Valence states of europium in CaAl 2O 4:Eu phosphors

Valence states of europium in CaAl2O4:Eu

phosphors

Nursen Avci,1,2

Katleen Korthout,1,2

Mark A. Newton,3 Philippe F. Smet,

1,2,*

and Dirk Poelman1,2

1LumiLab, Dept. Solid State Sciences, Ghent University, Krijgslaan 281-S1, 9000 Gent, Belgium

2Center for Nano- and Biophotonics (NB-Photonics), Ghent University, Belgium 3European Synchrotron Radiation Facility, F-38043 Grenoble, France

*[email protected]

Abstract: Persistent luminescent CaAl2O4:Eu2+

,Nd3+

powders were

prepared by a non-aqueous sol-gel technique. The crystallization of calcium

aluminate by heat-treatment of the sols is described in detail. After heat

treatment in air, the europium dopant ions are mainly in a trivalent state.

For the reduction to the divalent state post-annealing in a reducing

nitrogen-hydrogen atmosphere is used. The reduction of europium ions is

monitored by photoluminescence and x-ray absorption (XANES)

spectroscopy. The degree of reduction is strongly dependent on the

annealing temperature. Although for high temperature a strong

enhancement of the Eu2+

emission is observed, this also leads to powders

with a gray body color.

© 2012 Optical Society of America

OCIS codes: (160.5690) Rare-earth-doped materials; (160.6060) Solgel; (300.6280)

Spectroscopy, fluorescence and luminescence; (160.2900) Optical storage materials; (250.5230)

Photoluminescence; (300.6560) Spectroscopy, x-ray.

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#160742 - $15.00 USD Received 3 Jan 2012; revised 14 Feb 2012; accepted 18 Feb 2012; published 23 Feb 2012(C) 2012 OSA 1 March 2012 / Vol. 2, No. 3 / OPTICAL MATERIALS EXPRESS 321

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1. Introduction

Calcium aluminates are the most widely used members of the alkaline earth aluminates

MxAl2yOx + 3y (M = Ca, Sr and Ba) [1,2]. They are commonly used as cements in a wide range

of applications, from construction to dental restoration [1–4]. In addition to this, rare earth

doped CaAl2O4 shows interesting luminescence properties [5–16]. More specifically,

CaAl2O4:Eu is one of the few materials exhibiting a long-lasting afterglow after excitation

has ended, a phenomenon known as persistent luminescence [8,9,12,17]. The afterglow

duration can be improved by co-doping with other rare earths. The best co-dopant has been

reported to be neodymium, which increases the afterglow intensity more than an order of

magnitude compared to non-co-doped CaAl2O4:Eu [18]. Afterglow durations of over 5 hours

have been reported in literature [14,19,20].

Europium can be incorporated into the CaAl2O4 lattice in its trivalent (Eu3+

) state, divalent

(Eu2+

) state, or both of them together [7]. Different kinds of techniques have been used to

prepare Eu2+

doped CaAl2O4 such as solid state reaction, co-precipitation, microwave,

Pechini, combustion and sol-gel synthesis [9–16,21,22]. Comparing these methods, sol-gel

synthesis possesses some benefits, namely, relatively low preparation temperature, easy

control of the stoichiometry, high levels of product homogeneity, and no need for the use of

expensive equipment. In most of the techniques a heat treatment under a reducing atmosphere

is necessary to obtain Eu2+

[10,17,23].

It was reported that both hexagonal and monoclinic CaAl2O4:Eu2+

can be synthesized by

sol-gel methods [17], in contrast to solid state methods which normally yield only the

monoclinic phase. The hexagonal CaAl2O4:Eu2+

phase shows a single photoluminescent (PL)

emission band at 448 nm, at slightly longer wavelength than that of the monoclinic one (440

nm) [17]. Nevertheless, the monoclinic phase is preferred for luminescence application due to

its higher emission intensity. The minimum reported temperature to obtain monoclinic

CaAl2O4 is around 1050°C [7]. In the present work, structural and luminescence properties of

CaAl2O4:Eu(,Nd) powders prepared by a non-aqueous sol-gel method were studied.


Post-annealing under H2/N2 (10%/90%) atmosphere was performed to reduce the

europium ions. To assess the valence state of the europium dopant ions, two techniques were

used and compared. Based on photoluminescence emission and excitation spectroscopy it is

straightforward to assess the presence of Eu2+

and Eu3+

emission centers, due to their distinct

luminescence properties. However, it is far from obvious to determine the fraction of both

species from PL spectroscopy. Therefore x-ray absorption spectroscopy (XAS) was applied

as it is a powerful tool to investigate the valence states of the rare earth dopant, i.e. europium,

in CaAl2O4:Eu. The white lines of the Eu LIII XANES spectra (x-ray absorption near-edge

structure), assigned to 2p3/2 → 5d transitions in europium, are separated according to the

valence state of the rare earth ion. Furthermore, the area of the absorption peaks allows to

calculate the relative concentration of Eu2+

and Eu3+

. To be able to separate the effects of the

reducing atmosphere on the valence state of the europium ions from the formation of the

polycrystalline CaAl2O4 out of the sols, we first performed an heat treatment in air. Then the

influence of the temperature during the post-annealing step in reducing atmosphere was

assessed. Consequently, the emission spectrum of the Eu ions, which already showed their

typical luminescence after the heat treatment in air, could be used for determining the valence

state, also at low post-annealing temperatures (e.g. from 500°C onwards).

2. Experimental

CaAl2O4:Eu2+

,Nd3+

luminescent powders were prepared via a non-aqueous sol-gel method.

First, we investigated the structural properties and the phase formation for undoped CaAl2O4

powder. In the second stage, luminescent powders were synthesized by adding appropriate

rare earth doping during the preparation process.

Undoped calcium aluminum oxide (CaAl2O4) powder was synthesized using 2mmol

calcium nitrate tetrahydrate (Ca(NO3)2.4H2O) (Alfa Aesar, 99%) and 4mmol aluminum sec-

butoxide (Al[O(CH3)CHC2H5]3) (Alfa Aesar, 95%) as precursor, 0.4mol n-butyl alcohol (n-

BuOH) (Alfa Aesar, 99.4%) as solvent and 4mmol acetylacetone (AcAcH) (Alfa Aesar, 99%)

as chelating agent. Initially, 2/3 of the n-butyl alcohol and acetylacetone were mixed, and

aluminum sec-butoxide was added to this mixture. Then the solution was stirred for 4 hours

at 40°C. At the same time calcium nitrate tetrahydrate was dissolved in the remaining 1/3 of

n-butyl alcohol at 40°C, which took around 20 minutes. Finally, calcium nitrate tetrahydrate

solution was added to the aluminum sec-butoxide solution and the mixture was stirred for 4

hours at 40°C. Stable and transparent solutions were readily obtained via this route. CaAl2O4

powders were obtained by heat treatment in air for 1 hour at different temperatures (800 to

1200°C), with a heating rate of 5°C/min.

In the second stage, europium and/or neodymium were incorporated in CaAl2O4. These

powders were prepared in the same way as in the first stage except from adding hydrated

europium nitrate and/or neodymium(III) nitrate hexahydrate (Alfa Aesar, 99.9%) into the

calcium nitrate tetrahydrate solution and dissolving at 40°C.

In order to reduce Eu3+

to Eu2+

in CaAl2O4 powders, which were already heat treated in

air, annealing under reducing H2/N2 (10%/90%) atmosphere was used. Powders were

annealed for 1 hour at different temperatures (500 to 1000°C) with a heating rate of 5°C/min

under H2/N2 (10%/90%).

X-ray diffraction measurements (XRD, Bruker D8-Discovery, Cu Kα radiation) were

employed to acquire information about the crystal structure of the undoped and doped

CaAl2O4 powders. Photoluminescence emission and excitation were measured with a FS920

luminescence spectrometer (Edinburgh Instruments). XANES spectra at the europium LIII

edge were recorded in fluorescence mode at the BM23 beamline of the 6 GeV ESRF

synchrotron (Grenoble, France) during a uniform filling mode, giving a typical storage ring

current of 200 down to 160 mA within one synchrotron injection. The synchrotron radiation

emitted by the bending magnet was monochromated with a double crystal Si(111)


monochromator. Reference measurements were performed using EuS and Eu2O3 powder

samples, for the divalent and trivalent state, respectively.

3. Results and discussion

3.1 Influence of heat treatment temperature

In previous investigations it was suggested that the strongest luminescence in CaAl2O4 is

obtained for the monoclinic phase [23]. Janakova et al. reported the start of the transition

from the hexagonal phase to the monoclinic phase at 1050°C (for a dwell time of one hour).

Higher temperatures or longer heating times were required to have a full conversion [7]. The

X-ray diffraction (XRD) spectra of undoped CaAl2O4 sols heat-treated at various

temperatures are shown in Fig. 1. Crystallization starts between 800 and 900°C with the

formation of the hexagonal CaAl2O4 phase, as reported by Aitasalo et al. [23]. The similarity

between the diffraction patterns for the hexagonal and the monoclinic phase, in combination

with the relatively broad diffraction peaks, makes it difficult to confidently deduce the

crystallographic structure from the x-ray diffraction patterns (Fig. 1). At 1100°C, the phase

transition to the monoclinic CaAl2O4 phase sets in. For 1100°C or higher, some minority

phases are observed as well. Doping with europium (up to 3% substitution of calcium) does

not affect the crystallization temperature (not shown).

Besides structural consideration, the luminescence properties upon doping with Eu should

be assessed, in terms of the emission intensity and spectrum, which relates to the degree of

incorporation into the lattice and the valence state of the Eu ions, respectively. As mentioned

before, europium ions can show luminescence in CaAl2O4 in divalent or trivalent state, or

even with both valence states present in the material, leading to simultaneous 5d-4f broad

band and 4f-4f line emission [7,13,23]. In order to identify the oxidation state of europium in

CaAl2O4, two different excitation wavelengths were used (260 and 330 nm), as in this way

Eu3+

and Eu2+

can be preferentially excited.

Fig. 1. XRD spectra of undoped CaAl2O4 sols heat treated at various temperatures, with part of

the spectra enlarged in the figure on the right. The reference spectrum for monoclinic CaAl2O4

is based on data from ICDD file no. 70-0134. The green triangles represent the peak positions

of Ca12Al14O33 (data from ICDD file no. 09-0413) and blue squares indicate the peak positions

of CaO(Al2O3)2 (data from ICDD file no. 89-3851). The reference spectrum for the hexagonal

phase is based on [7].

Figure 2 shows the excitation and emission spectra of CaAl2O4: Eu (1%), which was heat-

treated at 1000°C. The excitation spectrum in Fig. 2(a) was recorded at an emission

wavelength of 616 nm (i.e. at the main Eu3+

emission line). It contains a broad band related to


the Eu3+

-O2−

charge-transfer transition, and narrow peaks originating from transitions within

the 4f6 electronic configuration [7]. For the emission spectrum recorded at an excitation

wavelength of 260 nm, we can distinguish two different wavelength regions, connected to

Eu2+

and Eu3+

. A broad emission peak at 440 nm, which is hardly visible, is related to the

5d4f6-4f

7 transition of Eu

2+. In the Eu

3+ emission region, the peak with a maximum at around

578 nm is originating from the 5D0 to

7F0 transition and the peaks at 587 and 598 nm are

associated with 5D0 to

7F1 transitions. The transitions from

5D0 to

7F2 levels in Eu

3+ are

associated with the peaks located at 611 and 616 nm [24]. The last two peaks at 649 and 655

nm are assigned to 5D0 to

7F3 transitions. The presence of two kinds of transitions ∆J = 1

(magnetic-dipole) and ∆J = 2 (electric-dipole) suggests at least two different crystallographic

sites occupied by Eu3+

[7], although the peaks related to sites without inversion symmetry (∆J

= 2) are dominating the spectrum.

For the same sample, excitation and emission spectra are presented in Fig. 2(b) for λem =

440nm and λexc = 330nm, respectively. The two peaks at 275 nm and 330 nm in the excitation

spectrum are due to transitions from the 4f7 ground state to the crystal field split excited

levels of the 4f65d state of the Eu

2+ ions [13]. As seen in Fig. 2, we can divide the emission

spectra into two regions. Eu2+

emission clearly dominates over Eu3+

emission when the

sample is excited at 330 nm, contrary to when the emission spectrum is recorded at an

excitation wavelength of 260nm (Fig. 2(a)). Therefore, it is not straightforward to determine

the ratio between the number of Eu2+

and Eu3+

luminescence centers for the emission spectra,

as the relative excitation efficiency of both ions depends on the wavelength of the excitation

light.

The emission spectra of CaAl2O4: Eu (1%) heat-treated at different temperatures are

shown in Fig. 3. The emission properties of Eu2+

are significantly more affected by the

environment, in contrast to those of Eu3+

. Therefore, if there is a small change in the structure

of CaAl2O4 with annealing temperature, this change is likely to be reflected in the emission

spectrum of Eu2+

. As observed in Fig. 3, the Eu2+

emission spectrum is similar for all

annealing temperatures (emission peak at 440nm), apart from an additional shoulder around

390 nm for the higher temperature, pointing at the formation of impurity phases. The

photoluminescence measurements reveal that the optimum annealing temperature is 1000°C,

although crystallization already occurs at 900°C, as observed from the x-ray diffraction data

(Fig. 1).


Fig. 2. Normalized PL excitation (blue line) and emission (red line

(1%) heat treated at 1000°C. Excitation spectra for (a)

Emission spectra for (a) λ

Fig. 3. PL emission spectra (

1000 (red line) and 1100°C (green line).

3.2 Reduction of Eu3+

to Eu2+

For persistent luminescence applications, the europium dopant has to be in its divalent state.

Clearly, a large fraction of the europium ions is in a

in air used for the formation of crystalline CaAl

described in literature to reduce Eu

atmosphere being the most commonl

Fig. 2. Normalized PL excitation (blue line) and emission (red line) spectra of CaAl2O4:Eu

(1%) heat treated at 1000°C. Excitation spectra for (a) λem = 616nm and (b) λem = 440nm.

Emission spectra for (a) λexc = 260nm and (b) λexc = 330nm.

Fig. 3. PL emission spectra (λexc = 330nm) of CaAl2O4:Eu (1%) heat treated at 900 (blue line),

1000 (red line) and 1100°C (green line).

2+: H2/N2 post-annealing


Clearly, a large fraction of the europium ions is in a trivalent state after the thermal treatment

in air used for the formation of crystalline CaAl2O4 (Fig. 2). Different techniques are

described in literature to reduce Eu3+

to Eu2+

in CaAl2O4 with post-annealing under H

atmosphere being the most commonly used [10,14,17]. Now we discuss the effect of post

:Eu

= 440nm.

(blue line),


trivalent state after the thermal treatment

). Different techniques are

annealing under H2/N2

]. Now we discuss the effect of post-


annealing under H2/N2 (10%/90%) atmosphere and the influence of the post-annealing

temperature.

Fig. 4. Influence of the temperature during the post-annealing in H2/N2 on the fraction of Eu2+

emission with respect to the total emission intensity upon excitation at 260nm (red squares).

The reflectivity of the powder samples is measured at 580nm with respect to the diffuse

reflection of the sample without post-annealing (green dots). All data points are plotted against

the fraction of Eu2+ as derived from the XANES measurements.

Figure 4 presents the relative emission intensity of Eu3+

and Eu2+

in CaAl2O4:Eu (1%)

heat-treated in air at 1000°C as a function of post-annealing temperature under H2/N2. Note

that this was obtained under excitation at 260nm, which tends to favor the Eu3+

emission.

Before post annealing, only 1% of the total emission intensity is attributed to Eu2+

as can be

derived from Fig. 2(a). By increasing the post-annealing temperature this ratio increases,

showing the effective reduction of europium. For 900°C or higher, the emission spectrum is

dominated by Eu2+

emission. Note that high temperatures are required to obtain solely Eu2+

emission. Moreover, the body color of the powders, which is white before post-annealing,

turns to gray by post-annealing under H2/N2 and gets darker by increasing the temperature

(Fig. 4). A powder with a gray body color is not desirable for luminescence applications due

to the strong (internal) absorption of the luminescent emission. The gray body color is

presumably related to oxygen vacancies in the material due to annealing under H2/N2

atmosphere, although this was not analytically determined. Interestingly, the powders turn

already gray when the annealing temperature reaches 800°C, while for the reduction of

europium - as monitored by the photoluminescence spectrum - at least 900°C is required.

3.3 X-ray absorption near-edge spectroscopy

Deriving the underlying fraction of europium ions being in the trivalent or divalent state from

the photoluminescence spectra alone is not straightforward, due to the differences for both

ions in absorption cross-section and luminescence efficiency. Also, energy transfer between

ions might occur and finally non-luminescent centers can also be present. This incited us to

perform x-ray absorption measurements on these post-annealed powders, to assess the

relationship with the photoluminescence behavior and the underlying valence distribution.


Fig. 5. XANES spectra for CaAl2O4:Eu heat-treated at 1000°C as a function of the temperature

of the post-annealing in H2/N2. The inset shows the XANES spectra for the reference

compounds EuS and Eu2O3.

Figure 5 shows the europium LIII XANES spectra as a function of the post-annealing

temperature. The peaks located at 6.975 keV and 6.983 keV are assigned to the 2p3/2 → 5d

transitions in divalent, resp. trivalent europium ions (inset of Fig. 5). The peak at 6.975 keV

becomes apparent upon increasing the temperature of the annealing process, indicating that

Eu3+

ions are reduced to Eu2+

due to the annealing process in H2/N2.

The Eu LIII XANES spectra are then analyzed by fitting procedures in which the white

line transitions are approximated with Lorentzian curves and the absorption step by an

arctangent function [25,26]. The resulting percentage of Eu2+

as a function of the post-

annealing temperature is given in Fig. 4. A clear one-to-one relationship between the valence

distributions derived from photoluminescence measurements and from the XANES

measurements is not observed, as this would imply data points on the dotted line in Fig. 4.

When the fraction of Eu2+

is 40% or above, as derived from the XANES data, the

photoluminescence spectrum starts to be dominated by Eu2+

emission.

3.4. Afterglow in CaAl2O4:Eu,Nd

Given that CaAl2O4:Eu only shows a limited afterglow, sols were also prepared by adding

appropriate amounts of Nd. CaAl2O4:Eu,Nd powders were then prepared by a heat treatment

at 1000°C in air, followed by a post-annealing in reducing H2/N2 atmosphere at 1000°C.

Similar to CaAl2O4:Eu powders which received the same synthesis and annealing conditions,

the steady state emission spectrum is dominated by Eu2+

emission. Figure 6 shows the

afterglow emission intensity (monitored at 445nm) after excitation at 350nm during 6

minutes for CaAl2O4:Eu(1%),Nd(1%). Samples were kept in the dark for a sufficiently long

time prior to the experiment, so that all traps can be assumed to be empty.

The emission intensity during the excitation is not constant, but smoothly increases, until

saturation is reached. This build-up is associated to the competitive process of filling the traps

responsible for the afterglow. When the excitation ends, a considerable afterglow is observed,

with respect to the steady-state photoluminescence intensity. The shape of the decay profile is

typical for CaAl2O4:Eu,Nd, with a relatively slow afterglow after an initial faster decay. In

absolute terms, the afterglow intensity one minute after the excitation (for five minutes with

1000lx of unfiltered Xe arc light) is only 0.25mcd/m2, which is about two orders of

magnitude lower than commercially available CaAl2O4:Eu,Nd (GloTech Int.). This much

lower afterglow intensity is mainly related to the dark gray body color of the post-annealed

samples, which leads not only to a strong absorption of the excitation light by defects (instead

of by the europium ions) but also hampers the emitted light to leave the sample.


Fig. 6. Emission intensity monitored at 445nm for CaAl2O4:Eu,Nd powder post-annealed in

H2/N2 at 1000°C. The sample was excited at 350nm for 360s. The excitation ended at time t =

0s.

4. Discussion

It is clear that the proposed non-aqueous sol-gel method allows the synthesis of crystalline

and luminescent CaAl2O4:Eu powder phosphors. A heat treatment at 900°C in air is sufficient

to crystallize the powders, and at 1100°C the expected monoclinic phase of CaAl2O4 starts to

be formed. However it is not a priori clear from the x-ray diffraction analysis what phase is

formed below 1100°C. Considering the work by Aitasalo et al. [10] and Janakova et al. [7],

the transition from hexagonal to monoclinic phase occurs around 1050°C, although it is

dependent on the dopant concentration and the duration of the heat treatment [7]. Also, both

phases can be detected simultaneously over a quite broad temperature range. For our

synthesis method, it appears that the largest crystalline fraction consists of the hexagonal

phase, based on the absence of splitting of the diffraction peak at a 2θ value of 35.5° (Fig. 1).

More information on the phase formation can be derived from the photoluminescence

measurements. In the low-temperature range, the Eu2+

emission band peaks invariably at

440nm, which is characteristic for the monoclinic phase [23]. The strongest emission peak

within the 5D0-

7F2 transition of Eu

3+ lies at 616.5nm, with a second less-intense one at 611nm

(Fig. 2). Again, this points at the Eu3+

emission mainly originating from the monoclinic

phase, based on the spectra described in [7]. Also the excitation spectrum for the Eu3+

emission, with a strong CT-based excitation band, is typical for the monoclinic phase [7].

Hence we can conclude that even for heat treatments at relatively low temperature, i.e. 900 to

1000°C, the monoclinic phase for CaAl2O4:Eu3+

is already obtained to a considerable extent.

The absence of the monoclinic phase in the x-ray diffraction patterns could be due to a

limited long range order of the monoclinic phase.

In a second part of this work, we evaluated the reduction of Eu3+

by a post-annealing in a

reducing H2/N2 atmosphere. Already at a post-annealing temperature of 500°C, about one

fifth of the europium ions are reduced to Eu2+

, which is also reflected in the

photoluminescence spectrum (Fig. 4). Increasing the post-annealing temperature further

increases the fraction of Eu2+

. At 900°C, the emission spectrum is dominated by Eu2+

, even

for an excitation wavelength which preferentially excites Eu3+

, although XANES indicates

that less than half of the europium ions are reduced to Eu2+

. For the highest annealing

temperatures used, the emission spectrum hardly contains Eu3+

-related emission, while

according to the XANES results there is still a significant fraction of Eu3+

ions. This behavior

is not untypical for Eu-doped materials [27]. During a XANES study on monoclinic rare-


earth co-doped CaAl2O4:Eu, Hölsä et al. observed only the presence of Eu2+

in CaAl2O4:Eu

and small fractions of Eu3+

if co-dopants were used [9]. These powders were formed by

heating for 10h at 1350°C in a reducing atmosphere (N2 + 12% H2). The result on

CaAl2O4:Eu appears to be compatible with our results (Fig. 4).

As mentioned above, the main change in the emission spectrum occurs between 800°C

and 900°C. At slightly lower temperature, the body color of the powders has already turned

gray, presumably indicating the formation of oxygen vacancies. This blackening is

detrimental for the afterglow emission intensity of the CaAl2O4:Eu,Nd powder. However, the

shape of the decay profile and the afterglow intensity relative to the steady-state

photoluminescence intensity (Fig. 6) allows us to state that many relatively deep traps have

been created in the CaAl2O4:Eu,Nd powder. Further research is required to evaluate whether

oxygen vacancies formed during the reducing post-annealing are playing a role in the

trapping mechanism, as has been suggested [8,9]. In any case, the post-annealing step should

be modified to avoid this blackening of the powder.

As a follow-up of this research, alternative post-annealing techniques will be explored,

such as reducing atmosphere with different composition (e.g. lower hydrogen content) or

vacuum annealing. In this way it should be possible to keep a white body color, while still

efficiently reducing the europium ions, and thus enhancing the afterglow intensity in

CaAl2O4:Eu,Nd.

5. Conclusions

In this work we have shown that crystalline CaAl2O4:Eu powders can be obtained at

relatively low temperature by a non-aqueous sol-gel method. After a heat-treatment of the

sols in air, the emission spectrum is dominated by 4f-4f emission peaks originating from

transitions within Eu3+

ions. XANES analysis confirmed that in these powders the trivalent

state is dominant for the europium ions. By performing a post-annealing in a reducing H2/N2

atmosphere, a large fraction of the Eu3+

ions can be reduced to Eu2+

. On the one hand, a

relatively low post-annealing temperature of 900°C is sufficient to have mainly Eu2+

emission. On the other hand, for an annealing temperature of 800°C or more, the body color

of the powders turns gray. Finally, the ratio between both valence states of europium derived

from photoluminescence measurements deviates strongly from the ratio derived from x-ray

absorption spectroscopy. A considerable fraction of Eu3+

ions can be present in the powders,

although this is not necessarily reflected in the photoluminescence properties.

Acknowledgments

KK thanks FWO-Vlaanderen for a PhD research grant. This research is partially supported by

the Interuniversity attraction poles programme IAP/VI-17 (INANOMAT) financed by the

Belgian State, Federal science policy office. We gratefully thank Iolanda Cimieri, Manuel

Claeys Boùùaert, Jonas Botterman and Koen Van den Eeckhout for assisting in the

synchrotron data collection.


Valence states of europium in CaAl 2O 4:Eu phosphors

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