s
point symmetry of each of the Ba 2 1 sites in Ba 2 Mg 3 F 10
fulfils the condition of these mirror planes [7]. The two
local coordination environments of the host cations are
presented in Fig. 4.
The first BaF 102
cluster (A) is formed by a distorted
12
cube of eight fluorine ions (2 F1, 2 F2a, 2 F2b and 2 F8)
arranged symmetrically above and below the mirror plane
m. Among the four remaining ions, two F5 are also
symmetrically arranged while F49 and F59 are in the mirror
plane. The distance between the Ba 2 1 ion and the F2a ions
Table 3
Positions of the 77 K f–f emission lines in Ba 2 Mg 3 F 10 :Eu(2‰) (excitation at 33 300 cm 2 1 )
Peaks position (cm 2 1 )
Group No. 1
Group No. 2
Fig. 2. Experimental angular variation at Ka-band frequency (crosses) in
the (010) plane together with the results of the simulation (line, see text)
at T577 K and n 536.4 GHz.
27 768
27 841
27 886
27 916
27 784
27 804
27 823
27 849
J.M. Rey et al. / Journal of Alloys and Compounds 274 (1998) 164 – 168
Fig. 4. The two BaF 1 102
clusters in Ba 2 Mg 3 F 10 . The mirror plane m is
2
parallel to the paper and contains the central Ba 21 ions.
(260 pm), is by far the shortest, the others range from 279
to 313 pm. The second BaF 102
cluster (B) also consists in
12
a distorted cube of eight F 2 ions (2 F1, 2 F2, 2 F7 and 2
F8) symmetrically arranged with respect to the mirror
plane. The two remaining F4 ions are above and below the
mirror plane which contains F59 and F69. In this cluster the
Ba–F distances range from 272 to 303 pm.
Of the two EPR centers observed, the one with the
strongest crystal field splitting has its principal axis at 318
from the a crystal axis. The purely electrostatic point
charge model predicts that the crystal field splitting
increases under shortening of the ligand distances, though
a more realistic version considers also the possibility of
sign changes as a function of the metal–ligand distance
[6]. The first BaF 102
cluster (A) has by far the shortest
12
Ba–F distances. In particular Ba(F2a) 2 resembles a local
‘molecular’ complex. Furthermore, the projection of the
Ba–F2a direction onto the mirror plane corresponds to the
maximum splitting of the EPR center no. 1. Therefore, we
assign the EPR center no. 1 to an Eu 21 ion substituting a
Ba 2 1 on the A site of the crystal lattice and forming an
EuF 2 local cluster. The local symmetry group only constrains the Eu 2 1 to lie on m. Due to the substitution, small
shifts within the mirror plane with respect to the Ba sites
are expected for this ion. For this reason, the very good
agreement between the angle obtained by EPR and the
angle of the projection of Ba–F2a into the m plane is
slightly fortuitous. The second EPR center (no. 2) can now
be assigned to the Eu 2 1 ion situated on the B lattice site of
the crystal. Of course, the same remarks apply regarding
the precise position of the impurities with respect to the
ideal lattice site. The comparatively important temperature
dependence and the important absolute value of the
dominant b ml parameters indicate that the mechanisms of
almost complete balancing of the different components of
the crystal field observed in many Matlockite-type hosts
[12] does not seem to apply for the Ba 2 Mg 3 F 10 host. This
is also not really expected because there is no evident
partial compensation between octahedral and cubic contributions to the ligand field for the present host. This
167
situation is in good agreement with the similar one
encountered for Eu 21 in BaMgF 4 [6]. It is remarkable,
however, that the absolute sign of the dominant terms of
the crystal field is inverted for the two sites. There is
perhaps a compound in the BaF 2 –MgF 2 phase diagram
where the crystal field is nearly zero.
The 295 K emission spectrum shows two broad bands as
dominant features. We assign them to 4f 6 5d→4f 7 transition
of the two Eu 21 ions, similar to the situation in many Eu 21
doped halide crystals [13]. The fact that lowering the
temperature redirects the emission intensity onto the intra
f-shell 6 P7 / 2 – 8 S 7 / 2 is remarkable. It shows that the lowest
levels of the 4f 6 5d configuration are approximately degenerate with the 6 P7 / 2 manifold. Additionally, one has to take
into account that the ligand field shows a quite important
temperature dependence which affects much more strongly
the 4f 6 5d configuration than the 4f 7 one.
The excitation spectrum shows barely resolved structure
6
1
which is probably due to the lowest levels of the (4f , 5d )
multiplets of this excited configuration. The structure is in
part due to exchange coupling between the multiplets of
the 4f 6 configuration and the 5d electron. But note that the
dominant splitting is most likely due to this latter electron
because the intra-multiplet splittings of 4f 6 are not large.
Another contribution to the structure of the excitation
spectrum arises from the fact that the 4f 6 5d configuration
is not affected in the same manner for the two Eu 2 1
centers.
5. Conclusion
The EPR and luminescence studies presented above
show that the Ba 2 Mg 3 F 10 compound doped with EuF 2
leads to two different Eu 2 1 centers. Each one has an
important ground state splitting due to a ligand field with
Cs point symmetry. The low symmetry of this ligand field
lifts most of the degeneracy whereas its considerable
strength produces an important spread in energy of the
resulting level structure. Such a ligand field tends to lower
the bottom of the 4f 6 5d band and promotes the interconfiguration 4f 7 –4f 6 5d emission [14]. Despite this fact,
competition between the 4f 7 –4f 6 5d and 4f 7 –4f 7 luminescence emissions occurs in this system.
Acknowledgements
The authors thank Mr. D. Frauchiger for technical help.
This work was supported by the Swiss Priority Program
Optics 2 and the Swiss National Science Foundation.
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