Localization of Eu3+ in KGd1.9Eu0.1F7 and KGd1.9Eu0.1F6.97O0.015 by site-selective excitation and time-resolved spectroscopy

Article abstract of DOI:10.1016/S0925-8388(99)00168-1

This paper reports site-selective excitation (or observation) and time-resolved spectroscopy of the host matrices KGd₂F₇ and KGd₂F_6.97O_0.015 doped by Eu³⁺. The crystal structure of KGd₂F₇ derives from that of the fluorite-type, by an ordering of the cations and anions. The luminescence spectra and site-selective excitation in the ⁵D₀→⁷F₀ region, have allowed to identify several distributions of discrete sites for the Eu³⁺ ions, in which two sites are characterized by unusual spectra which are attributed to Eu-O bonds. Another kind of site, characteristic of fluorinated surroundings close to the centrosymmetry, exhibits a very long lifetime (6.8 ms) of the ⁵D₀ level. Although the accurate structures of these compounds are not yet known, they are very close to that of KGd_2.77F_9.32 previously solved and the spectroscopic results are in agreement with the number and the symmetries of the rare earth crystallographic sites.

Full text of DOI:10.1016/S0925-8388(99)00168-1

Journal of Alloys and Compounds 289 (1999) 71–80
L
by
3
1
Localization of Eu in KGd Eu F and KGd Eu F O
1
.9
0.1
7
1.9
0.1 6.97 0.015
site-selective excitation and time-resolved spectroscopy
a
a ,
a
a
b
*
A. Pierrard , P. Gredin , N. Dupont , A. de Kozak , B. Piriou
a
Laboratoire de Cristallochimie du Solide, Universit e´ P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
Laboratoire de Physicochimie Mol e´ culaire et Min e´ rale, U.R.A. CNRS 1907, Ecole Centrale de Paris, Grande Voie des Vignes,
2295 Chatenay-Malabry Cedex, France
b
9
Received 8 February 1999; received in revised form 22 March 1999
Abstract
This paper reports site-selective excitation (or observation) and time-resolved spectroscopy of the host matrices KGd F and
2
7
3
1
KGd2F6.97O0.015 doped by Eu . The crystal structure of KGd F derives from that of the fluorite-type, by an ordering of the cations and
anions. The luminescence spectra and site-selective excitation in the D → F region, have allowed to identify several distributions of
discrete sites for the Eu ions, in which two sites are characterized by unusual spectra which are attributed to Eu–O bonds. Another kind
of site, characteristic of fluorinated surroundings close to the centrosymmetry, exhibits a very long lifetime (6.8 ms) of the D level.
2
7
5
7
0
0
3
1
5
0
Although the accurate structures of these compounds are not yet known, they are very close to that of KGd2.77F9.32 previously solved and
the spectroscopic results are in agreement with the number and the symmetries of the rare earth crystallographic sites.
Science S.A. All rights reserved.
 1999 Elsevier
3
1
Keywords: Fluoride; Eu : KGd F ; Local probe; Eu–O luminescence; Site-selective excitation; Time-resolved spectroscopy
2
7
1
. Introduction
refractive index, low phonon frequencies giving long
lifetimes of the excited states, reduced cross-relaxation.
The aim of our investigation is the characterization of the
3
1
Among the trivalent rare earth ions, Eu is the most
appropriate for an application as a local probe. First, each
site occupied by Eu leads to one line for the D → F
transition, since it occurs between two non-degenerate
states. Second, the low degeneracy of the other sub-levels
3
1
host matrix KGd F with Eu as a local probe, using both
2
7
3
1
5
7
site-selective excitation (or observation) and time-resolved
spectroscopy. Three samples are investigated:
0
0
KGd1.9Eu0.1F7 synthesized at 6308C, KGd1.9Eu0.1F7 syn-
thesized at 8508C and KGd1.9Eu0.1F6.97O0.015 synthesized
at 8508C. For this latter compound, we try to evaluate the
influence of oxygen on the crystal structure and lumines-
cence properties.
7
7
5
usually involved in the observed transitions ( F , F , D ,
1
2
1
5
D ) leads to simple spectra which allow to get infor-
2
mations about the symmetry of the surroundings of a given
site. By using the site-selectivity of the emission and
excitation spectra, it is possible to extract the contribution
3
1
of different kinds of Eu sites in a structure. The time-
resolved spectroscopy, which distinguishes the emission
arising from levels with different lifetimes, increases the
discrimination between the spectra of the sites. We have
2. Syntheses
The starting fluorides were prepared in the laboratory.
The gadolinium and europium trifluorides are obtained by
reaction between their oxides (purity: 4 N) and anhydrous
hydrogen fluoride at 7508C. The potassium fluoride is
obtained by reaction between potassium carbonate K CO
3
1
focused our attention on the doped fluoride Eu :KGd F .
2
7
This work is a part of a general study on fluorinated
matrices for optical applications [1,2], particularly for laser
applications. Indeed, fluorides have several specific fea-
tures for this kind of application: weak crystal field, small
2
3
and an aqueous solution of 40% hydrofluoric acid. The
solution is evaporated at 1008C until precipitation of
KHF . This compound melts at 2398C and loses gaseous
*
Corresponding author.
2
E-mail address: gredin@ccr.jussieu.fr (P. Gredin)
HF from its melting point to its boiling temperature at
0
925-8388/99/$ – see front matter
 1999 Elsevier Science S.A. All rights reserved.
PII: S0925-8388(99)00168-1

7
2
A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
˚
3
518C [3]. At this temperature, solid KF crystallises. It is
and 4.021 A and the splitting of the strong lines, suggest
hygroscopic and must be protected from moisture in a
glove box placed under dry argon. KGd F and KEu F
are synthesized by direct solid state reaction. First, a
the existence of a surstructure. The X-ray powder pattern
can be indexed in a tetragonal cell with the parameters:
2
7
2
7
]
Œ
˚
˚
a58.088(2) A (¯a 2) and c511.737(3) A (¯a 32).
c
c
mixture of 1 KF12 REF (RE5Gd, Eu) is homogenized in
On the X-ray powder pattern of KGd1.9Eu0.1F7 syn-
thesized at 8508C (Fig. 2), many lines of weak intensity
and the splitting of the fluorite-type structure lines are
observed. This powder pattern is not yet indexed and the
crystal structure of K0.33Gd0.67F2.33 synthesized at 8508C
is not exactly known, but its accurate structural determi-
nation is in progress. This one may be very close to the
crystal structure of K0.265Gd0.735F2.47, which was solved
previously [5].
3
a mortar in a glove box, then introduced in a gold crucible
placed in a dry argon stream and heated at 6008C for 24 h.
Powdered samples of KGd1.9Eu0.1F7 are prepared by solid
state reaction of stoichiometric mixtures of KGd F and
2
7
KEu F at 8508C for 15 h for the first sample and 6308C
2
7
for 48 h for the other one. KGd1.9Eu0.1F6.97O0.015 is
prepared by solid state reaction of stoichiometric mixtures
of KGd F , KEu F and Gd O at 8508C for 15 h.
2
7
2
7
2
3
3
. Structural data and X-ray diffraction
4
. Optical measurements
Many reports are devoted to the investigation of the
KF-REF3 systems (RE5La to Lu) and the compound
4.1. Raman spectroscopy measurements
KRE F is highlighted in all. It was shown, particularly for
2
7
RE5Gd and Eu, that this fluoride must be renamed
K0.33RE0.67F2.33 [4], because it is just a special point of a
solid solution formulated K12xRExF112x (0.665#x#
The Raman spectrum of KGd F synthesized at 6308C,
2
7
was recorded using an argon laser with an excitation
wavelength of 514.5 nm. The spectrum shows a broad
2
1
0
.735). The crystal structure of one limit of the solid
band of weak intensity centered at 350 cm . For a strict
fluorite structure, only one active Raman mode is observed
(F ). The existence of a broad band is characteristic of a
solution, K0.265Gd0.735F2.47, was already solved [5].
All the X-ray powder patterns were recorded on a 17-cm
vertical Philips PW1050/25 diffractometer (iron an-
2
g
disordered structure as expected for an anion excess
fluorite-type structure with, moreover, a random distribu-
˚
ticathode: l51.9361 A).
3
1
1
The strongest lines encountered on the X-ray powder
tion of Gd and K on the cationic sites. The phonons
21
pattern of KGd1.9Eu0.1F7 synthesized at 6308C, can be
cut-off frequency can be fixed at 450 cm . This value is
in the range of those encountered for other fluorides (for
˚
indexed on the basis of a fluorite-type cell (a ¯5.7 A)
c
2
1
21
(
Fig. 1). Nevertheless, two weak lines observed at 5.814
example, 360 cm for SrF [6] or 350 cm for LaF [7])
2 3
Fig. 1. Powder diffraction pattern of KGd1.9Eu0.1F7 synthesized at 6308C.

A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
73
Fig. 2. Powder diffraction pattern of KGd1.9Eu0.1F7 synthesized at 8508C.
and is very low compared to the usual values observed for
necessary to avoid the saturation of the detector by the
excitation beam. So, the line noted by a star (*) on the
figures is an artefact due to the removal of the diaphragm
aperture.
2
1
oxides (700 cm for Y Al O [6]).
3
5
12
4
.2. Luminescence spectroscopy
The third harmonic of a Q-switched neodynium YAG
4.3. Non-selective spectra
laser (Spectra Physics) was used as non-selective excita-
tion source (l5355 nm). This laser was also used to pump
a tunable dye laser (Jobin-Yvon, LA04/E1T model) for
The non-selective spectra of the three samples:
KGd1.9Eu0.1F7 synthesized at 6308C, KGd1.9Eu0.1F7 syn-
thesized at 8508C (Fig. 3) and KGd1.9Eu0.1F6.97O0.015
synthesized at 8508C, were recorded using the excitation
wavelength of 355 nm. Fig. 4a–f show the spectra for the
site-selective excitation. The dyes coumarin 500 and
5
coumarin 470 were used for the excitation in the D and
1
5
D levels, respectively. A Fabry-Perot etalon out of the
2
5
7
dye laser cavity was used to narrow the spectral width of
three samples in the D → F region. On the spectrum of
0 0
2
1
an emission line at 0.3 cm . The spectral analysis of the
luminescence was achieved by a 80-cm double grating
monochromator (PHO coderg) spectrometer driven by a
computer able to collect and process data. The time-
resolved spectroscopy was performed by means of a digital
oscilloscope (Tektronix 2430) coupled with the computer
using home made programs [8]. Each luminescence decay
following the excitation pulse was analyzed in real time
during the scanning of the spectrum. Moreover, from this
set-up, the decay profiles were obtained by averaging over
some hundred decay measurements. The powdered sam-
ples were cemented with a non-fluorescent glue on a
copper cold finger. To get the best resolution, the measure-
ments were carried out at about 20 K by using a He
closed-cycle refrigerator (Cryophysics, CP62-STS). The
KGd1.9Eu0.1F7 synthesized at 6308C, at least two very
21
weak lines are evidenced at 17 270 and 17 490 cm . All
these lines have nearly the same time behaviour, on the
contrary of the two other compounds. The spectrum of
KGd1.9Eu0.1F synthesized at 8508C exhibits a very strong
7
2
1
line at 17 490 cm and at least four other lines between
2
1
17 270 and 17 370 cm . These last ones are more intense
that for the previous sample and can be splitted into
several sets with different time behaviours. At least four
lines, out of which three are intense, are observed between
2
1
17 280 and 17 350 cm
on the spectrum of
KGd1.9Eu0.1F6.97O0.015. The line localized on the other
2
1
spectra at very high frequency (17 490 cm ), is also
evidenced at short delay, but has here a weak intensity. To
conclude, at least six distributions of sites are evidenced in
5
7
5
relative intensities of the D → F lines were optically
different proportions depending on the sample. Their D
levels overlap in the 17 260–17 370 cm region.
0
0
0
2
1
reduced due to the presence of a diaphragm aperture

7
4
A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
Fig. 3. Emission spectra obtained under 355 nm excitation, at 20 K for KGd1.9Eu0.1F7 (synthesized at 8508C).
5
7
4
.4. Site-selective excitation and time-resolved
We have performed the D → F
emission under
0,1,2
0
7
5
spectroscopy
F → D selective excitation in the 17 260–17 360 and
0 0
21
1
7 410–17 520 cm range and selective observation in
5
7
5
Only KGd1.9Eu0.1F7 synthesized at 8508C and
KGd1.9Eu0.1F6.97O0.015 synthesized at 8508C, were studied
by site-selective excitation and time-resolved spectroscopy.
KGd1.9Eu0.1F7 synthesized at 6308C exhibits a too low
luminescence intensity to obtain spectra of good quality.
the D → F transition by excitation into the D level. In
0
0
1
opposite to glasses or disordered structures, no continuous
variations in the spectra occur when the D excitation
changes. For a small shift of the excitation frequency, few
cm , new lines appear then disappear, but the spectra
5
0
2
1
5
7
Fig. 4. Emission spectra in the D → F range obtained under 355 nm excitation, at 20 K for: KGd1.9Eu0.1F6.97O0.015 (synthesized at 8508C) (a) delay:
0
0
7
2
800 ms, gate: 8800 ms, (b) delay: 200 ms, gate: 980 ms; KGd1.9Eu0.1F (synthesized at 8508C) (c) delay: 7800 ms, gate: 8800 ms, (d) delay: 980 ms, gate:
000 ms; KGd1.9Eu0.1F7 (synthesized at 6308C) (e) delay: 990 ms, gate: 2000 ms, (f) delay: 200 ms, gate: 980 ms.
7

A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
75
2
1
Fig. 5. Emission spectra of KGd1.9Eu0.1F6.97O0.015 obtained under 17 283 cm excitation, at 20 K. The arrows indicate the lines with the same time
behaviour and characteristic to site A.
2
1
remain similar. For a given excitation frequency, often two
or three sites can be simultaneously excited. Nevertheless,
in these discrete distributions, three kinds of spectra noted
A, B and C are distinguished, mainly by their intensity
decays.
range 17 275–17 285 cm on KGd1.9Eu0.1F7 synthesized
at 8508C, are very similar to those obtained in the same
range for the sample synthesized with the presence of
gadolinium oxide. Few differences in the intensities of the
lines are observed. The values of the lifetimes are un-
modified. These facts tell in favour of exclusive fluorinated
surroundings for the site A.
4
1
.4.1. Site A
The site A was investigated under selective excitation at
2
1
7 283 cm
on KGd1.9Eu0.1F6.97O0.015. This optimal
selective excitation value was found by weak variations of
the excitation wavenumber around 17 280 cm . The
4.4.2. Sites B
2
1
The set of the sites B is characterized under excitations
2
1
spectrum is shown on Fig. 5. This site is characterized by a
between 17 295 and 17 340 cm . As indicated in the
technical part, we have used a Fabry-Perot etalon to
narrow the spectral width of the excitation emission. All
the spectra obtained in this range of frequencies are
similar. The most intense lines show a frequency shifting
which depends on the excitation wavelength. Nevertheless,
5
long lifetime of the D level, with a value of 6.8 ms (Fig.
0
2
1
1
1). The spectrum obtained at 17 283 cm exhibits very
5 7 5 7
intense D → F lines in comparison with the D → F ,
0
1
0
2
indicating unambiguously surroundings close to the cen-
trosymmetry. The energies of the levels deduced from the
transition lines are reported in Table 1.
5
7
5
7
the number of the D → F and D → F lines, respec-
0
1
0
2
The spectra, obtained under excitations located in the
tively at least six and ten on Fig. 6, tells in favour of a set
Table 1
2
1
Energy levels (in cm ) of the different sites studied (A, C1, C2) and for comparison in hydroxyapatite for the Eu–O site
Level
Site A
Site C2
Site C1
Hydroxyapatite [11]
5
D0
F1
17 283
270
17 421
208
17 494
175
17 437
164
7
4
4
16
33
238
888
766
783
1226
1484
1525
251
1119
751
307
1051
768
7
F2
932
95
125
9
763
811
1
1484
1583
1618
1382
1531
1547
–
–

7
6
A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
2
1
Fig. 6. Solid line: Emission spectrum of KGd1.9Eu0.1F6.97O0.015 obtained under 17 336 cm excitation, at 20 K. To remove the contribution of the sites
non selectively excited, the curve is the result of the difference between two spectra obtained with the following conditions: delay5100 ms and gate5500
ms for the first one, delay53900 ms and gate54400 ms for the second one. Dotted line: Emission spectrum of KGd1.9Eu0.1F7 (synthesized at 8508C)
2
1
obtained under an excitation at 17 336 cm and at 20 K. Difference curve between two spectra obtained with the following conditions: delay5100 ms and
gate5500 ms for the first one, delay53900 ms and gate54400 ms for the second one.
7
of sites with very close surroundings and which are
distributed. In fact, several sets of sites correspond to a
7). As usually observed, the splitting of the F sub-levels
1
5
increases with the D energy.
0
5
7
same D → F transition. These distributions can be
The spectra of the sites B show some differences at the
same excitation frequency, between KGd1.9Eu0.1F7
0
0
7
5
characterized by F sub-levels versus the D level (Fig.
1
0
7
31
5
Fig. 7. Plot of the F1 sub-level energies of Eu in KGd1.9Eu0.1F6.97O0.015 at 20 K, versus the D0 level energy.

A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
77
Table 2
(
8508C) and KGd1.9Eu0.1F6.97O0.015 (8508C) (Fig. 6). The
5
21
Energies of the D1 Stark sub-levels (in cm ) for the sites B1 and B2
frequencies of the lines are similar, but their relative
intensities are modified. This suggest that the oxygen plays
a leading part in the surroundings of this kind of sites.
Level
B1
B2
2
1
21
Energy in cm
Energy in cm
5
The spectra of KGd1.9Eu0.1F6.97O0.015 (8508C) obtained
D0
D1
17 320
19 088
17 320
19 080
19 061
19 013
5
5
by excitation in D level are very intricate and involve at
least two sites. The one obtained with an observation at
1
8
1
1
9 041
2
1
19 028
7 320 cm shows two sets B1 and B2 of three lines (Fig.
5
). The degeneracy of the D level for the sites B1 and
1
B2, is fully lifted. These two sets of sites are unambigu-
splitted into two subgroups: C1 and C2. The optimal signal
for site C1 was performed by weak variations of the
ously distinguished by their time behaviour. Table 2 gives
5
21
the energies of the D sub-levels for the two sites. We can
excitation wavenumber around 17 490 cm . The spec-
1
5
21
observe that for a given D level, two sites characterized
trum obtained under an excitation at 17 494 cm is given
0
5
5
7
7
by their D sub-levels, are identified. This fact shows the
on Fig. 9. In the range of the D → F , F transitions, six
1
0 1 2
difficulty to obtain good selectivity by excitation in a given
bands are observed with an unusual arrangement. The
identification of the lines is performed by analogy with the
5
D level
0
5
The lifetimes of the D levels for the two samples are
spectra obtained for Eu-doped fluoroapatites [9]. The
0
7
respectively: 3.5 ms for KGd1.9Eu0.1F6.97O0.015 and 3.8 ms
energies of the F sub-levels deduced from the transition
1,2
for KGd1.9Eu0.1F (Fig. 11). These values are measured on
lines are reported in Table 1. The site C1 is particularly
enhanced for the sample KGd1.9Eu0.1F7 (8508C). The
spectrum of site C2 (Fig. 10) obtained under an excitation
at 17 421 cm , exhibits the same unusual features than
site C1. Nevertheless, on the contrary of the site C1, the
7
5
7
the most intense D → F lines of the spectra obtained
0
2
2
1
under an excitation at 17 336 cm
.
5
7
21
For both samples, the intensities of the D → F lines
0
2
5
7
are greater than the intensities of the D → F , indicating
0
1
the low symmetry of the sites.
site C2 was not observed for KGd1.9Eu0.1F7 (8508C).
5
7
The relative intensities of the D → F lines observed
0
0
4
.4.3. Sites C1 and C2
These sites correspond to D0 levels of high energy,
on Figs. 9 and 10 were optically reduced as previously
5
5
indicated. Some spectra performed by excitation in the D
1
2
1
distributed from 17 460 to 17 530 cm . They can be
levels, allow to measure the real relative intensities of the
2
1
5
7
21
Fig. 8. Excitation spectrum between 19 138 and 18 950 cm for the emission D → F at 17 320 cm of KGd1.9Eu0.1F6.97O0.015 synthesized at 8508C
0
0
(T520 K). The line noted S is relative to a site with a slow emission intensity decay.

7
8
A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
2
1
Fig. 9. Emission spectrum of KGd1.9Eu0.1F7 (synthesized at 8508C) obtained under an excitation at 17 494 cm , at 20 K (site C1). The line noted v is
attributed to a vibronic transition. The assignment of the lines is performed according to Ref. [10].
2
1
Fig. 10. Emission spectrum of KGd1.9Eu0.1F6.97O0.015 obtained under an excitation at 17 421 cm , 20 K (site C2). As for Fig. 5, the curve is the
difference spectrum between those obtained with the following conditions: delay5100 ms and gate5500 ms for the first one, delay51500 ms and
gate52500 ms for the second one (v: vibronic transition).

A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
79
5
7
D → F lines: these ones are approximately 1.5 stronger
these two compositions suggest that the crystal structures
of these ones are very close. We can assume that the
surroundings of the gadolinium are the same. In
K0.265Gd0.735F2.47, space group Immm (n8 71), the
gadolinium cations are localized in seven sites which
correspond to three different sets. The first set is character-
ized by an eight-fold fluorinated pseudo-cubic surround-
ings. The second set is composed by five sites character-
ized by analogous eight-fold fluorinated pseudo-antipris-
matic surroundings. The last site, randomly occupied by
potassium and gadolinium, corresponds to ten-fold fluori-
nated surroundings. We feel that the two first sets of sites
are globally unmodified in the crystal structure of KGd2F7
synthesized at 8508C.
0
0
5
7
than the most intense D → F lines.
0
2
The peculiar spectra obtained for the two sites C1 and
C2 can be related to those of Eu-doped apatite [9,10] and
some other oxide compounds [11,12]. It was suggested that
privileged Eu–O bonds are responsible for this kind of
spectra. This assumption is in agreement with previous
studies on some fluorides [13], in which similar spectra
3
1
22
attributed to Eu –O
features of these spectra are due to a strong anisotropic
associates were reported. The
7
crystal field which induces a large splitting of the F and
1
7
F2 manifolds leading to overlap their components. In a
similar way, the odd parameters of the crystal field matrix
lead to the considerable oscillator strength of the forbidden
5
7
D → F transition.
The pseudo-cubic site is very close to the ideal local
0
0
5
The lifetime of the D level for site C1 is very short,
symmetry O , but slight distortions due to variations in the
0
h
measured at 0.5 ms, value comparable to that for calcium
Eu–F bond lengths lead to a real local symmetry C or C .
s
1
5
oxyapatites [10]. The lifetime of the D level for site C2
This observation is in agreement with the characteristics of
0
5 7
is longer, with a measured value of 1.8 ms (Fig. 11).
the site A. Indeed, the strong intensities of the D → F
0 1
5 7
lines compared with those of the D → F , are in favour
0
2
4
.5. Relations with the crystal structure
of a site close to the centrosymmetry. However, degen-
7 7
eracy of the F and F levels deduced from the number
1
2
5
7
The accurate crystal structures of KGd F , also formu-
of components of the D → F transitions, indicates a
2
7
0 1,2
lated K0.33Gd0.67F2.33
,
synthesized at 8508C and
lower local symmetry. Moreover, the great lifetime of the
5
KGd2F6.97O0.015 are not exactly known. Nevertheless, the
isotipy between the X-ray powder patterns of
K0.265Gd0.735F2.47 [5] and KGd2F7 synthesized at high
temperature and the existence of a solid solution between
D level (6.8 ms) is in agreement with the hypothesis of
0
fluorinated surroundings. Nevertheless, this kind of value
was already measured for oxygenated matrices but only in
3
1
the case of perfect centrosymmetric Eu sites [14–16].
3
1
Fig. 11. Decay curves of the Eu
emission obtained at 20 K under selective excitation for the different sites: A (KGd1.9Eu0.1F6.97O0.015), B
(
KGd1.9Eu0.1F6.97O0.015), B (KGd1.9Eu0.1F7 synthesized at 8508C), C1 (KGd1.9Eu0.1F7 synthesized at 8508C) and C2 (KGd1.9Eu0.1F6.97O0.015).

8
0
A. Pierrard et al. / Journal of Alloys and Compounds 289 (1999) 71–80
5
7
The spectra of these sites exhibit then only D → F
The sites B appear to be a set of very close discrete
sites. Nevertheless, each site is distributed as indicated by
0
1
5
7
5
7
lines, never D → F and D → F . For low symmetry
sites, this type of value for lifetimes is commonly observed
for fluorinated matrices.
0
2
0
0
7
7
the shift of the energies of the F1 and F2 sub-levels
5
versus the energy of the D0 level. These assertions
The sites B correspond to a set of several distributed
sites with very close surroundings. That is the main reason
why we have attributed these ones to the set of five sites
with analogous eight-fold fluorinated pseudo-antiprismatic
surroundings. The ideal local symmetry of this kind of
neighbourhood is the non-centrosymmetric group D .
indicate that the perturbations which generate the dis-
tributions are limited and that a long range ordering of the
cations and anions exists in the crystal structure. The
spectra of sites B exhibit some differences between
KGd1.9Eu0.1F7 and KGd1.9Eu0.1F6.97O0.015 synthesized at
8508C, indicating the part played by oxygen in the
4
d
3
1
However distortions of the polyhedra lead to a real local
surroundings of Eu . The characteristic lifetime of the
5
symmetry C or lower. These polyhedra are more distorted
D level is about 3.5 ms.
4
0
than the previous (pseudo-cubic) surroundings and far
removed from the centrosymmetry. This fact is in agree-
The last sites C1 and C2 exhibit unusual spectra
5
characterized by a high position of the D level, a strong
0
5
7
5
7
7
7
ment with the strong intensities of the D → F lines
D → F line and a large splitting of the F and F
0
2
0
0
1
2
5
7
compared with those of the D → F .
manifolds leading to overlap their components. These
features are related to a strong anisotropic crystal field due
0
1
In the crystal structure of KGd2F6.97O0.015 the presence
of oxygen affecting the previously described sites, induce
new kinds of sites characterized by the association of an
oxygen ion, a rare earth ion and a vacancy (sites C1 and
to privileged Eu–O bonds. The characteristic lifetimes of
5
the D level are in the range 0.5–1.8 ms. The site C1
0
appears to be a defect created by UV-radiation.
5
7
5
7
C2). The degeneracy of the D → F and D → F
We have observed that the global intensity of lumines-
cence is strongly enhanced by the presence of a low rate of
oxygen in the crystal structure. This increase of the
0
1
0
2
transitions is in agreement with a pseudo local symmetry
C`v for which the levels belonging to the E representations
are just splitted into doublets (more explanations are given
in Refs. [9–11]). In these sites, the crystal field is
controlled mainly by the oxygen ions. The two sites C1
luminescence does not induce a decrease of the average
5
lifetime of the D level, particularly due to the high value
0
of the lifetime (1.8 ms) observed for the oxygenated site
C2. This observation is in favour of the use of oxyfluori-
nated, rather than fluorinated matrices, for potential laser
applications.
and C2 are characterized by different Eu–O bond dis-
5
tances. Short distances increase the energy of the D level
0
7
7
and the splitting of the F and F levels. In this way the
1
2
Eu–O bond lengths are shorter for site C1 than for site C2.
In spite of a careful synthesis under a dehydrated argon
stream, the existence of a site C1 in the structure of
KGd1.9Eu0.1F7 synthesized at 8508C suggests the presence
of oxygen. Nevertheless, a careful investigation of the
non-selective spectra obtained under UV-excitation of
5KGd Eu0.1F7 synthesized at 8508C, shows that the
References
[1] A. Pierrard, Thesis, University of Paris VI, France, 1998.
[2] A. Pierrard, P. Gredin, N. Dupont, A. de Kozak, B. Viana, P.
Ashehoug, D. Vivien, Solid State Sci. 36 (1999) in press.
1
.9
7
21
[3] A. Pierrard, P. Gredin, A. de Kozak, Powder Diffr. 11 (1996) 121.
[4] A. Cousson, Thesis, University of Clermont-Ferrand, France, 1973.
[5] Y. Le Fur, S. Aleonard, M.F. Gorius, M.T. Roux, Z. Kristallogr. 182
(1988) 281.
D → F transition at 17 490 cm , characteristic of the
0
0
site C1, appears only after some experiments under 355 nm
excitation. So, we feel that the site C1 is an oxygenated
surface-defect created during the spectroscopic measure-
ments by an UV radiation from the oxygen adsorbed in the
powder.
[6] L.A. Reiseberg, H.W. Moos, Phys. Rev. 174 (1968) 429.
[7] J.M.F. Van Dijk, M.F.H. Schuurmans, J. Chem. Phys. 78 (1983)
5
317.
8] B. Piriou, J. Dexpert-Ghys, S. Mochizuki, J. Phys. Condens. Matter
(1994) 7317.
9] Y.K. Voronko, G .V . Maksimova, A.A. Sobol, Opt. Spectrosc. 70
1991) 203.
[
[
6
(
5
. Conclusion
[10] B. Piriou, D. Fahmi, J. Dexpert-Ghys, A. Taitai, J.L. Lacourt, J.
Lumin. 39 (1987) 97.
This characterization of the sites in the host matrix
[11] B. Piriou, M. Richard-Plouet, J. Parmentier, F. Ferey, S.Vilminot, J.
Alloys Compd. 262–263 (1997) 450.
KGd F is not exhaustive. The disorder in the cationic and
2
7
anionic frameworks induces the existence of a lot of sites
with very close surroundings. It is difficult to obtain, in
these conditions, spectra with a very good selectivity.
Sometimes great differences in the spectra are observed,
only with weak variations of 2 or 3 cm . Nevertheless
three main kinds of sites are evidenced: A, B, C.
The site A is characterized by rather centrosymmetric
[12] B. Piriou, Y.F. Chen, S.Vilminot, Eur. J. Solid State Inorg. Chem. 32
1995) 469.
13] D. Van der Voort, G.J. Dirksen, G. Blasse, J. Phys. Chem. Solids
3-2 (1992) 219.
14] J. Heber, K.H. Hellwege, U. Kobler, H. Murmann, Z. Phys. 237
(1970) 189.
(
[
[
5
2
1
[15] M. Buijs, A. Meyerink, G. Blasse, J. Lumin. 37 (1987) 9.
[16] B. Piriou, M. Fedoroff, J. Jeanjean, L. Bercis, J. Colloid Interface
Sci. 194 (1997) 440.
fluorinated surroundings and exhibits the longest lifetime
5
of the D level with a value of 6.8 ms.
0