Crystal structure, electronic structure, and luminescence of Cs 2KYF6:Pr3+

Article abstract of DOI:10.1002/zaac.200500316

The crystal structure, electronic states and VUV spectroscopic behaviour of Cs₂KYF₆ doped with Pr³⁺ ions have been investigated both by experimental and theoretical methods. Cs ₂KYF₆ (Fm3m, Z = 4, a = 945.5(3) pm, R1_all = 0.0297) crystallizes with the cubic elpasolite type of structure. The local relaxation of the activator ions in the host lattices has been calculated by the projector augmented wave method (computer code VASP). The electronic states have been calculated using a spin density functional procedure based on the atomic sphere approximation (computer code ASW). VUV spectroscopic measurements show fast 4f¹ 5d¹ → 4f² emission of nanosecond duration as well as slow 4f² → 4f² emission depending on the excitation energy which indicates the occupation of different sites in the host lattice. This assumption was verified by a recently developed quantum mechanical method. The combination of the experimental and the theoretical results show that the Pr³⁺ ions are occupying three different sites, namely the Y³⁺ and the Cs⁺ site as well as the K⁺ site.

Full text of DOI:10.1002/zaac.200500316

3؉
Crystal Structure, Electronic Structure, and Luminescence of Cs KYF :Pr
2
6
1
1
1,
2
3
Daniel Schiffbauer , Claudia Wickleder , Gerd Meyer *, Marco Kirm , Michael Stephan , and Peter C.
3
,
Schmidt **
1
Köln, Institut für Anorganische Chemie der Universität
Tartu, Institute of Physics, University of Tartu, Estonia
Darmstadt, Eduard-Zintl-Institut der Universität
2
3
Received July 25th, 2005.
th
Dedicated to Professor Rüdiger Kniep on the Occasion of his 60 Birthday
2
2
Abstract. The crystal structure, electronic states and VUV spectros-
emission of nanosecond duration as well as slow 4f Ǟ 4f emission
depending on the excitation energy which indicates the occupation
of different sites in the host lattice. This assumption was verified
by a recently developed quantum mechanical method. The combi-
nation of the experimental and the theoretical results show that the
doped with Pr3 ions have been inve-
KYF
Fm3m, Z ϭ 4, a ϭ 945.5(3) pm, R1all ϭ 0.0297) crystallizes with
ϩ
copic behaviour of Cs
2 6
KYF
stigated both by experimental and theoretical methods. Cs
(
2
6
¯
the cubic elpasolite type of structure. The local relaxation of the
activator ions in the host lattices has been calculated by the projec-
tor augmented wave method (computer code VASP). The electronic
states have been calculated using a spin density functional proce-
dure based on the atomic sphere approximation (computer code
3
ϩ
3ϩ
Pr ions are occupying three different sites, namely the Y and
ϩ
ϩ
the Cs site as well as the K site.
Keywords: Cesium; Potassium; Yttrium fluoride; Praseodymium;
1
1
2
ASW). VUV spectroscopic measurements show fast 4f 5d Ǟ 4f
Luminescence; Electronic structure
Introduction
ment of mercury free discharge lamps containing quantum
cutters would also be an improvement with regard to en-
vironmental aspects.
Currently, there is a large interest in the development of
new phosphors with a quantum efficiency larger than unity
As an alternative for Hg a Xe plasma which emits radi-
ation at mainly 172 nm can be used as an excitation source
for the visible phosphors [3]. To utilise the high energy of
these photons absorbed, luminescent ions have to be found
that yield two photon emission after VUV excitation. This
process is called quantum cutting or downϪconversion [4].
Trivalent rare earth ions are excellent candidates for this
purpose. In fact, Pr3 ions exhibit the so called two-pho-
ton-cascade (TPC) which is due to an emission in two steps,
(quantum cutters) under VUV (vacuum ultraviolet) exci-
tation. These materials would enhance the efficiency of dis-
charge lamps significantly, in spite that they are one of the
most efficient light sources today. In this case UV radiation
is generated by a Hg plasma which emits radiation of
mainly 254 nm wavelength. This radiation is used to excite
three different phosphors containing rare earth ions: typi-
ϩ
2
ϩ
3ϩ
3ϩ
cally BaMgAl O :Eu , Y O :Eu and CeAl O :Tb
1
0
17
2
3
11 19
which emit blue, red, and green radiation, respectively. In
sum, white light is observed [1]. The average emission wave-
length of these three colour lamps is at 555 nm, therefore a
loss of more than 50 % must be taken into account due to
the difference of the two wavelengths [2]. Due to the toxicity
of mercury and the enhancement of efficiency, the develop-
1
1
3
3
3
3
namely S Ǟ I , P followed by blue P Ǟ F , H emis-
0
6
J
0
J
J
1
1
1
3
sion or S Ǟ D followed by orange D Ǟ H emission,
0
2
2
J
respectively (Fig. 1a).
For an efficient absorption of the xenon excimer emission
2
1
1
the parity allowed very strong 4f Ǟ 4f 5d transitions are
very suitable. As the position of these bands depends very
strongly on the coordination sphere, host lattices have to be
found which shift these absorption bands to 172 nm. More-
*
Prof. Dr. G. Meyer
1
1
1
over, if the 4f 5d state is located below the S state, no
0
Institut für Anorganische Chemie
Universität zu Köln
1
1
TPC but single photon 4f 5d emission to several low lying
4f states is observed (Fig. 1b) [5].
2
Greinstraße 6
D-50939 Köln
Fax: 0221 470 5083
E-Mail: gerd.meyer@uni-koeln.de
To elucidate the influence of the structure of a host lattice
1
1
for the position of the 4f 5d state and thus the efficiency
of the TPC process after VUV excitation, we present the
investigation of the luminescence properties of Pr3 ions
ϩ
** Prof. Dr. P. C. Schmidt
Eduard-Zintl-Institut für Anorganische und Physikalische Chemie
TU Darmstadt
Petersenstraße 20
D-64287 Darmstadt
Fax: 06151/16 6015
doped in Cs KYF6 which exhibits a small coordination
2
number (six) of the rare earth ions and thus very small RE-
F distances. For this purpose we have performed single
crystal X-ray investigations and prepared Pr3 doped phase
pure samples. Emission and excitation spectra were meas-
ϩ
E-Mail: p.c.schmidt@tu-darmstadt.de
3046
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
DOI: 10.1002/zaac.200500316
Z. Anorg. Allg. Chem. 2005, 631, 3046Ϫ3052

2 6
Crystal Structure, Electronic Structure, and Luminescence of Cs KYF
:Pr3
ϩ
3
ϩ
1
1
Fig. 1 Schematic representation of the electronic situation and emission mechanisms in Pr ions if the 4f 5d state is located (a) above
1
0
and (b) below the S state.
ured in the VUV range using synchrotron radiation. For a
detailed interpretation of the measurements the electronic
structure and the local relaxation of the activator ion was
calculated together with a simulation of the excitation spec-
tra resulting from these calculations.
phase purity of all samples used for luminescence measurements
was checked by X-ray powder diffraction (Stoe and Cie., Stadi P).
VUV Spectroscopy
The experiments were performed at the DESY Synchrotron in
Hamburg (Germany) using the SUPERLUMI experimental station
Table 1 Crystallographic data for Cs
nation
2
KYF
6
and their determi-
Experimental Section
a)
Synthesis and Crystal Structure Determination
lattice constant
molar volume
no. of formula units
crystal system
space group
diffractometer
radiation
temperature
theta range
rotation range, ϕ-increment 0° < ϕ < 200°, 2°
index range
a ϭ 945.5(3) pm
507.83 cm /mol
4
3
as well as Pr3ϩ doped samples (0.5 mol %) were
2 6
KYF
Pure Cs
obtained from solid state reactions of the binary fluorides CsF
99.9 %, Merck), KF (p. a., Merck), YF (99.9 %, Chempur) and
PrF . For the preparation of the latter, Pr 11 (99.9 %, Chempur)
was dissolved in aqueous HCl and a few ml of hydrogen peroxide
cubic
(
3
¯
Fm3m (no. 225)
3
6
O
Stoe IPDS
α
Mo-K (Graphite-Monochrom., λϭ71.07 pm)
293 K
4
ϩ
2 2 3
(30 % H O ) were added to accelerate the reduction of Pr . PrF
5° < 2θ < 56°
was precipitated by adding hydrofluoric acid (40 % HF) at 80 °C.
After filtration the precipitate was washed with a mixture of HF,
ethanol and water, and finally dried. In order to remove any traces
of oxygen, the precipitate was thoroughly ground with an excess of
Ϫ12 Յ h Յ 12
Ϫ12 Յ k Յ 12
Ϫ12 Յ l Յ 12
100
5 min
60 mm
no. of images
exposure time
detector distance
absorption correction
μ
4 2 4 2
NH HF , heated in a platinum crucible until all of NH HF was
removed, and finally dried in vacuo at 400 °C for twelve hours. For
the solid state reactions, appropriate molar ratios of the binary flu-
orides were sealed in silica-jacketed arc-welded Monel ampoules
and heated to 870 °C with the aid of a resistance furnace. After
slow cooling, the moisture sensitive single crystals were selected in
a glove box. They were sealed in glass capillaries (л ϭ 0.1 mm) for
X-ray investigations, and their quality was checked by means of
orientation images taken with an image plate diffractometer (STOE
IPDS). The same device was used to collect diffraction data of
the best specimen. The structure was solved and refined using the
programs SHELXS-97 and SHELXL-97 [6], respectively. Numeri-
cal absorption corrections were applied with the aid of the pro-
grams X-red and X-shape [7]. Details of the data acquisition and
the crystallographic data are summarized in the Tables 1Ϫ3. The
numerical after crystal shape optimization [7]
Ϫ1
15.908 m
unique reflections
71
56
0.1468
with I
o
> 2σ (I)
R
int
structure determinations
scattering factors
goodness of fit
SHELXS-97 and SHELXL-97 [6]
Intern. Tables, Vol. C [8]
1.003
0.0228; 0.0431
0.0297; 0.0449
R1; wR2 (I
o
> 2σ(I))
R1; wR2 (all data)
a)
Crystallographic data are available from the Fachinformations-
zentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (crysdat-
a@FIZ-Karlsruhe.de) referring to the deposition number CSD
413206.
Z. Anorg. Allg. Chem. 2005, 631, 3046Ϫ3052
zaac.wiley-vch.de
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
3047

D. Schiffbauer, C. Wickleder, G. Meyer, M. Kirm, M. Stephan, P. C. Schmidt
of HASYLAB. Details of the experimental setup are described in
10]. The excitation light was dispersed through a 2 m normal inci-
dence monochromator with a holographic MgF -coated Al grating
blazed at 120 nm (1200 gr/mm) providing a typical resolution of
.32 nm in excitation. The excitation spectra were corrected for the
photon flux of the excitation beam using the excitation spectrum of
sodium salicylate as a standard. The emission in the 200Ϫ800 nm
spectral range was detected using a 0.3 m imaging spectrograph
Pr(6s,6p,5d,4f), F(2s,2p). The v. Barth-Hedin exchange-correlation
potential [14] and a 12x12x12 k-mesh in the full Brillouin zone
were applied. Tests with finer k-meshes and different exchange-cor-
relation potentials have proven this choice to be sufficient.
[
2
0
Pr doped compounds are simulated by replacing one metal atom
of the unit cell by one Pr atom. For Pr on an alkali metal site we
have additionally included a possible charge compensating defect
(
replacing Y by an alkali ion) in the structure. We took care that
(Acton Research Corporation, SpectraPro-308) with a CCD cam-
the defects do not influence each other. In order to take local relax-
ation effects around the substitutionally defect Pr into account we
have performed a structure optimisation using the VASP (Vienna
ab initio simulation package) program [15]. VASP is a quantum
mechanical molecular dynamic simulation code using the projected
augmented waves [16]. For the exchange correlation potential we
have used GGA Ϫ the generalised gradient correction approxi-
mation [17]. Details can be found in an earlier publication [18].
era (Princeton Instruments Inc.). Additionally, the time-resolved
emission and excitation spectra were recorded using a photomultipl-
ier (Hamamatsu R6358) mounted on the second exit arm of the
same spectrograph. Pulsed nature of synchrotron radiation (a rep-
etition rate of 5 MHz at HASYLAB) allows conveniently to record
time-resolved spectra delayed by δt in respect of the excitation pulse
in a time window with a length of Δt. Also the decay kinetics for
the fast emissions with nanosecond duration was recorded. The
spectral resolution achieved was about ϳ1 nm when using the CCD
camera and in measurements with photomultiplier was typically
Results and Discussion
5
nm. Emission spectra were corrected for wavelength dependent
detection efficiency using the best available curves. The samples
were cleaved in air before mounting onto a sample holder attached
to a cold finger of the liquid helium cryostat (CryoVac GmbH)
working in the temperature range of 5-300 K.
Crystal Structure
Cs KYF crystallises with the face-centred cubic elpasolite
2 6
type of structure (space group Fm3m, Table 1). The struc-
¯
ϩ
ture can be described as a cubic closest packing of Cs and
Ϫ
F
ions with all of the octahedral voids formed by fluoride
Computational Details
3ϩ
ϩ
ions filled by Y and K ions (Fig. 2). Thus, the coordi-
nation numbers are 6 for these smaller cations and 12 for
The electronic band structure has been computed applying the aug-
mented spherical wave (ASW) method [11] in a new scalar-relativis-
tic implementation [12]. This method is a density functional (DFT)
method based on the atomic sphere approximation with spherically
symmetric potentials within the Wigner-Seitz spheres centered at
the nuclei. To fill the inter-atomic space in the open crystal struc-
ture of the fluoride materials, so-called empty spheres are inserted.
They can be considered as pseudo atoms without nuclei and are
used to model the correct shape of the crystal potential within large
voids. We have applied the sphere geometry optimization (SGO)
algorithm [13], which determines optimal empty sphere positions
as well as radii of all spheres automatically.
ϩ
Cs . The highly symmetric structure provides special posi-
tions for all of the atoms (Tab. 2), with only one variable
parameter for the F ion on the Wyckoff site 24e. In Table
Ϫ
3
the metal-fluoride distances for the undoped and doped
crystals are summarized. The Pr-F distances are calculated
by VASP, see below. Within the [YF ] and [KF ] octahedra
6
6
six equal distances of 215.5(8) pm and 257.2(8) pm, respec-
tively, are observed. For the cuboctahedrally coordinated
ϩ
Cs ions, a distance of 335.0(1) pm is found matching
pretty well the expected value for this type of coordination.
For Cs KYF :Pr the Pr-F distance is calculated to be
2
6
Y
This method requires only a minimal basis set. In the present case,
the basis sets include K(4s,4p), Cs(6s,6p), Y(5s,5p,4d), Gd/
larger than the original Y-F distance (Tab. 3). A change of
about 10 pm is expected from the difference in the ionic
radii of sixfold coordinated Y3 and Pr ions [19]. For Pr
on the alkali metal site, the Pr-F distances are found to be
smaller than the Cs-F or K-F distances as is expected by
ϩ
3ϩ
Table 2 Atomic positions and equivalent displacement parameters
for Cs
2
KYF
6
U
eq·10Ϫ1/pm2 a)
Atom
Site
x/a
y/b
z/c
the respective ionic radii. The Pr -F distance is larger than
Cs
the Pr -F distance mainly because of the different coordi-
nation numbers.
Y
Cs
K
Y
F
8c
4b
4a
1/4
1/2
0
1/4
0
0
1/4
0
0
24.6(6)
9(1)
9.0(8)
45(2)
24e
0.772(1)
0
0
a)
eq ϭ 1/
3
(U11 ϩ U22 ϩ U33) [9]
Photoluminescence
U
3
ϩ
The excitation spectrum of Cs KYF :Pr at 10 K monitor-
2
6
1
1
Table 3 Metal-fluoride distances /pm in Cs
2
KYF
6
and Cs
2
KYF
6
:Pr,
ing the S Ǟ I emission at 408 nm is shown in Fig. 3. A
broad band is observed which can be assigned to 4f ( H )
Ǟ 4f 5d transitions of Pr . The broad band splits into
0 6
left: from X-ray structure analysis, right: Pr-F distances calculated
by VASP.
2 3
4
1
1
3ϩ
five transitions peaking at about 194, 175, 155, 139 and
Cs
2
KYF
6
2 6
Cs KYF :Pr
Ϫ1
1
32 nm (51546, 57143, 64516, 71942 and 75757 cm ). No
3
1
Y-F (6x)
Cs-F (12x)
K-F (6x)
215.5(8)
335.0(1)
257.2(8)
Pr
PrCs-F
Pr -F
Y
-F
226.0
271.5
245.5
fine structure and no H Ǟ S excitation is observed. The
4 0
latter can be too weak due to the forbidden character of
K
n
the intraconfigurational 4f transition but one cannot ex-
3048
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2005, 631, 3046Ϫ3052

2 6
Crystal Structure, Electronic Structure, and Luminescence of Cs KYF
:Pr3
ϩ
ϩ
2 6
Fig. 4 Emission spectrum of Cs KYF
:Pr3 at 10 K, λex ϭ 180 nm.
1
1
ditional UV emissions are at 270 nm ( S Ǟ G ) and at
0
4
1
3
Fig. 2 Perspective view of the crystal structure of Cs
100].
2
KYF
6
along
250 nm ( S
0
Ǟ F
4
) while in the high resolution spectra the
S Ǟ D transition could not be identified on the back-
1
1
[
0
2
ground forming structure hinting at the presence of broad
emission band. We note that measurements with the moder-
ate resolution using PMT revealed indeed such a lumi-
3
ϩ
nescence band. This is in contrast to CaF :Pr [20] and
2
3ϩ
1
especially to LaZr F :Pr [21] where relatively strong S0
Ǟ D emission is observed. In the visible region, the emis-
sion peaks can be assigned to transitions from the P state
3
15
1
2
3
0
to lower states. The assignment of the transitions around
3
3
1
6
00 nm to P Ǟ H is more ambiguous because the D
0
6
2
3
3ϩ
Ǟ H transitions lie at similar energies. In the Pr doped
4
alkaline earth sulfates MSO (M ϭ Ca, Sr, Ba) a broad
4
band at 600 nm is ascribed to these transitions [22]. In con-
trast to the present case they are much broader, and the
3
3
intensity is much larger compared to P Ǟ H . Addition-
0
4
1
1
ally, there is a remarkable intensity of S Ǟ D in the UV
0
2
1
region; therefore the D state is well populated, while for
Cs KYF :Pr the D population can only be due to radi-
ationless relaxation from the P level. The latter process
2
3ϩ
1
2
6
2
3
0
:Pr3ϩ at 10 K, λem
2 6
KYF ϭ
has a rather low probability because the energetic gap be-
Fig. 3 Excitation spectrum of Cs
08 nm.
Ϫ1
tween these two states is about 4000 cm and the highest
4
Ϫ1
phonon energy for fluorides is of less than 500 cm . Due
to these reasons the given assignment is more meaningful
3
ϩ
clude the possibility that it is partially masked by tran-
sitions arising from other sites (see discussion below). The
onset of the excitation band for emission at 408 nm is at
about 208 nm, being slightly shorter than that observed in
and in consistence with this for CaF :Pr [20], but the
2
1
3
small peak at 582 nm may be assigned to D Ǟ H .
2
4
There is a significant change of the shape of the spectra
at different excitation and emission energies. In Fig. 5 the
excitation spectra monitoring emissions at 320 nm and
230 nm are depicted. In this case several broad excitation
bands between 125 and 235 nm are observed which can be
2
1
YF (213 nm) [4]. Therefore, the 4f ( S ) level should be lo-
3
0
1
1
cated below the lowest 4f 5d level and quantum cutting is
expected for Pr in this site.
3
ϩ
2
3
1
1
3ϩ
In fact, the high-resolution emission spectrum at the
again assigned to 4f ( H ) Ǟ 4f 5d transitions of Pr
4
1
1
2
1
80 nm excitation shows no broad 4f 5d Ǟ 4f emission
ions. The excitation spectrum of 320 nm luminescence splits
in two large bands peaking at about 220 and 153 nm and
some smaller ones at 212, 172 and 134 nm. For the 230 nm
emission major excitation peaks centred at 212 and 182 nm
2
2
but numerous narrow 4f Ǟ 4f emission peaks as is de-
picted in Fig. 4. In the UV region the strongest one is at
1
1
about 407 nm ( S Ǟ I ) as is expected from theory. Ad-
0
6
Z. Anorg. Allg. Chem. 2005, 631, 3046Ϫ3052
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© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
3049

D. Schiffbauer, C. Wickleder, G. Meyer, M. Kirm, M. Stephan, P. C. Schmidt
ϩ
2 6
KYF
:Pr3 at 10 K, solid line:
Fig. 5 Excitation spectra of Cs
em ϭ 320 nm, symbols: λem ϭ 230 nm.
:Pr3ϩ at 10 K,
2 6
KYF
Fig. 6 Time resolved emission spectra of Cs
ex ϭ 211 nm. Solid line is used for time-integrated spectrum, sym-
bols depict emission in the short time-window (δtϭ11 ns and Δtϭ
λ
λ
33 ns) and dashed line in the long time-window (δtϭ63 ns and Δtϭ
1
15 ns). The inset shows the decay of the 230 nm emission excited
with minor structures at 172, 156 and 121 nm were found.
In both cases no fine structure was revealed. The energy of
these transitions discussed is lower compared to those
at 211 nm.
1
1
shown in Fig. 3, and, thus, the lowest 4f 5d state should
1
1
1
2
also a small M-F distance is required and a substitution of
the Y3 site can be expected to be responsible for these
spectra due to the larger radii of the K ions (K : 152 pm,
be located below the S state and 4f 5d Ǟ 4f emission is
0
ϩ
expected instead of TPC.
ϩ
ϩ
This is indeed the case as it is shown in the emission
spectrum after excitation at 211 nm (Fig. 6). No line struc-
tures typical for f-f transitions are observed but broad emis-
sion bands which can be attributed to transitions from the
3ϩ
Y
: 104 pm in sixfold coordination [19]). In contrast, the
excitation spectrum which is observed with 408 nm emis-
sion points to small crystal field strength and low site sym-
metry. The latter is not found for the cation positions in
1
1
2
lowest 4f 5d state to several lower 4f states. Moreover,
time-resolved emission spectra show the presence of several
emissions with different decay times. This situation is com-
3ϩ
Cs KYF , and it can not be assumed that the Pr ions
2
6
occupy interstitial sites. A better explanation would be the
assumption that the high symmetry is distorted due to the
occupation with a cation of smaller size than the regular
one. In this case the Pr ion would be located most prob-
ably on the Cs site because of the large M-F distances
3
ϩ
parable to that of CaF :Pr and the shapes of the emission
2
spectra are similar. On the other hand, the widths of the
bands are broader and the energy difference of the maxima
3ϩ
ϩ
3
ϩ
is larger than for CaF :Pr and the determination of the
2
1
1
and, thus, the small crystal field strength.
final states is not unique. But it can be granted that 4f 5d
Ǟ H , F transitions are included in the broad band be-
3
3
For a detailed investigation of the origin of the different
spectra some calculations were carried out and the results
are described in the next paragraph.
J
J
tween 260 and 400 nm. Moreover, a one exponential-fit of
the decay curve shown in the inset of Fig. 6 gave a value of
3
ϩ
1
4.8 ns which is a typical life-time for Pr d-f emissions.
These observations lead to the assumption that the Pr3
ϩ
Electronic states
ions occupy several different crystallographic sites in
Cs KYF . In general, the number of 5d levels observed in
The ordering of the electronic states of Cs KYF :Pr can be
2
6
2
6
the excitation spectra is correlated with the point symmetry
of the site. The splitting in two bands as it is observed for
seen from the density of states (DOS) and the partial DOS
(PDOS) which we have calculated with the spin polarised
version of the ASW procedure. The DOS and PDOS of
Cs KYF :Pr with Pr occupying the Y-site is shown in
Fig. 7a. The upper valence band edge EF is chosen to be
the zero of the energy scale. The low energy peaks of the
partial DOS at about Ϫ19.0 eV are the 2s like states of flu-
oride ions. Above these bands are the valence states due to
mostly 2p-like states of F with a band width of 3.0 eV. The
3
20 nm emission (Fig. 5) points to the symmetry O or T
h d
3
ϩ
which is in fact the symmetry of the three cations (Y
,
2
6
ϩ
ϩ
K : O ; Cs : T ). Additionally, the lowest 5d state is at low
h
d
energy and the splitting of about 20.000 cmϪ1 is rather
large. Therefore, the crystal field strength should be large
and the coordination number should be small which is true
3
ϩ
ϩ
for the Y and the K site. Because of this large splitting
3050
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
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2 6
Crystal Structure, Electronic Structure, and Luminescence of Cs KYF
:Pr3
ϩ
2 6 Y
Fig. 7 (a) Density of states (DOS) and partial DOS of Cs KYF :Pr . The partial DOS are shown for the ions and additionally for Pr the
spin up and spin down contributions to the 4f and 5d states. (b) Partial DOS of the Pr 4f and 5d states in Cs KYF :PrCs and Cs KYF :Pr .
2 6 2 6 K
bonding character is predominantly ionic. The Pr 4f states
are located inside the band gap at 5 to 6 eV. The lower con-
duction states between 8.0 and 14.0 eV are hybrid states of
mostly Y 4d and Pr 5d character, but also with contri-
butions of Cs and K states. The d-states of Pr and Y at
about 9 to 13 eV show a clear splitting into two levels,
which is expected for an Oh symmetry. The distance be-
tween the f-states and the d-states (Δε
)
is 3.3 eV for
fϪd calc
Pr . In the case of Cs KYF :Pr with Pr occupying the Cs/K-
Y
2
6
site (Δε
)
is increasing to 4.0 and 3.7 eV for (Pr ) and
fϪd calc Cs
(
Pr ) as can be seen from the partial d and f DOS in
K
Fig. 7b.
These values of (Δε
)
do not give directly the exci-
fϪd calc
tation energies ΔEfϪd of the first f Ǟ d transitions discussed
in the previous paragraph because there are systematic dif-
ferences between the calculated one-electron states and
measured optical properties. For example, the band gaps
calculated by DFT are systematically too small and the
band structure calculation does not give the multiplett
structure of the f states. Nevertheless, it is shown in a recent
paper [18] that one finds an empirical relation between the
energy of the first f->d transition ΔEfϪd and the values
Fig. 8 Calculated ε
2
(λ) in arbitrary units for the Pr f-d transitions
KYF :PrCs
for Cs KYF :Pr and Cs
2
6
Y
2
6
.
and to Cs
KYF :Pr . Further, the calculated values of
6 K
2
1
(ΔEfϪd
)
calc show that only in the case of PrCs the S
state
is located below the Pr-d states (214 nm ( S Ǟ H )) and
0 J
0
1
3
(Δε
)
, namely
fϪd calc
therefore the emission spectrum in Fig. 4 should belong to
(ΔEfϪd
)calc / eV ϭ 1.06·(ΔεfϪd
)calc / eVϩ1.77.
Pr .
Cs
To underline this correspondence between theory and
experiment not only for the onset of the f Ϫ d transitions
we have calculated the imaginary part of the dielectric con-
Using this equation and the values of (Δε
above one predicts the following values for the first f-d tran-
)
given
fϪd calc
sition for Pr at the three different sites: λ(Pr ) ϭ 235 nm;
stant ε for the f-d transitions of PrY and PrCs The results
Y
2
λ(Pr ) ϭ 206 nm and λ(Pr ) ϭ 218 nm. All these values
are displayed in Fig. 8 where the shift of 1.77 eV is taken
into account. We can not expect a one-to-one correspon-
dence between the excitation spectra of Fig. 3 and 5 and
Cs
K
agree quite well with the onset of the spectra of Fig. 3 and
. This is a first hint that the spectrum in Fig. 3 belongs to
Cs KYF :Pr and the spectra in Fig. 5 to Cs KYF :Pr
5
the ε (λ) curves of Fig. 8 because of the limitations of the
2
6
Cs
2
6
Y
2
Z. Anorg. Allg. Chem. 2005, 631, 3046Ϫ3052
zaac.wiley-vch.de
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
3051

D. Schiffbauer, C. Wickleder, G. Meyer, M. Kirm, M. Stephan, P. C. Schmidt
theoretical method [23] and because the excitation spectra
should be different from ε (λ). Moreover, the strong f-d ab-
[2] T. Jüstel, H. Nikol, C. Ronda, Angew. Chem. 1998, 110, 3250;
Angew. Chem. Int. Ed. 1998, 37, 3084.
[3] T. Kamegaya, H. Matsuzaki, M. Yokozawa, IEEE Trans. Elec-
tron. Devices 1978, 25, 1094.
2
3
ϩ
sorption of Pr ions located at different cationic sites in-
troduces additional modulations in the spectra recorded.
Namely, the penetration depth of the exciting photons is
controlled by the absorption coefficient leading to the com-
petition between different centres in the spectral regions
[4] J. L. Sommerdijk, A. Bril, A. W. de Jager, J. Lumin. 1974, 8,
341; W. W. Piper, J. A. DeLuca, F. S. Ham, J. Lumin. 1974,
8, 344.
[
[
5] K. H. Yang, J. A. DeLuca, Phys. Rev. B 1978, 17, 4246.
6] G. M. Sheldrick, SHELXS-97, Program fort he Solution of
Crystal Structures, Göttingen 1997; G. M. Sheldrick,
SHELXL-97, Program for the Refinement of Crystal Struc-
tures, Göttingen 1997.
3
ϩ
where all kind of Pr ions can absorb exciting radiation
i.e. λexc < 206 nm). On the other hand the peak structure
(
of the spectra should be comparable. In Fig. 8 the spectrum
of Cs KYF :Pr shows a clear splitting into two peaks at
2
6
Y
[
7] Fa. Stoe & Cie, X-RED 1.07, Data Reduction for STAD4 and
IPDS, Darmstadt 1996; Fa. Stoe & Cie, X-SHAPE 1.01, Crys-
tal Optimisation for Numerical Absorption Correction,
Darmstadt 1996.
about 220 nm and 140 nm, as we expect for octahedral co-
ordination. The spectrum for Cs KYF :Pr has much
2
6
Cs
more structure and a maximum at 200 nm. Therefore the
assumption that ε (λ,Cs KYF :Pr ) corresponds to Fig. 3
2
2
6
Cs
[
[
8] Th. Hahn (ed.), International Tables for Crystallography, Vol.
C, D. Reidel Publishing Company, Dordrecht, Boston 1983.
9] R. X. Fischer, E. Tillmanns, Acta Crystallogr. 1988, C44, 775.
and ε (λ,Cs KYF :Pr ) to Fig. 5 is supported. Further-
2
2
6
Y
more, the emission peaked at 230 nm can be tentatively as-
3
ϩ
ϩ
cribed to the Pr ions at the K site.
[
[
[
10] G. Zimmerer, Nucl. Instrum. Methods Phys. Res. A 1991,
08, 178.
11] A. R. Williams, J. Kübler, C. D. Gelatt Jr., Phys. Rev. B 1979,
9, 6094.
12] V. Eyert, Int. J. Quant. Chem. 2000, 77, 1007.
3
Conclusions
1
We have investigated the crystal structure and the lumi-
3
ϩ
[13] V. Eyert, K.-H. Höck, Phys. Rev. B 1998, 57, 12727.
nescent properties of Cs KYF :Pr in order to look for
2
6
[
14] U. v. Barth, L. Hedin, J. Phys, C: Solid State Phys. 1972, 5,
629.
candidates of quantum cutting lamp phosphors. Surpris-
ingly, the measured excitation spectra for monitoring emis-
sion at 408, 320, and 230 nm are very different and can not
1
[
15] G. Kresse, J. Hafner, Phys. Rev. B 1993, 48, 13 115; G. Kresse,
J. Hafner, ibid. 1994, 49, 14 251; G. Kresse, J. Furthmüller, J.
Comput. Mater. Sci. 1996, 6, 15; G. Kresse, J. Furthmüller,
Phys. Rev. B 1996, 54, 11169.
3
ϩ
3ϩ
be explained by assuming that Pr ions replace only Y
.
3ϩ
To get more insight into the structure of Cs KYF :Pr we
2
6
have supplemented our experimental work by band struc-
ture investigations. From the combined experimental and
[
16] P. E. Blöchl, Phys. Rev. B 1994, 50, 17953.
[17] D. C. Langreth, M. J. Mehl, Phys. Rev. B 1983, 28, 1809.
[18] M. Stephan, M. Zachau, M. Gröting, O. Karplak, V. Eyert,
K. C. Mishra, P. C. Schmidt, J. Lumin., 2005, 114, 255.
3
ϩ
ϩ
theoretical results we can conclude that Pr replaces K ,
3
ϩ
ϩ
3ϩ
Y
, and Cs ions and only for Pr ions located on the
ϩ
1
[19] R. D. Shannon, C. T. Prettwitt, Acta Crystallogr. 1969, B25,
Cs site the d states are lying above the S state and there-
0
9
25; R. D. Shannon, Acta Crystallogr. 1976, A32, 751.
20] K. D. Oskam, A. J. Houtepen, A. Meijerink, J. Lumin. 2002,
7, 107.
fore only for PrCs we can expect a quantum cutting process.
[
[
[
Acknowledgments. This work was supported by the BMBF-Ver-
bund project “VUV-Leuchtstoffe für effiziente quecksilberfreie
Entladungslampen” #03N8019D,E,G.
9
21] E. van der Kolk, P. Dorenbos, C. W. E. van Eijk, Opt. Com-
mun. 2001, 197, 317.
22] E. van der Kolk, P. Dorenbos, A. P. Vink, R. C. Perego, C. W.
E. van Eijk, A. R. Lakshmanan, Phys. Rev. B 2001, 64,
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3052
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2005, 631, 3046Ϫ3052