Journal of Luminescence 132 (2012) 2398–2403

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence

0022-23

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Yellow persistent luminescence of Sr2SiO4:Eu2þ ,Dy3þ

Danuta Dutczak a,b, Alexander Milbrat a, Arturas Katelnikovas a,c, Andries Meijerink b,Cees Ronda b,d, Thomas Justel a,n

a Department of Chemical Engineering, Munster University of Applied Sciences, Stegerwaldstr. 39, D-48565 Steinfurt, Germanyb CMI, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80 000, 3508 TA Utrecht, The Netherlandsc Department of General and Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuaniad Philips Research Europe – Eindhoven, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands

a r t i c l e i n f o

Article history:

Received 19 December 2011

Received in revised form

2 March 2012

Accepted 26 March 2012Available online 3 April 2012

Keywords:

Afterglow

Persistent luminescence

Strontium ortho-silicate

Photoluminescence spectroscopy

13/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jlumin.2012.03.055

esponding author. Tel.: þ49 2551 962205; fa

ail addresses: tj@fh-muenster.de, dutczak@fh

a b s t r a c t

This paper reports the photoluminescence and afterglow of Sr2SiO4 doped with Eu2þ and Dy3þ . Factors

governing the formation of the monoclinic or orthorhombic phase of this ortho-silicate are described

and the impact of the crystallographic modification on the luminescence and afterglow under UV and

VUV excitation are discussed and insight in factors limiting the efficiency of this yellow afterglow

material is given.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Persistent luminescence phosphors find application in manyareas, such as security signs, toys, watch dials, interior decorationand optical data storage [1–3]. Especially since the discovery ofSrAl2O4:Eu2þ ,Dy3þ in the middle of the 1990s [4] extensiveresearch on different afterglow materials has been conductedaimed at tuning their emission color, prolong their persistentluminescence and provide fundamental understanding of theafterglow mechanism. Different synthesis methods, various co-dopants, and fluxes were investigated [5–7]. Those studies havebrought new interesting afterglow phosphors emitting mostly inthe blue-green spectral range. Until now, the most efficientafterglow phosphors are blue-green emitting Eu2þ doped alumi-nates and silicates, viz. Sr2MgSi2O7:Eu,Dy [8], Sr4Al14O25:Eu,Dy[9], SrAl2O4:Eu,Dy [10]. There are rather few yellow and redemitting afterglow phosphors, e.g. Sr2SiO4:Eu,Dy [11], Y2O2S:Eu,-Ti,Mn [12], Ca2Si5N8:Eu,Tm [13], and Mg2SiO4:Dy,Mn [14] butmost of them show short and weak persistent luminescence.Thus, the development of efficient yellow and red emitting after-glow phosphors is still an ongoing challenge [15].

One of the yellow emitting afterglow phosphors is strontiumortho-silicate (Sr2SiO4) doped with Eu2þ and Dy3þ . The Eu2þ

activated ortho-silicates were first reported by Barry [16] and

ll rights reserved.

x: þ49 2551 962502.

-muenster.de (T. Justel).

Blasse et al. in 1968 [17] and were intensively studied as phos-phors for color-tunable white light emitting diodes (LEDs). Theluminescence of divalent europium doped ortho-silicates is attrib-uted to the [Xe]4f65d1–[Xe]4f7 transition and is strongly influencedby the local coordination in the host lattice [18]. Strontium ortho-silicate exists in two crystallographic modifications, viz. a0-Sr2SiO4

(orthorhombic) and b-Sr2SiO4 (monoclinic) [19]. The transitionbetween the b-phase and the high temperature a0-phase occursat 358 K involves the rearrangement of SiO4 tetrahedra withoutdisconnection of bonds [20–22]. The a0-Sr2SiO4 phase can bestabilized at room temperature by partial replacement of stron-tium ions by barium [23,24].

This paper reports the luminescence and persistent lumines-cence of yellow emitting Sr2SiO4:Eu2þ ,Dy3þ and is focused on thecorrelation between the phase transition and persistent lumines-cence of this phosphor. The Eu2þ luminescence in strontiumortho-silicate will be discussed on the basis of crystal structure,covalent interaction and crystal-field strength.

2. Experimental

2.1. Synthesis

Ortho-silicates with different concentrations of co-dopantswere prepared by conventional high-temperature solid statepreparation techniques. Stoichiometric amounts of the startingmaterials were ground with acetone in an agate mortar to obtain

Fig. 1. (a) Projection of crystal structure of a0-Sr2SiO4 (orthorhombic) along the

(0 1 0) plane. (b) Projection of crystal structure of b-Sr2SiO4 (monoclinic) along the

(0 1 0) plane.

D. Dutczak et al. / Journal of Luminescence 132 (2012) 2398–2403 2399

a homogeneous mixture. As starting materials SrCO3 (Aldrich,99.9%), BaCO3 (Alfa Aesar 99.8%), nanosized SiO2 (Merck), Eu2O3

(Treibacher, 99.99%) and Dy2O3 (Shin-Etsu Chemical Co., 99.9%)were used. The mixed powders were pre-annealed at 1000 1C inair and finally at 1350 1C in a reducing atmosphere (5% H2þ95%N2). In order to obtain a single phase orthorhombic Sr2SiO4 and toprevent the formation of the monoclinic phase, a fast coolingprocedure was applied. After the final calcination step thesamples were taken out of the furnace at 600 1C and cooled downquickly.

Monoclinic Sr2SiO4:Eu2þ ,Dy3þ was prepared via the sol–gelmethod with ethylene glycol as a polymerizing agent. Sr(NO3)2

(Sigma Aldrich, Z99.0%), nanoscale SiO2 (Merck), Eu2O3 (Treiba-cher, 99.99%) and Dy2O3 (Shin-Etsu Chemical Co., 99.9%) wereused as starting materials. The rare earth oxides Eu2O3 and Dy2O3

were dissolved in a small amount of HNO3, followed by theaddition of Sr(NO3)2 and SiO2. Sr(NO3)2 dissolved in the solution,while SiO2 remained undissolved in the resulting suspension.Finally, ethylene glycol was added and the suspension was stirredfor an hour at 70 1C and then put into a drying oven (130 1C) overnight. The molar ratio of ethylene glycol to total metal cationswas 2:1. The resulting intermediates were ground and pre-annealed at 1000 1C in air. The second calcination step wasperformed in a CO atmosphere at 1200 1C and the final calcinationstep was performed at 1350 1C in 5% H2þ95% N2 atmosphere.

2.2. Characterization

The X-ray powder diffraction measurements were performedon a Rigaku MiniFlex II, operated in the Bragg–Brentano geometryand equipped with a Cu-anode X-ray source. The XRD patterns ofall obtained samples showed that single phase monoclinic Sr2SiO4

(JPCDS 38-0271) or orthorhombic Sr2SiO4 (JPCDS 39-1256) wasobtained.

The room temperature UV/VIS excitation and emission spectrawere recorded on an Edinburgh Instruments FSL900 fluorescencespectrometer equipped with a 450 W Xenon discharge lamp, amonochromator TMS300 (Czerny-Turner optics), and a cooled(�20 1C) photo-multiplier tube (Hamamatsu R2658P) operatingin the single photon counting mode. The spectra were correctedfor the instrumental response and wavelength dependent lampoutput. For the excitation and emission spectra at liquid nitrogentemperature (�77 K) a ‘‘MicrostatN’’ cryostat from Oxford Instru-ments has been used with the spectrofluorometer.

All VUV spectroscopy measurements were carried out on anEdinburgh Instruments FS920 spectrometer equipped with a VUVmonochromator VM504 from Acton Research Corporation (ARC)and deuterium lamp as excitation source. The sample chamberwas flushed with dry nitrogen to prevent VUV radiation absorp-tion by oxygen and water.

To record reflection spectra of undoped Sr2SiO4 samples in therange from 150 to 350 nm a BaMgAl10O17:Eu2þ (BAM:Eu) coatedintegration sphere was mounted into the sample chamber of theVUV spectrometer. In order to obtain the reflection spectra thefollowing procedure was conducted: firstly, an excitation scanfrom 150 to 350 nm with BAM:Eu is recorded. Subsequently, theexcitation scan of the sample is recorded (please note that bothscans contain the same information about the excitation ofBAM:Eu, as the integrated sphere is also coated with BAM:Eu).For this reason, the excitation scan can be divided by theexcitation scan of BAM:Eu monitored under the same conditionsto obtain the reflection spectrum.

Persistent luminescence was recorded on the EdinburghInstruments FS900 spectrometer equipped with a 450 W Xenondischarge lamp, monochromator TMS300 (Czerny-Tuner optics),an integration sphere coated with barium sulphate, and a cooled

(�20 1C) photo-multiplier tube (Hamamatsu R2658P) operatingin a single photon counting mode.

3. Results and discussion

According to the structural data, two different strontium siteswith coordination numbers 9 and 10, respectively, exist in bothphases of Sr2SiO4 (Fig. 1a and b). Because of the similar ionic radii,Eu2þ ions replace Sr2þ ions leading to two different types ofluminescent Eu2þ centers. The distances between oxygen andstrontium at the 10-fold coordinated site are larger than at the9-fold coordinated one. Consequently the covalent bonding andcrystal field strength are weaker for the 10-fold coordinated sitethan for the 9-fold site resulting in shorter wavelength emissionfor Eu2þ on the 10-fold coordinated site.

3.1. Different concentration of dopants

As reported by Park et al. [25] and Sun et al. [26] the Eu2þ

doping level has a strong influence on the phase formation ofSr2SiO4:Eu2þ . If the Eu2þ concentration is small, Sr2SiO4:Eu2þ

exists in the monoclinic b-phase. The orthorhombic a-phase isobtained if the Eu2þ concentration is higher than 0.5% [26].

XRD patterns of the synthesized Sr2SiO4:Eu2þ ,Dy3þ samplesand reference patterns of the monoclinic and orthorhombicSr2SiO4 phases are presented in Fig. 2. The XRDs of the monoclinicand the orthorhombic modification of Sr2SiO4 are very similar buta clear identification is possible by the reflections around 2y¼271and the 32.41 reflection which appears in the monoclinic phase.The Sr2SiO4:0.1%Eu2þ ,0.05%Dy3þ sample consists mostly of themonoclinic phase with small traces of the orthorhombic phase,while the Sr2SiO4:1%Eu2þ ,0.5%Dy3þ consists of the orthorhombicphase with small impurities of the monoclinic phase. The XRDpatterns of Sr2SiO4:2%Eu2þ ,1%Dy3þ are in good agreement withthe reference pattern of orthorhombic Sr2SiO4 and are, therefore,regarded to be of single phase.

Fig. 3 shows normalized excitation and emission spectra ofSr2SiO4:Eu2þ ,Dy3þ with different doping levels of Eu2þ and Dy3þ

measured at liquid nitrogen temperature and room temperature.Under UV excitation Eu2þ doped strontium silicates show strongluminescence due to transitions from the lowest crystal fieldcomponent of [Xe]4f65d1 configuration of Eu2þ to the 8S7/2

Fig. 2. Powder XRD patterns of (a) Sr2SiO4:0.1%Eu2þ ,0.05%Dy3þ , (b) Sr2SiO4:

1%Eu2þ ,0.5%Dy3þ and (c) Sr2SiO4:2%Eu2þ ,1%Dy3þ . The reference diffraction

patterns of (d) monoclinic Sr2SiO4 and (e) orthorhombic Sr2SiO4 are included for

comparison. Results from Rietveld refinement, performed for the samples con-

sisting of more than one phase, are added to the graph and give information on the

phase composition.

Fig. 3. Normalized emission and excitation spectra of Sr2SiO4:0.1%Eu2þ ,

0.05%Dy3þ (a) at 77 K and (b) at 300 K; Sr2SiO4:1%Eu2þ ,0.5%Dy3þ (c) at 77 K

and (d) at 300 K; Sr2SiO4:2%Eu2þ ,1%Dy3þ (e) at 77 K and (f) at 300 K. The phase

composition is indicated in the figure for the different Eu-concentrations.

Fig. 4. Emission (for 310 nm excitation) and excitation (for 570 nm emission)

spectra of Sr2SiO4:2%Eu2þ ,1%Dy3þ at 77 K and at 300 K recorded under identical

alignment conditions, allowing for a comparison of the absolute intensities at the

two temperatures.

D. Dutczak et al. / Journal of Luminescence 132 (2012) 2398–24032400

([Xe]4f7) ground state. Since Eu2þ ions replace Sr2þ ions twodifferent types of luminescent centers in both phases can beexpected. The emission spectra of Sr2SiO4:Eu2þ ,Dy3þ do not showany transitions, which could be attributed to Dy3þ .

At low temperature and low Eu2þ concentration the emissionfrom both Eu2þ centers is clearly observed. The emission spec-trum of Sr2SiO4:0.1%Eu2þ ,0.05%Dy3þ measured at 77 K consistsof two bands located around 470 and 550 nm. Based on covalencyand crystal field arguments [18], the high energy emission bandof Eu2þ can be assigned to the higher (10) coordinated site andthe low energy band to the lower (9) coordinated site. It isgenerally observed that d–f emission from Eu2þ is at higherenergies for Eu2þ on a larger site with higher coordinationnumbers. Two factors contribute to this shift: the larger Eu-liganddistances on the larger sites reduce the covalent interactionswhile also the crystal field splitting is smaller. Both covalency and

crystal field splitting shift the lowest 5d state, from whichemission occurs, to lower energies. As a result, a higher coordina-tion number gives a smaller shift of the emitting 5d state to lowerenergies, and higher energy d–f emission is observed.

The emission spectra of Eu2þ doped strontium silicates changewith excitation wavelength, the dopant concentration and tem-perature (see Fig. 3 and Refs. [27,28]).

The relative intensity of the two emission bands stronglydepends on the excitation wavelength [28]. The excitation spectraof Sr2SiO4:Eu2þ ,Dy3þ recorded for the two types of emissioncenters demonstrate different shapes and positions of the max-ima. The excitation spectrum recorded for the emission at 480 nmconsists of a broad band with a maximum at 330 nm, while theexcitation spectrum recorded for the low energy emission(560 nm) has a maximum at around 370 nm. At low dopantconcentrations, when energy transfer from the blue emittingto the yellow emitting Eu-site is absent, Sr2SiO4:0.1%Eu2þ ,0.05%Dy3þ shows blue luminescence under short wavelengthexcitation and yellow luminescence under long wavelengthexcitation (see Fig. 3a and b). A slight shift towards longerwavelengths can be observed for both emission bands as theEu2þ concentration increases due to re-absorption.

At room temperature more efficient energy transfer betweenthese sites occurs due to the increasing spectral overlap betweenthe high energy emission band (peaking at 480 nm) and theexcitation band of the emission band peaking at 560 nm.

The increasing spectral overlap is caused by thermal broad-ening of the bands. Consequently, energy transfer from Eu(1) tothe Eu(2) center becomes more efficient, which leads to adecrease in the short wavelength emission band and an intensityenhancement of the long wavelength emission band. In order toconfirm the change in relative intensities due to energy transferand not temperature quenching of the blue emission, emissionspectra for Sr2SiO4:2%Eu2þ ,1%Dy3þ at 77 K and at 300 K wererecorded under identical conditions (see Fig. 4). It is clear that theintensity ratio of the two emission bands changes with tempera-ture while the integrated intensity remains the same.

Fig. 5 depicts luminescence intensity decay curves ofSr2SiO4:Eu2þ ,Dy3þ samples after irradiation at 370 nm for 120 s.All phosphors first show a rapid intensity decrease and then a longerlasting yellow luminescence. The longest afterglow was observed forthe Sr2SiO4:1%Eu2þ ,0.5%Dy3þ sample, which consists of both theorthorhombic and monoclinic phase. Significantly shorter afterglow

Fig. 5. Persistent luminescence of Sr2SiO4:0.1%Eu2þ ,0.05%Dy3þ , Sr2SiO4:1%Eu2þ ,

0.5%Dy3þ and Sr2SiO4:2%Eu2þ ,1%Dy3þ after 370 nm irradiation for 120 s at room

temperature.

Fig. 6. Powder XRD patterns of (a) monoclinic Sr2SiO4:1%Eu2þ ,0.5%Dy3þ prepared

via the sol–gel method; (b) orthorhombic Sr2SiO4:1%Eu2þ ,0.5%Dy3þ prepared by

the solid state method and (c) (Sr0.95Ba0.05)2SiO4:1%Eu2þ ,0.5%Dy3þ prepared by

the solid state method. The reference patterns of (d) monoclinic Sr2SiO4 and (e)

orthorhombic Sr2SiO4 are included for comparison.

Fig. 7. Normalized emission and excitation spectra of monoclinic Sr2SiO4:

1%Eu2þ ,0.5%Dy3þ (a) at 77 K and (b) at 300 K; orthorhombic Sr2SiO4:1%Eu2þ ,

0.5%Dy3þ (c) at 77 K and (d) at 300 K; orthorhombic (Sr0.95Ba0.05)2SiO4:

1%Eu2þ ,0.5%Dy3þ (e) at 77 K and (f) at 300 K.

D. Dutczak et al. / Journal of Luminescence 132 (2012) 2398–2403 2401

was observed for the samples with lower dopant concentrations(mainly monoclinic phase) and higher dopant concentrations (pureorthorhombic phase).

3.2. Comparison of the luminescence of orthorhombic and

monoclinic Sr2SiO4:Eu,Dy

In order to compare the luminescence and persistent lumines-cence properties of the orthorhombic and monoclinic Sr2SiO4:Eu2þ ,Dy3þ and exclude the effect of the dopant concentration,single phase orthorhombic and monoclinic Sr2SiO4:Eu2þ ,Dy3þ

with identical concentrations of dopants, were synthesized. Theorthorhombic phase of Sr2SiO4:1%Eu2þ ,0.5%Dy3þ was synthe-sized via the solid state method, with small addition of bariumor applying the rapid temperature quenching of the sample afterthe calcinations. According to Pieper et al. [24] 5% of Sr2þ wassubstituted by Ba2þ in Sr2SiO4:Eu2þ ,Dy3þ to stabilize the orthor-hombic Sr2SiO4 phase. Moreover, it was found that the rapidquenching of the sample prevents formation of the low tempera-ture monoclinic phase and leads to formation of pure orthorhom-bic Sr2SiO4. The monoclinic phase was obtained using the sol–gelmethod.

The XRD patterns, shown in Fig. 6, reveal that the preparationprocedure strongly influences the Sr2SiO4 phase formation. Sam-ples prepared by the sol–gel method consist of the pure mono-clinic phase, while the pure orthorhombic phase was obtained insamples prepared by the solid state method by Ba2þ addition orapplying a rapid quenching procedure. It can also be observedthat the XRD patterns of (Sr0.95Ba0.05)2SiO4:Eu2þ ,Dy3þ shifttowards slightly smaller angles in comparison to Sr2SiO4:Eu2þ ,Dy3þ . This effect is caused by the expansion of the latticesince the ionic radius of Ba2þ is larger than the ionic radius ofSr2þ .

The maximum of the emission spectra of monoclinicSr2SiO4:1%Eu2þ ,0.5%Dy3þ is shifted towards slightly longerwavelengths (�3 nm), compared to the orthorhombic phase(see Fig. 7). This effect is caused by slight differences in symmetrybetween these two forms [6], resulting in different covalentinteraction, crystal-field strength and Stokes Shift. Moreover, asmall red shift (�6 nm) for the Eu2þ emission in Sr2SiO4:1%Eu2þ ,0.5%Dy3þ can be observed if 5% of Sr2þ is substitutedby Ba2þ . This observation is different from the results of Poort[27]. The shift is likely a combined effect of the two contributions,viz. covalency and crystal-field strength. On one hand, the

substitution of smaller Sr2þ cations by larger Ba2þ cations causesan increase in covalent character of the Eu–O bonds. The highercovalency reduces the energy difference between the [Xe]4f7

ground state and the [Xe]4f65d1 excited state and a red shift ofthe 5d–4f emission of Eu2þ is observed. On the other hand, as thelarger ions replace the smaller ions, the bond length increases andthe crystal-field strength is reduced causing a blue shift of theEu2þ emission. It can also be observed that the influence of thesetwo effects is different for 10-fold and 9-fold coordinated sites.The shift of high energy emission band of Eu2þ (CN¼10) isstronger than that for low energy band (CN¼9). The high energyband shifts roughly by 13 nm and low energy band shifts by 7 nmat 77 K compared to room temperature.

The luminescence afterglow curves for monoclinic and orthor-hombic Sr2SiO4:Eu2þ ,Dy3þ are shown in Fig. 8. Both the mono-clinic and orthorhombic Sr2SiO4:Eu2þ ,Dy3þ show persistent

Fig. 8. Persistent luminescence of monoclinic Sr2SiO4:1%Eu2þ ,0.5%Dy3þ , orthor-

hombic Sr2SiO4:1%Eu2þ ,0.5%Dy3þ and orthorhombic (Sr0.95Ba0.05)2SiO4:1%Eu2þ ,

0.5%Dy3þ compared with orthorhombic Sr2SiO4:1%Eu2þ , after 370 nm irradiation

for 120 s at room temperature.

Fig. 9. Persistent luminescence of monoclinic and orthorhombic Sr2SiO4:

1%Eu2þ ,0.5%Dy3þ compared with orthorhombic Sr2SiO4:1%Eu2þ , after 160 nm

irradiation for 300 s (T¼RT). The inset shows persistent emission spectra after

160 nm irradiation for 300 s at room temperature.

D. Dutczak et al. / Journal of Luminescence 132 (2012) 2398–24032402

luminescence, which is clearly stronger in comparison to thesample without Dy3þ co-doping.

Considerably stronger afterglow is observed for all investi-gated silicates after irradiation by supra-bandgap 160 nm excita-tion, as depicted in Fig. 9. This observation implies that variouslattice defects, which are present in the host and can be filled byirradiation with 160 nm radiation, play an essential role in theafterglow. The persistent luminescence emission spectra of thephosphors are similar to steady state emission spectra. Theposition and shape of the emission bands remains the same overtime. The time dependence of the afterglow for monoclinic andorthorhombic Sr2SiO4:1%Eu2þ ,0.5%Dy3þ is almost identical,which suggests a similar depth of electron traps in these twophosphors. This is consistent with the fact that the crystallo-graphic differences between the two modifications are small. Theslightly different intensities (�factor 2) of the afterglow intensityfor monoclinic and orthorhombic modifications imply a differentdensity or trapping efficiency for the traps. It is interesting toobserve that the afterglow intensity is much stronger after supra-

bandgap excitation. This shows that there is a high probability oftrapping free charge carriers in traps that slowly release thecharge carriers at room temperature resulting in a bright yellowafterglow with a decay time of tens of minutes. A bright yellowafterglow with this decay time is promising for application andmakes this material a candidate for a yellow afterglow material tocomplement existing green and blue persistent luminescencecommercial afterglow phosphors. Unfortunately, the afterglow isless intense after near-UV or visible excitation in the Eu2þ 4f65dstates. Apparently, charge liberation from the fd states excited inthis spectral region into the conduction band is not efficient andlimits the afterglow intensity.

Sr2SiO4:1%Eu2þ also shows the persistent luminescence,though it is much weaker compared to the samples co-dopedwith Dy3þ . This confirms that Dy3þ ions play a very importantrole in the physical processes leading to afterglow. However, thedetailed persistent luminescence mechanism is not known. As weobserve Eu2þ emission, Eu2þ acts as hole trap. Very likely Dy3þ

ions are involved in electron trapping, and for this reason thepersistent emission is stronger when Dy3þ is added. The fact thattwo Dy3þ ions can replace the Sr2þ ions, the Dy3þ ions maypromote the formation of defects that act as electron traps,possibly oxygen vacancies. Since we observe Eu2þ emission, weconclude that the electron release is the rate determining step inthe afterglow. The electron is subsequently captured by Eu3þ ,rendering a Eu2þ ion in the excited state, followed by the Eu2þ

emission. Without Dy3þ added, the electron traps are of adifferent nature, e.g. vacancies created by the presence of smallamounts of Eu3þ in the material.

4. Conclusions

In this work it is demonstrated how the formation of differentcrystalline modifications (monoclinic or orthorhombic) of stron-tium orthosilicate can be controlled by the synthesis conditions oraddition of dopants, such as Eu2þ or Ba2þ . An unexpected red-shift of the Eu2þ 4f5d luminescence was observed for partialreplacement of Sr2þ by Ba2þ in orthorhombic Sr2SiO4. Introduc-tion of Dy3þ as a co-dopant strongly enhances the afterglowintensity for both crystalline modifications. The persistent lumi-nescence is about two times stronger for the monoclinic phaseafter supra-bandgap VUV excitation and is much more intensethan after sub-bandgap near-UV or visible excitation. This showsthat direct creation of free charge carriers under supra-bandgapexcitation is followed by trapping and slow release of chargecarriers resulting in a strong yellow afterglow. The absence of thisstrong afterglow under sub-bandgap excitation in the 4f5d-stateof Eu2þ indicates that charge liberation from these excited 4f5dstates is not very efficient in Sr2SiO4.

Acknowledgments

We would like to acknowledge M. Strobele and Prof. H.-J.Meyer from Eberhard-Karls-Universitat Tubingen for Rietveldrefinement.

References

[1] C. Feldmann, T. Justel, C.R. Ronda, P.J. Schmidt, Adv. Funct. Mater. 13 (2003)511–516.

[2] C.R. Ronda, Luminescence: From Theory to Applications, Wiley-VCH, Wein-heim, 2008.

[3] A.M. Srivastava, H.S. Nalwa, in: H.S. Nalwa (Ed.), Handbook of Luminescence,Display Materials and Devices, 2003.

[4] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143(1996) 2670–2673.

D. Dutczak et al. / Journal of Luminescence 132 (2012) 2398–2403 2403

[5] R. Chen, Y. Wang, Y. Hu, Z. Hu, C. Liu, J. Lumin. 128 (2008) 1180–1184.[6] J. Niittykoski, T. Aitasalo, J. Holsa, H. Jungner, M. Lastusaari, M. Parkkinen,

M. Tukia, J. Alloys Compd. 374 (2004) 108–111.[7] Z. Qiu, Y. Zhou, M. Lu, A. Zhang, Q. Ma, Acta Mater. 55 (2007) 2615–2620.[8] Y. Lin, Z. Tang, Z. Zhang, X. Wang, J. Zhang, J. Mater. Sci. Lett. 20 (2001)

1505–1506.[9] Y. Lin, Z. Tang, Z. Zhang, Mater. Lett. 51 (2001) 14–18.

[10] F. Clabau, X. Rocquefelte, S. Jobic, P. Deniard, M.H. Whangbo, A. Garcia, T. LeMercier, Chem. Mater. 17 (2005) 3904–3912.

[11] N. Lakshminarasimhan, U.V. Varadaraju, Mater. Res. Bull. 43 (2008)2946–2953.

[12] Z. Hong, P. Zhang, X. Fan, M. Wang, J. Lumin. 124 (2007) 127–132.[13] K. Van den Eeckhout, P.F. Smet, D. Poelman, J. Lumin. 129 (2009) 1140–1143.[14] L. Lin, M. Yin, C. Shi, W. Zhang, J. Alloys Compd. 455 (2008) 327–330.[15] J. Holsa, Electrochem. Soc. Interface 18 (2009) 42–45.[16] T.L. Barry, J. Electrochem. Soc. 115 (1968) 1181–1184.[17] G. Blasse, W.L. Wanmaker, J.W. ter Vrugt, A. Bril, Philips Res. Rep. 23 (1968)

189–200.

[18] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin,1994.

[19] M. Catti, G. Gazzoni, G. Ivaldi, G. Zanini, Acta Crystallogr. Sect. B: Struct. Sci.B39 (1983) 674–679.

[20] M. Catti, G. Gazzoni, G. Ivaldi, Acta Crystallogr. Sect. C: Cryst. Struct.Commun. C39 (1983) 29–34.

[21] B.G. Hyde, J.R. Sellar, L. Stenberg, Acta Crystallogr. Sect. B: Struct. Sci. B42(1986) 423–429.

[22] L. Stenberg, B.G. Hyde, Acta Crystallogr. Sect. B: Struct. Sci. B42 (1986)417–422.

[23] J.H. Lee, Y.J. Kim, J. Ceram. Process. Res. 10 (2009) 81–84.[24] G. Pieper, W. Eysel, T. Hahn, J. Am. Ceram. Soc. 55 (1972) 619–622.[25] J.K. Park, M.A. Lim, C.H. Kim, H.D. Park, J.T. Park, S.Y. Choi, Appl. Phys. Lett. 82

(2003) 683–685.[26] X. Sun, J. Zhang, X. Zhang, Y. Luo, X. Wang, J. Rare Earths 26 (2008) 421–424.[27] S.H.M. Poort, W. Janssen, G. Blasse, J. Alloys Compd. 260 (1997) 93–97.[28] W. Zhi-Jun, Y. Zhi-Ping, G. Qing-Lin, L. Pan-Lai, F. Guang-Sheng, Chin. Phys. B

18 (2009) 2068–2071.