Electronic structure and photoluminescence properties of Eu2+-activated Ca2BN2F

Article abstract of DOI:10.1016/j.jssc.2009.09.028

The electronic structure of undoped and luminescence properties of Eu²⁺-doped Ca₂BN₂F have been investigated. First-principles calculations for Ca₂BN₂F show that the valence band is mainly composed of

Full text of DOI:10.1016/j.jssc.2009.09.028

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Journal of Solid State Chemistry 182 (2009) 3299–3304
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Journal of Solid State Chemistry
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Electronic structure and photoluminescence properties of
2
+
Eu -activated Ca BN F
2
2
a,b,Ã
c
b
a,d
a
a,d
Y.Q. Li
, C.M. Fang , Y. Fang , A.C.A. Delsing , G. de With , H.T. Hintzen
a
Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands
College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China
b
c
Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ, Delft, The Netherlands
d
Materials and Devices for Sustainable Energy Technologies, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB,
Eindhoven, The Netherlands
a r t i c l e i n f o
a b s t r a c t
2
+
Article history:
2 2
The electronic structure of undoped and luminescence properties of Eu -doped Ca BN F have been
Received 14 June 2009
Received in revised form
investigated. First-principles calculations for Ca
2
BN
2
F show that the valence band is mainly composed
BN F is a
of F and N 2p, B 2s and 2p orbitals, while the Ca 4s and 3d are almost empty, indicating that Ca
2
2
7
September 2009
very ionic compound. The valence band close to the Fermi level is dominated by the N 2p states, and the
bottom of the conduction band is determined by the Ca 3d and N/B 3s orbitals. The direct energy gap is
calculated to be about 3.1 eV, in fair agreement with the experimental data of ꢀ3.6 eV derived from the
Accepted 20 September 2009
Available online 30 September 2009
Keywords:
diffuse reflection spectrum. Due to the high degree of ionic bonding of the coordinations of Eu with (N,
Calcium boron–nitride–fluoride
First principles
2+
2 2
F) on the Ca sites, Ca BN F:Eu shows strong blue emission with a maximum at about 420 nm upon UV
excitation in the absorption range of 330–400 nm.
Electronic structure
Europium
&
2009 Elsevier Inc. All rights reserved.
Luminescence
1
. Introduction
properties of rare-earth doped nitrogen containing compounds
having relatively ‘‘soft’’ elements, like Al and B. It is also
interesting to understand the effect of the nitrogen atoms mixed
with other types of anions besides oxygen (e.g., beyond the
oxynitride compounds), such as F and Cl, on the luminescence
properties when these atoms are involved into the first nearest
neighbors of the alkaline earth ions. In between ionic and covalent
Alkaline earth nitride compounds can be roughly classified into
three catalogs: metal nitride, ionic nitride and covalent nitride,
according to the bonding characteristics [1–6]. The luminescence
properties of Eu in the binary alkaline earth metal nitrides (e.g.,
Ca , SrN , BaN ) hardly can be observed due to a small band gap
2
+
3
N
2
2
2
and significantly higher absorption of the host lattice. Whereas in
the covalent nitrides the alkaline earth ions are normally located
nitrides, Ca
study combined with its good chemical stability in air.
Ca BN F crystallizes in the orthorhombic system with space
2 2
BN F just provides a nice example for our present
2
+
within the three-dimensional structure of Si–N and Al–N. Eu
-
2
2
doped alkaline-earth silicon nitride/oxynitride materials are the
typical examples, which are well-known wavelength conversion
phosphors for applications in white LED lighting [7–18]. Among
such kinds of hosts, there are more opportunities to find novel
phosphors emitting in green to red spectral range having unusual
long-wavelength excitation bands (370–480 nm). This is attrib-
uted to the presence of nitrogen due to high degree of covalent
bonding and large crystal field splitting of the 5d levels of the Eu
ion as well as the rigid lattices [19]. Apart from the silicon nitride-
based compounds, it is interesting to know the luminescence
group of Pnma and its structure is built by isolated linear
3
2
ꢁ
nitridoborate anions BN
and [CaCa3/3F] units [20]. There are
two different Ca crystallographic sites having sixfold coordination
with the nitrogen and fluoride ions in different N/F ratio (N/F=5/1
and 3/3 for Ca1 and Ca2, respectively). The shortest bond lengths
˚
are 2.341 and 2.339 A, respectively, for Ca–N and Ca–F. Addition-
3
2
ꢁ
ally, the B–N distances in the ðBN
Þ cluster are significant shorter
2
+
˚
(i.e., 1.316–1.35 A). It is well known that the degree of covalent
bonding between the alkaline earth ion and ligand is significantly
decreased going from N via O to F due to an increase in the
electronegativity in a sequence of N (3.04)oO (3.44)oF (3.98)
21]. Therefore, in contrast to rigid lattices of alkaline-earth silicon
nitride compounds (e.g., MSiN [22], M Si [5] and MAlSiN
14–16] (M=Ca, Sr, Ba)) and alkaline-earth silicon oxynitride
[
2
2
5
N
8
3
Ã
Corresponding author at: Laboratory of Materials and Interface Chemistry,
[
Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The
Netherlands.
2 2 2 2 2
compounds (e.g., MSi O N (M=Ca, Sr, Ba) [9,17,23]), Ca BN F is a
more ionic compound due to the presence of ðBN ꢁÞ and Fꢁ anions
3
E-mail address: li.yuanqiang@nims.go.jp (Y.Q. Li).
2
0
022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.09.028

ARTICLE IN PRESS
3
300
Y.Q. Li et al. / Journal of Solid State Chemistry 182 (2009) 3299–3304
around Ca [20]. In addition, similar to alkaline earth silicon
oxynitride, the addition of F would result in the emission band of
spectra were automatically corrected for the variation in the lamp
intensity by a second photomultiplier and a beam-splitter. All the
spectra were recorded with a scan speed of 200 nm/min at room
temperature.
2
+
2+
Eu shifting to shorter wavelength if Eu experiences similar
2
+
environment as expected. It has been found that with Eu in pure
2
+
fluorine environment CaF
2
:Eu
shows blue emission at about
4
25 nm upon UV excitation [24]. Under N/B environment,
2.3. Density functional theory (DFT) calculations
2
+
BN:Eu
shows yellow emission with the maximum emission
band at 550–560 nm when excited by 365 nm light [25]. Nano-
First-principles calculations [28,29] were carried out in the
2
+
tube BN:Eu , however, has been found to give blue–green
cathode-luminescence at about 490 nm [26]. In contrast with
the covalent nitride compounds, so far less attention has been
paid to the ionic nitride compounds as the potential luminescent
hosts. Our interesting point is concentrated on the luminescence
generalized gradient approximation (GGA) [30] and the local
density approximation (LDA) [31] using the projector-augmented
wave (PAW) method [31,32], which were preformed by the ab
initio quantum-mechanical molecular dynamics simulation code
of VASP package based on the DFT computations [32]. The
electronic wave functions were sampled on a (4 ꢂ10 ꢂ 4) mesh
in the irreducible Brillouin zone (BZ) [33]. The cut-off energy of
the wave functions was 850 eV. The augment cut-off energy was
2
+
properties of Eu
environment aside from only nitrogen atoms, e.g., a combination
of CaF and BN. As far as we know, no relevant paper about the
luminescence properties of Eu -doped Ca
that is incorporated into the complex
2
2
+
2
2
BN F has been
900 eV. Convergence of the total energy with the number of the
published so far. Therefore, in the present study, we firstly
performed first-principles calculations to investigate the electro-
k-mesh and the plane wave cut-off energy has been checked.
nic structures of Ca
bonding and the luminescence properties. Then we synthesized
Ca2ꢁxEu BN F compounds by a solid-state reaction under a N /H
atmosphere. Subsequently, the structural characteristics, optical
2 2
BN F for a better understanding of chemical
3. Results and discussion
x
2
2
2
3.1. Electronic structure calculations
x 2
and photoluminescence properties of Ca2ꢁxEu BN F were studied
in detail.
2 2
The optimized lattice parameters of Ca BN F within both GGA
and LDA are listed in Table 1. Compared with our experimental
data for powders and literature data for single crystal [20], the
GGA values are slightly larger while the LDA values are slightly
smaller. As the GGA values are much closer to the experimental
data, thus we used the GGA for next studies.
2
. Experimental procedures
2.1. Synthesis
Fig. 1 shows the partial and total density of states (DOS) for
2
+
Undoped and Eu -doped Ca
a solid state reaction. Ca (Alfa, purity 498%), BN (Cedimx,
9%), CaF (Aldrich, 499.5%) and EuN were used as starting
2
BN
2
F powders were prepared by
2 2
Ca BN F from the GGA. The calculated distribution curves along
3
N
2
the high symmetry lines are shown in Fig. 2. The semicore level 2s
orbitals of the F and N atoms form narrow bands (of band-widths
of typically 0.5 eV), which are about 23.5 eV (F 2s), or about
13.0 eV (N atoms) below the Fermi level which is set to be the top
of the valence band. Also the semicore level Ca 3p electrons are
calculated to be about 20 eV below the Fermi level. The valence
bands are mainly composed of F/N 2p, B 2s and 2p orbitals, as
shown in Figs. 1 and 2. The F 2p orbitals form a rather narrow
band with three peaks at about ꢁ5.2, ꢁ4.8, and ꢁ4.5 eV with the
same intensity. The N 2s orbitals, which are strongly hybridized
with the N 2p electron, form a band in the energy range between
ꢁ4.3 and ꢁ3.7 eV. These properties are very similar to two
9
2
materials. The binary nitride EuN was pre-synthesized by
nitridation of Eu metal (Csre, ꢀ99%) at 850 1C for 10 h in a
horizontal tube furnace under flowing dried nitrogen. Subse-
3 2 2
quently, the stoichiometric amounts of Ca N , BN and CaF as well
as EuN were weighed out, and then thoroughly mixed and ground
in an agate mortar in a dry glovebox flushed with high purity
nitrogen because of air and moisture sensitivity of Ca
3
N
2
and EuN.
2
+
For Eu -doped Ca
.5 mol% with respect to the Ca
mixtures were then fired in a molybdenum crucible at 1050–
100 1C for 6–8 h in a horizontal tube furnace under a N –H (10%)
2 2
BN F, the Eu concentrations were set at 0.5–
2
+
1
2 2
ion in Ca BN F. The powder
1
2
2
similar ionic compounds of Mg
energy levels of the F 2p and N 2s, while the N 2p states in Mg
and Mg NF
are narrowly distributed in the ꢁ2.5 to 0 eV range
34]. The relatively high intensity of the Ca2 3d orbitals are
overlapping with the F 2p orbitals within the range of ꢁ5.5 to
4.2 eV (Fig. 1), giving an evidence that the chemical bonding
2
NF and Mg
3
NF
3
regarding to the
atmosphere. After heating, the samples were gradually cooled
down in the furnace.
2
NF
3
3
[
2
.2. Characterizations
ꢁ
between the Ca2 and F atoms is relatively stronger than that
The obtained samples were checked by X-ray powder diffrac-
tion (Rigaku, D/MAX-B) using CuK
with a graphite monochromator in the range of 101 and 801 2
a step scan mode (a step size of 0.021 in 2 and a count time of
s). The structural parameters were determined by the Rietveld
a
radiation at 40 kV and 30 mA
Table 1
y
by
Lattice parameters for Ca2ꢁxEu
x
BN
2
F (x=0 and 0.02).
y
2
Ca2ꢁxEu
x
2
BN F
Lattice parameters
method using GSAS package [27].
The diffuse reflectance, emission and excitation spectra of the
samples were measured at room temperature by a Perkin Elmer LS
a ( A˚ )
b ( A˚ )
c ( A˚ )
V ( A˚ 3
)
a
Calc. (x=0)
GGA
5
0B spectrophotometer equipped with a Xe flash lamp. The
reflection spectra were calibrated with the reflection of black felt
reflection 3%) and white barium sulfate (BaSO reflection
100%) in the wavelength region of 230–700 nm. The emission
9.2281
8.9526
3.6718
3.5574
10.0136
9.7571
339.3
LDA
310.74
Exp. (x=0, powder)
9.1812(7) 3.6575(2)
9.182(2) 3.649(2)
9.1870(4) 3.6603(2)
9.9657(7) 334.66(4)
9.966(2) 333.9(1)
9.9667(5) 335.15(3)
(
ꢀ
4
,
b
Exp. (x=0, single crystal)
Exp. (x=0.02, powder)
spectra were corrected by dividing the measured emission
intensity by the ratio of the observed spectrum of a calibrated
W-lamp and its known spectrum from 300 to 900 nm. Excitation
a
First-principles calculation data based on PAW-GGA and PAW-LDA.
Single crystal data from Ref. [20].
b

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Y.Q. Li et al. / Journal of Solid State Chemistry 182 (2009) 3299–3304
3301
conduction band is mainly determined by the unoccupied N 3s
2
+
3ꢁ ꢁ
states [35]. It is reasonable to assert that for Ca (BN
bottom of the conduction band is composed of Ca 3d and very
weak N/F 3s states. This is largely different with that of Mg NF and
Mg NF , where the bottom of the conduction band is determined
2
)
F
the
2
3
3
mainly by the 3s states of F and N in the magnesium nitride–
fluorides, and the Mg 2p states are deep in energy below the Fermi
level (ꢀ40 eV) [34].
3.2. Phase formation and structural characters
Fig. 3 shows the typical X-ray powder diffraction (XRD) pattern
2
+
of Ca
2
BN
2
F:Eu (1 mol%). It can be clearly seen that the prepared
BN F is a nearly single phase compound, well matching
with the calculated XRD data or a JCPDS 89-4591 card. As a result,
the lattice parameters of the powder samples of undoped Ca BN
Ca2ꢁxEu
x
2
2
2
F
are in good agreement with the single crystal data and close to the
results calculated by first-principles approaches (Table 1).
2
+
Moreover, as expected, the lattice parameters of Eu -doped
Ca BN F are slightly increased compared with undoped Ca BN
due to the replacement of small Ca ion (1.01 A, C.N.=7) by the
2
2
2
2
F
2
+
˚
2
+
2+
˚
larger Eu ion (1.28 A, C.N.=7) [36], indicating that the Eu ion
have been incorporated into the Ca BN F lattice and formed a
solid solution. Based on the fact that Sr BN F is isostructural with
F [20], the Eu ions should occupy on the two individual
2
2
2
2
Fig. 1. Partial and total DOS for the valence bands of Ca
BN
2 2
F.
2+
2 2
Ca BN
2
+
Ca sites as shown in Fig. 4 due to similar ionic size of Eu and
Sr . Accordingly, in the structure of Ca
on the Rietveld refinement of the powder XRD data, the Eu ion is
2
+
2+
2
BN
2
F:Eu (1 mol%) based
1
Ca
directly coordinated with one F and five N atoms with the average
2
Ca
˚
bond length of about 2.505 A; while the Eu ion is coordinated
with three F and three N atoms with a shorter average bond length
˚
of about 2.416 A. Both the two EuCa sites have the point group of C
and the calculated coordination-polyhedron volume [37] for
1
1
2
3
˚
Eu N5F1 and Eu N3F3 are about 20.59 and 17.62 A ,
Ca
Ca
respectively, which has more significant difference in volume
upon the two crystallographic sizes. Consequently, the Eu–(N, F)
bond has more ionic character in comparison with the Eu–N bond
because the electronegativity of F (3.98) is higher than that of N
2
Ca
(
3.04). Specifically, Eu –(N, F) has higher degree of ionic bonding
1
than that of Eu –(N, F) due to the presence of more coordinating
Ca
2
F
atoms with shorter Eu ꢁF bond distances (Fig. 4) in
Ca
Ca2ꢁxEu
x 2
BN F. Therefore, it would be expected that these local
structure characters would have greatly influences on the
2
+
luminescence properties of Eu
.
Fig. 2. Dispersion curves close to the Fermi level along the high symmetry lines.
between the Ca1 and F atoms. The B 2p orbitals are also strongly
hybridized with N 2p states, and form a band between about ꢁ3.5
and ꢁ2.2 eV. That indicates that the B and N atoms are strongly
3
ꢁ
chemically bonded and can be regarded as (BN
2
)
clusters.
Beyond hybridizing with the B 2s and 2p, the remaining N 2p
orbitals occupied by electrons form a band at the top of the
valence band from about ꢁ1.4 eV to the Fermi level. The
calculated electronic configurations also showed that the Ca 4s
and 3d orbitals are almost empty (forming the conduction band).
2 2
Therefore, Ca BN F is a ionic compound which can be formulated
2
+
3ꢁ ꢁ
to be Ca (BN
2
)
F .
As shown in Figs. 1 and 2, there is a direct energy gap at
G
in
the BZ between the top of the valence band and the conduction
band. The energy gap is calculated to be about 3.1 eV. The upper
part of the valence band is mainly composed of N 2p states, while
the bottom of the conduction band consists of mainly Ca 3d
characters. However, the former calculations for the ionic
2+
Fig. 3. Observed and calculated X-ray powder diffraction patterns of Ca
2 2
BN F:Eu
compounds such as Mg
3
N
2
showed that the bottom of the
(1 mol%) and Ca BN F.
2
2

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Y.Q. Li et al. / Journal of Solid State Chemistry 182 (2009) 3299–3304
2
+
2 2
Fig. 6. Excitation and emission spectra of Ca BN F:Eu (0.5 mol%).
calculations in the description of the excitation states in solids,
this underestimated band gap is understood for the first-
principles calculations.
2
+
2 2
Fig. 4. Projection of the crystal structure of Ca BN F:Eu and EuCa-(N, F) bond
length ( A˚ ).
2
+
Eu -doped Ca
imposed on the reflection spectrum of undoped Ca
covering the visible range from 330 to 660 nm. This broad
2
BN
2
F has a strong absorption band super-
2
BN F, broadly
2
2
+
absorption band is readily assigned to the Eu ion because both
3
+
the host and the Eu ion do not have absorption in this range.
2
+
Accordingly, the observed daylight color of Ca
2 2
BN F:Eu powder
is orangish-yellow pigment with the CIE color points at
2
+
(
Ca
0.41, 0.38) and (0.48, 0.39) for Ca
2
BN
2
F:Eu
(0.5 mol%) and
2
+
2
2
BN F:Eu (1 mol%), respectively.
2
+
Even though there is a strong Eu
absorption band in the
visible range of 360–650 nm centered at about 450 nm (Fig. 5), the
absorbed energy does not lead to excitation (Fig. 6) probably
2
+
because this Eu center has been quenched at room temperature.
2
+
As a result, this Eu
center cannot convert efficiently the
absorbed energy into electromagnetic radiation of a different
wavelength. The excitation spectrum consists of two major
excitation bands located in the UV (200–280 nm) peaking at
about 229 and 245 nm, and the visible light range (320–450 nm)
peaking at 339, 351, 367, 378 nm having a large separation with
the UV ones in wavelength. According to the reflection spectra
(
Fig. 5), the first excitation band at 229 nm could be assigned to
the host excitation band as this band is deeply inside the
absorption edge of Ca BN F. Other excitation bands in the
2
+
2 2
Fig. 5. Diffuse reflection spectra of undoped and Eu -doped Ca BN F.
2
2
2
+
visible range are ascribed to the Eu ions. On the other hand,
the excitation band clearly shows efficient excitation at about
2
+
3
2 2
.3. Luminescence properties of Ca BN F:Eu
2
+
3
39 nm, indicating a second Eu center, in agreement with the
Fig. 5 illustrates the diffuse reflection spectra of undoped and
presence of two Ca crystallographic sites in the host lattice [20].
Because this Eu center (ꢀ339 nm) does not show up
outstandingly in the reflection spectrum, this suggests that the
concentration of this Eu center maybe very low. On the basis of
those facts, the long wavelength absorption at around 450 nm (not
2
+
2 2 2 2
Eu -doped Ca BN F. The reflection intensity of undoped Ca BN F
is as high as ꢀ90% in the spectral range of 450–700 nm, and then
shows a gradual decrease within 450–310 nm, implying a weak
absorption of the host in the visible range. A sharp drop can be
clearly observed approximately from 310 to 260 nm in the UV
observable in the excitation spectrum) is probably ascribed to the
1
partition of the reflection spectrum of Ca
2
BN
2
F originating from
covalent Eu N5F1 center, while the short wavelength excitation
Ca
2
Ca
the electron transition from the valence to the conduction band.
This is in agreement with the observation of a grayish-white
band at about 339 nm could be ascribed to the ionic Eu N3F3
center.
2
+
daylight color for undoped Ca
2
BN
2
F powder. The optical band gap
Nitrogen seems to be having large impact on the Eu ions,
since the intensity of the dominant excitation bands of Eu in the
range of 320–420 nm is much higher than that at around 230 nm
2
+
of Ca BN F was estimated to be about 3.6 eV from the diffuse
2
2
reflection spectrum (the intersection point of two extrapolation
lines of the reflection spectrum in the ranges of 480–700 and
2
+
for Ca
2
BN
2
F:Eu
even in the presence of the higher electro-
2
50–320 nm, as shown in Fig. 5), roughly in correspondence with
negative F atoms. From the excitation spectra, the estimated
crystal field splitting (CFS, the energy difference between highest
the value (ꢀ3.1 eV) obtained by first-principles calculations (see
2
+
Section 3.1). Based on the fact that the limitation of the DFT
and lowest observed 5d excitation bands of Eu
in energy) is

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Y.Q. Li et al. / Journal of Solid State Chemistry 182 (2009) 3299–3304
3303
Table 2
Luminescence characters of Eu doped Ca
2
+
2 2 2
BN F, calcium silicon nitrides and oxynitride as well as CaF .
max
exc
max
em
ꢁ1
ꢁ1
ꢁ1
Materials
Excitation
l
(nm)
Emission
l
(nm)
CFS (cm
)
COG (cm
)
Stokes shift (cm
)
Ca
CaSiN
Ca Si
CaSi
CaF
2
BN
2
F
339
360
422
14 360
14 000
13 500
15 700
N/A
31 000
25 100
25 800
29 000
N/A
2800
5000
5100
4500
ꢀ5040
2
[38]
630–655
605–620
560
2
5
N
8
[11]
[17]
394
395
ꢀ350
2
N
2
O
2
2
[24]
425
ꢁ
1
about 14 360 cm , and the center of gravity (COG, average energy
2
+
ꢁ1
of the observed 5d excitation levels of Eu ) is about 31000 cm
.
2
+
In comparison with CaSiN
2
:Eu
[38], their CFS values are
2
+
comparable, while the COG value of Ca
2
2
BN F:Eu is much higher
2
+
ꢁ1
than that of CaSiN
bonds of EuCa-(N, F) in Ca
Similarly, as compared with other calcium–silicon–nitride/oxyni-
2
:Eu (ꢀ25100 cm ), reflecting more ionic
2+
2
BN
2
F:Eu
as described above.
2
+
tride phosphors, Ca
having similar CFS values, as listed in Table 2. Ca
shows strong deep blue emission peaking at about 422 nm which
2 2
BN F:Eu also has the highest COG values
2
+
2
2
BN F:Eu
6
7
2+
is ascribed to the transition of 4f 5d-4f (8S7/2) of Eu [39]. In
2
+
addition, the position of the emission band of Eu
is nearly
independent of the excitation wavelengths (see Fig. 6) in
2
Ca
agreement with an assumption that only the Eu N3F3 center
does show luminescence. Just from the positions of the emission
2
+
and excitation bands of Eu , it seems that the luminescence
2
+
characteristics of Ca
2
BN
2
F:Eu
are much similar to that of
2
+
2+
CaF
2
:Eu , which further implies that the overall Eu lumine-
scence properties of the Eu N3F3 center in Ca
dominated by the F ions due to its significantly shorter distances
e.g., EuCa–F for Eu 1 mol%: 2.325, 2.325, and 2.379 A) than that
ions. It is worth noting that generally the emission
bands of Eu are located at nearby or above the absorption edge
2
Ca
2+
BN
2
F:Eu
are
2+
2+
2
Fig. 7. Excitation and emission spectra of Ca
2 2
BN F:Eu
at different Eu
ꢁ
concentrations.
2
+
˚
(
3
ꢁ
of the N
ꢁx x 2
obtained Ca2 Eu BN F powders are nearly single phase solid
2
+
solutions. First-principles calculations showed that the valence
bands mainly consist of F-2p, N-2p, and B-2s2p orbitals (top of the
valence band) and the conduction bands are composited of Ca-3d
states with the calculated energy band gap being about 3.1 eV, in
fair agreement with the experimental value of ꢀ3.6 eV. Ca BN F is
2
+
2+
of Eu
CaSiN
(in wavelength), as found in
M
2
Si
F:Eu
5
N
8
2+
:Eu
and
2
+
2
2 2
:Eu , etc. [11,38]. However, for Ca BN
both the
dominant excitation band (ꢀ339 nm) and the emission band
2
+
(
ꢀ422 nm) of Eu
350–650 nm) of Ca
are just embedded in the absorption band
2
2
2
+
(
2
BN
2
F:Eu . Thus only energy absorbed in
confirmed to be an ionic compound because the calculated
short-wavelength range can be partially transferred to the
luminescent centers and then emit blue light. Furthermore,
electronic configurations of Ca-4s and Ca-3d are almost empty
2
+
in the valence bands. Ca BN F:Eu has strong absorption in the
2
2
2
+
2+
Ca
800 cm
phosphors (Table 2).
Fig. 7 shows the relationship between the Eu concentration
2
BN
2
F:Eu
possesses a small Stokes shift of approximately
in comparison with calcium–silicon–nitride/oxynitride
visible spectral range; however, an absorption center by Eu
ꢁ
1
1
Ca
2
itself ðEu N5F1Þ at about 450 nm does not show luminescence.
2+
Ca BN F:Eu
2
2
emits bright deep blue light under excitation at
2
+
339 nm, exhibiting a broad emission band at about 422 nm due to
2
+
6
7
2+
and the photoluminescence behaviors of Ca
2
BN
2
F:Eu . Both the
the transition of 4f 5d-4f of Eu . This short-wavelength
2
+
excitation and emission profiles have not changed markedly with
emission of Eu
is probably mainly attributed to more ionic
2
+
2
increasing the Eu
concentration from x=0.01 to 0.03. The
Eu N3F3 center.
Ca
2
+
luminescence intensity of Eu is significantly decreased by ꢀ65%
2
+
caused by a higher degree of concentration quenching of Eu due
to a high ratio of Ca/B=2 from the composition viewpoint.
References
2
+
Normally, the emission band of Eu can be shifted more or less
depending on the composition/structure by increasing the Eu
concentration [7,11,14,38,39]. Nevertheless, the emission band is
almost fixed at about 422 nm in Ca
independent of the Eu
[
[
1] N.E. Brese, M. O’Keeffe, Crystal chemistry of inorganic nitrides, in: Structure &
Bonding, vol. 79, Springer, Berlin, Heidelberg, 1992, pp. 307–378.
2] R. Marchand, F. Tessier, A. Le Sauze, N. Diot, Int. J. Inorg. Mater. 3 (2001) 1143.
2
+
2
+
[3] B.V. Beznosikov, J. Struct. Chem. 44 (2003) 885.
4] R. Niewa, F.J. DiSalvo, Chem. Mater. 10 (1998) 2733.
2
BN
2
F:Eu , say almost
[
2
+
concentration. This unusual pheno-
[5] W. Schnick, H. Huppertz, Chem. Eur. J. 3 (1997) 679.
[6] D.H. Gregory, J. Chem. Soc. Dalton Trans. (1999) 259.
2
+
menon supports the argument that the solubility of Eu on the
Ca2 site is very low. Additionally, probably the overall possibility
[
[
[
7] H.T. Hintzen, J.W.H. van Krevel, G. Botty, European Application Patent, EP1104
99 A1, 1999.
7
2
+
of energy transfer between Eu to the host is much higher than
¨
8] H.A. Hoppe, H. Lutz, P. Morys, W. Schnick, A. Seilmeier, J. Phys. Chem. Solids 61
(2000) 2001.
2
+
2+
that in between Eu ions themselves in Ca
2
BN
2
F:Eu
.
9] R. Mueller-Mach, G. Mueller, M.R. Krames, H.A. H o¨ ppe, F. Stadler, W. Schnick,
T. J u¨ estel, P. Schmidt, Phys. Status Solidi (A) 202 (2005) 1727.
[
[
10] [a] R.-J. Xie, N. Hirosaki, Sci. Techno. Adv. Mater. 8 (2007) 588;
4
. Conclusions
[
b] R.-J. Xie, N. Hirosaki, M. Mitomo, Oxynitride phosphors, in: W.M. Yen, S.
Shionoya, H. Yamamoto (Eds.), Phosphor Handbook, second ed., CRC
Press, Taylor & Francis Group, Boca Raton, London, New York, 2006.
2
+
Undoped and Eu -doped Ca
temperature solid-state reaction under a N
sphere. X-ray powder diffraction analysis indicated that the
2
BN
2
F were synthesized by high
11] Y.Q. Li, J.E.J. van Steen, J.W.H. van Krevel, G. Botty, A.C.A. Delsing, F.J. DiSalvo,
G. de With, H.T. Hintzen, J. Alloy Compd. 417 (2006) 273.
2
–H (10%) atmo-
2
[12] Y.Q. Li, G. de With, H.T. Hintzen, J. Solid State Chem. 181 (2008) 515.

ARTICLE IN PRESS
3
304
Y.Q. Li et al. / Journal of Solid State Chemistry 182 (2009) 3299–3304
[
[
[
13] Y.Q. Li, N. Hirosaki, R.J. Xie, T. Takeda, M. Mitomo, Chem. Mater. 20 (2008)
704.
14] K. Uheda, N. Hirosaki, Y. Yamamoto, A. Naito, T. Nakajima, H. Yamamoto,
Electrochem. Solid-State Lett. 9 (2006) 4.
[25] U.D. Dzhuzeev, P.E. Ramazanov, Russ. Phys. J. 12 (1969) 890.
[26] H. Chen, H.Z. Zhang, Y. Chen, Y. Liu, C.P. Li, J.S. Williams, J. Aust. Ceram. Soc. 44
(2008) 68.
6
[27] [a] A.C. Larson, R.B. Von Dreele, Report LAUR 86-748, Los Alamos, National
Laboratory, Los Alamos, NM, 2000;
15] K. Uheda, N. Hirosaki, Y. Yamamoto, H. Yamamoto, Phys. Status Solidi (A) 203
(2006) 2712.
[b] B.H. Toby, J. Appl. Cryst. 34 (2001) 210.
[
[
[
16] H. Watanabe, H. Yamane, N. Kijima, J. Solid State. Chem. 182 (2008) 1848.
17] Y.Q. Li, A.C.A. Delsing, G. de With, H.T. Hintzen, Chem. Mater. 17 (2005) 3242.
18] [a] R.J. Xie, M. Mitomo, K. Uheda, F.F. Xu, Y. Akimune, J. Am. Ceram. Soc. 85
[
[
[
[
[
28] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 58.
29] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169.
30] J.P. Perdew, S. Burke, M. Ernenrhof, Phys. Rev. Lett. 77 (1996) 3865.
31] P.E. Bl o¨ chl, Phys. Rev. B 50 (1994) 17953.
(2002) 1229;
[
b] J.W.H. van Krevel, J.W.T. van Rutten, H. Mandal, H.T. Hintzen, R. Metselaar,
J. Solid State Chem. 165 (2002) 19.
32] [a] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758;
[
[
[
[
b] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558;
c] G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251;
d] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 6 (1996) 15;
e] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169.
[
19] J.W.H. van Krevel, H.T. Hintzen, R. Metselaar, A. Meijerink, J. Alloys Compd.
68 (1998) 272.
2
[
[
20] F.E. Rohrer, R. Nesper, J. Solid State Chem. 135 (1998) 194.
21] Linus Pauling, Nature of the Chemical Bond, third ed., Cornell University
Press, Ithaca, New York, 1960.
22] A.G. Zolt a´ n, M.M. Phillip, J.O. Heston, J.C. Simon, Inorg. Chem. 43 (2004) 3998.
23] H.A. H o¨ ppe, F. Stadler, O. Oeckler, W. Schnick, Angew. Chem. Int. Ed. 43 (2004)
[
[
33] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188.
34] C.M. Fang, K.V. Ramanujachary, H.T. Hintzen, G. de With, J. Alloy Compd. 351
[
[
(2003) 72.
[
35] C.M. Fang, R.A. de Groot, R.J. Bruls, H.T. Hintzen, G. de With, J. Phys. Condens.
Matter 11 (1999) 4833.
5
540.
[
24] [a] T. Tsuboi, P. Silfsten, J. Phys. Condens. Mater. 3 (1991) 9163;
[
36] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.
[
b] W.M. Yen, M.J. Waber, Inorganic Phosphors—Composition, Preparation
and Optical Properties, CRC Press, Boca Raton, London, New York,
Washington, DC, 2004;
[37] T. Balic Zunic, I. Vickovic, J. Appl. Cryst. 29 (1996) 305.
[38] Y.Q. Li, N. Hirosaki, R.J. Xie, T. Takada, K. Shioi, Y. Yamamoto, M. Mitomo, J.
Appl. Ceram. Technol., 2009, doi:10.1111/j.1744-7402.2009.02393.x.
[39] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994.
[
[
c] U. Fritzler, G. Schaackz, J. Phys. C Solid State Phys. 9 (1976) L23;
d] M. Itoh, T. Sakurai, T. Yamakami, J. Fu, J. Lumin. 112 (2005) 161.