Optical Materials 36 (2014) 1146–1152

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Optical Materials

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Luminescence properties of Sm3+-doped alkaline earth ortho-stannates

http://dx.doi.org/10.1016/j.optmat.2014.02.0180925-3467/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +370 67757147.E-mail address: andrius.stanulis@chf.vu.lt (A. Stanulis).

Andrius Stanulis a,⇑, Art�uras Katelnikovas b,c, David Enseling b, Danuta Dutczak b, Simas Šakirzanovas d,Marlies Van Bael e,f, An Hardy e,f, Aivaras Kareiva a, Thomas Jüstel b

a Department of Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuaniab Department of Chemical Engineering, Münster University of Applied Sciences, Stegerwaldstr. 39, D-48565 Steinfurt, Germanyc Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuaniad Department of Physical Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuaniae Institute for Materials Research, Laboratory of Inorganic and Physical Chemistry, Hasselt University, Agoralaan D, B-3590 Diepenbeek, Belgiumf IMEC, Division IMOMEC, Agoralaan D, B-3590 Diepenbeek, Belgium

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 November 2013Received in revised form 12 February 2014Accepted 19 February 2014Available online 24 March 2014

Keywords:StannatesPhosphorsLuminescenceOptical materials

A series of Sm3+ doped M2SnO4 (M = Ca, Sr and Ba) samples were prepared by a conventional high tem-perature solid-state reaction route. All samples were characterized by powder X-ray diffraction (XRD)analysis, photoluminescence (PL), photoluminescence thermal quenching (TQ) and fluorescence lifetime(FL) measurements. The morphology of synthesized phosphor powders was examined by scanning elec-tron microscopy (SEM). Moreover, luminous efficacies (LE) and color points of the CIE 1931 color spacediagram were calculated and discussed. Synthesized powders showed bright orange-red emission underUV excitation. Based on the results obtained we demonstrate that Sm3+ ions occupy Ca and Sr sites in theCa2SnO4 and Sr2SnO4 ortho-stannate structures, respectively. In contrast, Sm3+ substitutes Sn in the bar-ium ortho-stannate Ba2SnO4 structure.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The development of flat panel displays, such as field emissiondisplays (FEDs), plasma display panels (PDPs) and thin film elec-tro-luminescent devices (TFEL) has always been accompanied byimprovements in the applied phosphors. Many efforts have beenmade to discover novel host materials as well as activators withhigh performance for phosphor applications [1,2].

From a practical point of view, oxide phosphors are more attrac-tive than the traditional sulfide or halide phosphors due to theirresistance to moisture. During last decade, alkaline earth ortho-stannates (M2SnO4, where M = Ca, Sr and Ba) have been drawingmore and more attention as host matrix for new phosphorsbecause of their stable crystalline structure and high physicaland chemical stability [3]. Incorporation of optically active lantha-nide ions into the stannate host matrix resulted in phosphorspossessing photoluminescence (PL) [4–6] and long lasting phos-phorescence (LLP) [7–9] properties.

The performance of LLP phosphors emitting in the blue [10] andgreen [11] spectral region almost met the requirement for practicalapplications and are commercially widely available. However,

orange to red LLP phosphors are still out of reach from a practicalpoint of view. Therefore, there is still a strong need for the devel-opment of novel host materials for LLP phosphors emitting at long-er, i.e. in the orange to red, wavelength range [12].

In general, the properties of LLP phosphor can be adjusted byusing a host-mixing method to change the original constituentsof the lattice or introducing different co-dopant [13]. Therefore, itis necessary to systematically investigate each host material indi-vidually to learn more about the mechanisms governing the photo-luminescence phenomenon. Important performance factors are e.g.the band gap, the energetic distance of the excited state to the con-duction band and the type and energetic location of defects.

The purpose of this work is to investigate photoluminescenceproperties of Sm3+-doped Ca2SnO4, Sr2SnO4 and Ba2SnO4 phos-phors prepared by a high temperature solid-state reaction method.The influence of Sm3+ concentration on phase formation peculiari-ties and luminescence properties are discussed. Temperaturedependent luminescent behavior of Sm3+ ions in three differentmatrixes was also comparatively studied.

2. Experimental

Samarium-doped calcium, strontium and barium ortho-stan-nate (M2SnO4:Sm3+, M = Ca, Sr and Ba) powder samples were

Fig. 1. (a) Crystal structure of Ba2SnO4, (b) BaSnO3 and (c) the unit cell of the rock salt type BaO structure.

Fig. 2. Powder XRD patterns of Ca2SnO4:Sm3+.

Fig. 3. Powder XRD patterns of Sr2SnO4:Sm3+.

A. Stanulis et al. / Optical Materials 36 (2014) 1146–1152 1147

synthesized by a typical solid-state reaction method. The rawmaterials were CaCO3 (P99.995% Aldrich), SrCO3 (P99.9% Al-drich), BaCO3 (99.98% Aldrich), SnO2 (P99% Merck) and Sm2O3

(99.9% Alfa-Aesar). Three series of samples, with Sm3+ concentra-tions ranging from 0.1 to 4 mol% of M2+ ions in M2SnO4, were pre-pared. For this purpose, stoichiometric amounts of raw materialswere thoroughly ground in an agate mortar (acetone was used asmedium) and then sintered at 1400 �C for 10 h in air (heating rate5 �C/min).

Powder X-ray diffraction (XRD) analysis has been carried outemploying a Rigaku MiniFlex II diffractometer working in theBragg–Brentano (h/2h) geometry. The data were collected within2h angle from 10� to 80� at a step size of 0.02� and an integrationtime of 1 s using Ni-filtered Cu Ka line.

Excitation (excitation slit 0.5 nm and emission slit 1.5 nm) andemission (excitation slit 5.0 nm and emission slit 0.5 nm) spectrawere recorded in the ranges of 250–550 and 500–800 nm, respec-tively, on the Edinburgh Instruments FSL920 fluorescence spec-trometer equipped with 450 W Xe arc lamp, mirror optics forpowder samples and a cooled (�20 �C) single-photon photomulti-plier (Hamamatsu R2658P). The photoluminescence emissionspectra were corrected by a correction file obtained from tungstenincandescent lamp certified by the NPL (National Physics Labora-tory, UK) and excitation spectra were corrected by reference detec-tor. The reflection spectra were recorded on the same modelspectrometer equipped with an integration sphere coated withbarium sulfate. BaSO4 (99% Sigma–Aldrich) was used as a reflec-tance standard. Photoluminescence lifetimes were measured withthe Edinburgh Instruments FSL920 spectrometer equipped withthe mF900 flash lamp and a Hamamatsu extended red sensitivityphotomultiplier tube. Data were acquired in a gated single photoncounting (MCS) mode. All measurements were performed at roomtemperature in air.

For thermal quenching (TQ) measurements a cryostat ‘‘Micro-statN’’ from Oxford Instruments has been applied to the abovementioned spectrometer set-up. Liquid nitrogen was used as acooling agent. Measurements were carried out from 100 to 500 Kin 50 K steps.

Fig. 4. Powder XRD patterns of Ba2SnO4:Sm3+.

1148 A. Stanulis et al. / Optical Materials 36 (2014) 1146–1152

3. Results and discussion

The refinement of the crystal structure revealed that Ca2SnO4

belongs to the Sr2PbO4-type structure with an orthorhombic unitcell and space group Pbam (#55). In this structure type, the Caatoms occupy a 4h site (x, y, 1/2) and are coordinated by seven oxy-gen atoms. The Sn atoms occupying a 2a site (0,0,0) are coordi-nated by six oxygen atoms [14,15]. On the other hand, Sr2SnO4

and Ba2SnO4 adopt layered structures derived from the K2NiF4

structure type. Ba2SnO4 crystallizes in the undistorted tetragonalspace group I4/mmm (#139). In the undistorted K2NiF4 structure,the larger cations are surrounded by nine anions. This coordinationnumber is, however, reduced to seven in Sr2SnO4 due to the titlingof the SnO6 octahedrons and leads to an orthorhombic distortion ofthe K2NiF4 structure at room temperature [16,17]. In both structuretypes, SnO6 octahedra are connected in low-dimensional form;

Fig. 5. Excitation, emission, and reflection spectra of (a) Ca2SnO

SnO6 octahedra are linked by sharing edges with each other andforming one-dimensional chains in Ca2SnO4 and two-dimensionalperovskite-like layers in Sr2SnO4 and Ba2SnO4 [5]. In this kind oflow-dimensional structure, it is easy to implant other ions intothe host lattice and create traps located at suitable depths thatcan store the excitation energy and emit light upon re-activationat room temperature [18,19]. Considering the ionic radii it can beassumed that Sm3+ ions (1.10 Å ? CN = 6; 1.16 Å ? CN = 7 and1.27 Å ? CN = 9) have a tendency to replace closer in size Ca2+

(1.2 Å ? CN = 7) and Sr2+ (1.35 Å ? CN = 7) ions, rather than occu-py Sn4+ sites (0.83 Å ? CN = 6) [20]. However, in Ba2SnO4 structureBa2+ is coordinated by nine oxygen ions (1.61 Å ? CN = 9). We as-sume that Sm3+ ions will occupy closer in size Sn4+ octahedral sitein Ba2SnO4 structure according to the Vegard’s law [3].

Moreover, a closer look at the Ba2SnO4 structure (Fig. 1a) showsthat it consists of an intergrowth of perovskite-like blocks ofcomposition BaSnO3 and fragments of BaO (NaCl type). Both blockscomposing the structure are drawn in Fig. 1b and c, whereas theBaO fragment is outlined in the unit cell of the rock salt BaOstructure in Fig. 1c. The other component of the structure, theBaSnO3 blocks, are formed by slightly distorted cubes of Ba ions(4.14 � 4.14 � 3.88 Å), centered by the Sn4+ ions. The topologyand distances of the basic units BaO and BaSnO3 remain unalteredin the more complex compounds [21].

Wang et al. [22] demonstrated on the basis of atomistic semi-empirical simulations and density functional theory (DFT) calcula-tions for the BaSnO3 structure that trivalent dopants, Sc, In, and Yhave relatively favorable solution energies at the Sn site. For cat-ions larger than Sm3+, some partial substitution on Ba-site cannotbe discarded.

The substitution of Sm3+ ion for M2+ is a non-equivalent substi-tution and can be expressed by the following equation [23]:

SmM $ Sm�M þ e0 ð1Þ

To fulfill the charge neutrality requirements an excess ofpositive charge in the lattice must be compensated. There are

4:1% Sm3+, (b) Sr2SnO4:1% Sm3+, and (c) Ba2SnO4:2% Sm3+.

Fig. 6. Emission intensity integrals as a function of Sm3+ concentration in differentalkaline earth stannate phosphors.

Fig. 7. (a) Normalized emission integrals and (b) emission decay values as afunction of temperature of the Ca2SnO4:1% Sm3+ sample.

Fig. 8. (a) Normalized emission integrals and (b) emission decay values as afunction of temperature of the Sr2SnO4:1% Sm3+ sample.

A. Stanulis et al. / Optical Materials 36 (2014) 1146–1152 1149

two possible pathways of charge compensation: electronic and va-cancy compensation. In the first case, there is a possibility thatsome of the Sn4+ ions capture electrons liberated in Eq. (1) andchange their valence state to Sn3+ and/or Sn2+ resulting in the for-mation of structure defects with localized charge carriers (elec-trons), which can act as hole trapping centers. The other possiblepathway of charge compensation is that two Sm3+ ions replacethree M2+ ions, which create two Sm�M positive defects and oneV 00M (cation vacancy) negatively charged defect [8,23]:

2Sm3þ þ 3M2þ ! 2Sm�M þ V 00M ð2Þ

where Sm�M and V 00M act as the electron and hole trap sites, respec-tively. In the present case, trivalent Sm3+ ions not only play the roleof activator, but also serve as an aliovalent auxiliary dopant to cre-ate defects.

The powder XRD patterns of Ca2SnO4:Sm3+ samples annealed at1400 �C for 10 h in air are given in Fig. 2. All the peaks can be in-dexed to the orthorhombic Ca2SnO4 system and are in a goodagreement with standard powder XRD data of ICDD#04-008-2918. A minor impurity phase of CaSnO3 is present in all samplesas well as in undoped host material. This result is consistent withthe previous reports [24,25].

Fig. 9. (a) Normalized emission integrals and (b) emission decay values as afunction of temperature of the Ba2SnO4:2% Sm3+ sample.

1150 A. Stanulis et al. / Optical Materials 36 (2014) 1146–1152

Fig. 3 shows the powder XRD patterns of Sr2SnO4:Sm3+. Thedoping of Sm3+ does not make any noticeable changes in the XRDpatterns and all the reflection peaks can be assigned to a singlephase Sr2SnO4 (ICDD#04-010-4448). The absence of impurityphases indicates that the trivalent samarium ions are well incorpo-rated into the crystal lattice.

Powder XRD analysis revealed that single phase Ba2SnO4:Sm3+

phosphor can be obtained once the samarium content is higher

Fig. 10. Decay curves of M2SnO4:Sm3+ a

than 2 mol% (Fig. 4). Furthermore, a small amount of Ba2SmSnO5.5

(ICDD#04-002-8282) impurity phase is observed if the dopantconcentration exceeds 4 mol%.

The photoluminescence spectra of Ca2SnO4:1% Sm3+,Sr2SnO4:1% Sm3+, and Ba2SnO4:2% Sm3+ are depicted in Fig. 5a, b,and c, respectively. The excitation spectra of Sm3+ doped Ca2SnO4

sample was recorded by monitoring the emission at 610 nm(Fig. 5a). The strongest excitation band is located at 408 nm(6H5/2 ?

4L13/2 + 6P3/2 + 4F7/2) and some peaks at 346 nm (6P7/2),362 nm (5D3/2 + 4F9/2 + 6P5/2), 378 nm (6P7/2 + 4K13/2), 416 nm (6P5/2),477 nm (4I9/2 + 4M15/2) are ascribed to the electronic transitionsfrom the ground state to the high energy excited levels of Sm3+.The strongest emission peaks observed at 616 nm and 646 nmoriginate from the well-known intra-4f-shell transitions of Sm3+

from the excited state 4G5/2 to the ground state levels 6HJ (J = 5/2,7/2, 9/2, 11/2). The given excitation spectra of the Sr2SnO4:1%Sm3+ sample (Fig. 5b) is rather similar to Ca2SnO4:1% Sm3+.However, the most intensive emission band of Sr2SnO4:1% Sm3+

phosphor is located at 646 nm and corresponds to the 4G5/2 ?6H9/2

transition. The excitation spectra of Ba2SnO4 sample doped with 2%Sm3+ as monitored at 610 nm is depicted in Fig. 5c. The dominatingbroad band centering at 283 nm can be attributed to the chargetransfer (CT) transition from the O2� ligand to Sm3+ and/or Sn4+,whereas direct 4f5–4f5 transitions of Sm3+ are very weak andbarely visible. In accordance to what we say, in calcium and stron-tium ortho-stannates Sm3+ ions are located on the Ca2+ and Sr2+

sites, respectively. Only in the Ba2SnO4 structure Sm3+ goes tothe Sn4+ site, which is much smaller. It means that charge transferfrom oxygen to Sm3+/Sn4+ is much easier in barium ortho-stannates.

The broadening of PL emission lines (Fig. 5a and b) is governedby the Stark effect caused by the crystal field interaction. The 4forbitals of the Sm3+ ion are only partially filled by five electrons(4f5) and the resulting unpaired electrons give twice Kramerdegeneration in any crystal-field lower than cubic. The maximumnumber of Stark sublevels for Sm3+ ion with 2S+1LJ multiplets is(2J + 1)/2 due to the Kramer degeneracy of its odd 4f5 electron con-figuration. The energy level splitting effect is observed clearly inthe emission spectra of Ba2SnO4:2% Sm3+ sample (Fig. 5c). As ex-pected, 3 and 4 sharp spectral lines are observed for J = 5/2 and7/2, respectively [26].

The dependence of the integral of the emission intensity as afunction of activator concentration is depicted in Fig. 6. The photo-luminescence intensity of Ca2SnO4:Sm3+ (kex = 408 nm) andSr2SnO4:Sm3+ (kex = 407 nm) phosphors increases with the Sm3+

content, reaches its maximum at a dopant concentration of about1 mol%, and then significantly decreases with a further increaseof the Sm3+ concentration due to concentration quenching. This

s a function of Sm3+ concentration.

A. Stanulis et al. / Optical Materials 36 (2014) 1146–1152 1151

phenomenon is frequently observed in rare-earth doped lumines-cent materials as the dopant level increases and is often ascribedto a non-radiative energy transfer from one activator to the adja-cent ones. As the probability of this process is distant dependentit increases with an increase of the activator concentration [27].In contrast to that, concentration quenching is not observed forBa2SnO4:Sm3+ up to a samarium concentration of 4 mol%. More-over, the emission integral intensity values for 2 and 4 mol%Sm-doped Ba2SnO4 (kex = 283 nm) phosphors are very similar.However, powder XRD analysis revealed that the Ba2SnO4 sampledoped with 4 mol% Sm3+ is not of single phase (Fig. 4), thus a sam-ple doped by 2 mol% Sm3+ was selected for the further studies.

Fig. 7a represents normalized integrated values of Ca2SnO4:1%Sm3+ emission spectra, recorded in 100–500 K temperature range.For the calculations of TQ1/2 values (the temperature, at which theluminescent sample loses half of its light output) the Boltzmannsigmoidal fit of the temperature dependent emission integralswas employed. The obtained results revealed that Ca2SnO4:1%Sm3+ phosphors loses half of its efficiency at around 622 K (349 �C).

All temperature dependent decay curves were fitted by a doubleexponential decay function (Marquardt–Levenberg algorithm)[27]:

IðtÞ þ I0 þ A1eðt=s1Þ þ A2eðt=s2Þ ð3Þ

where I and I0 are the luminescence intensity at time t and 0,respectively; t is time; A1 and A2 are constants; s1 and s2 are thedecay times. The calculated decay constants s revealed that the de-cay curve consists of a fast decay process (s1) and a subsequent slowdecay process with a long tail (s2). The estimated TQ1/2value forCa2SnO4:1% Sm3+ is 622 K (�349 �C), however, this result was de-rived from a measurement between 100 and 500 K (Fig. 7a), whichmeans that the uncertainty is rather large. Temperature dependentdecay values for Ca2SnO4:1% Sm3+ are plotted in Fig. 7b. It is evidentthat both decay constants decrease gradually and correlate wellwith the temperature dependent emission integrals of Ca2SnO4:1%Sm3+ specimen. The first exponential term (s1) is not from the bulkbecause it is too fast (0.03–0.07 ms). It is most likely due to Sm3+

located close to the surface. According to vast literature data it isknown that some deviation from single exponential decay couldbe due to surface nonradiative processes [28–30]. The second (dom-inating) exponential term (s2) values fall into 1.0–1.1 ms range.

The photoluminescence thermal quenching behavior ofSr2SnO4:1% Sm3+ phosphor is depicted in Fig. 8a. The emission inte-grals only slightly decrease even at temperatures as high as 500 K.Such thermal stability is a desirable property for application in var-ious fields, for instance, in solid state lighting [31]. Unfortunately,the estimation of TQ1/2 from temperature dependent emissionintegrals for Sr2SnO4:1% Sm3+ phosphor cannot be done because

Fig. 11. CIE1931 color points of: (a) Ca2SnO4:Sm3+, (b) Sr2SnO4:Sm3+, and (c) Ba2SnO4:Smconcentration are given in the inset tables. The inset pictures show digital images of thcolour in this figure legend, the reader is referred to the web version of this article.)

Boltzmann sigmoidal fit curve does not overpass the turning point(half of the value in this case). The temperature dependent lifetimevalues for Sm3+ emission in Sr2SnO4 host are plotted in Fig. 8b. It isinteresting to note, that the decay time values (s2 = 1.1-1.17 ms)become longer at higher temperatures, indicating an increase ofthe internal quantum efficiency of the activator.

Fig. 9a clearly shows that the integrals of the emission intensityintegral increases from 100 to 250 K for Ba2SnO4:2% Sm3+ sample.The further increase of the sample temperature leads to a decreasein emission intensity and finally nearly all emission vanishes iftemperature reaches 500 K. The employed Boltzmann sigmoidalfit revealed that Ba2SnO4:2% Sm3+ phosphor losses half of its effi-ciency at around 376 K (�103 �C) if measured from 100 to 500 K.The temperature dependent photoluminescence decay time valuesof Ba2SnO4:2% Sm3+ specimen are given in Fig. 9b. The observedtrend is very similar to the emission intensity integrals and decaytimes increase with increasing temperature up to 250 K. We as-sume that the peak at 250 K is a sort of glow peak, i.e. the extraor-dinarily long decay time is caused by the thermal activation oftraps. If we excite Ba2SnO4:Sm3+ at 283 nm and assume that thisexcitation band is a O2�–Sn4+ CT, forming a trapped electron ontothe Sn4+ site (‘‘Sn3+’’), the reactivation of such electron will be tem-perature dependent and optimal at a temperature, where the ther-mal energy is as large as the depth of the defect. This theory is alsosupported by the observation of the long afterglow in the decaycurves of Ba2SnO4:Sm3+, which is completely lacking for the otherstannates.

Several concentration dependent decay curves of Sm3+ dopedalkaline earth stannates phosphors are shown in Fig. 10. It is obvi-ous that the derived decay times for Ca2SnO4:Sm3+ (Fig. 10a) andSr2SnO4:Sm3+ (Fig. 10b) become shorter with higher activator con-centration. This can be related with an increase of energy transferbetween Sm3+ ions at the cost of radiative relaxation. However, thedecay curves of all Ba2SnO4:Sm3+ samples are nearly identical (seeFig. 10c), suggesting a similar internal quantum efficiency of Sm3+

ions at all concentrations.Fig. 11a, b and c represent the fragments of CIE 1931 chromatic-

ity diagram with color points of Ca2SnO4:Sm3+, Sr2SnO4:Sm3+, andBa2SnO4:Sm3+, respectively. The color points are shifted towardslower x values (from red to orange region) according to the se-quence Ca ? Sr ? Ba. This goes hand in hand with calculated LEvalues which increase following the same trend. Since the humaneye is more sensitive to orange light this leads to higher LE valuesif the emission shifts to the orange spectral range. Moreover, it isalso obvious that color points are located very close to the edgeof the CIE 1931 diagram thus indicating the high color saturation.

The morphology of synthesized phosphor samples wasexamined by taking SEM pictures, which are shown in Fig. 12. It

3+for different Sm3+concentrations. Exact color points and LE values for each Sm3+

e respective compounds excited at 254 nm. (For interpretation of the references to

Fig. 12. SEM images of: (a) Ca2SnO4:1% Sm3+, (b) Sr2SnO4:1% Sm3+, and (c)Ba2SnO4:2% Sm3+ powders sintered at 1400 �C under magnification of 10 K.

1152 A. Stanulis et al. / Optical Materials 36 (2014) 1146–1152

is obvious that Ca2SnO4:1% Sm3+ sample is composed of slightlyagglomerated irregularly shaped particles with a particle sizedistribution ranging from 2 to 5 lm. Similar morphologicalfeatures were observed for the Sr2SnO4:1% Sm3+ sample. The maindifference is that smaller particles (0.5–2 lm) have formed in thiscase. However, these smaller particles tend to form prolongatedagglomerates. The size of these ‘‘necked to each other’’ Sr2SnO4:1%Sm3+ crystalline grains is about of 2–5 lm. Interestingly, the

smallest particles (0.25–1 lm) among three host materials wereobtained for Ba2SnO4:2% Sm3+ sample. Obviously, the particle sizeand shape of Sm3+-doped alkaline earth ortho-stannates aredependent on the type of alkaline earth metal.

4. Conclusions

Single phase Sm3+-doped alkaline earth ortho-stannatesM2SnO4:Sm3+ (M = Ca, Sr and Ba) were obtained at low substitu-tional level of samarium. The particle size and shape of Sm3+-dopedalkaline earth ortho-stannates were found to be slightly dependenton the nature of alkaline earth metal. All samples were character-ized by photoluminescence (PL), PL thermal quenching (TQ) andfluorescence lifetime (FL) measurements. The strongest emissionpeaks of Ca2SnO4:1% Sm3+ (kex = 408 nm) and Sr2SnO4:1% Sm3+

(kex = 407 nm) samples are located at 616 nm and 646 nm, respec-tively, and correspond to 4G5/2 ?

6H9/2 transitions. However, theenergy level splitting effect was observed clearly in the emissionspectra of Ba2SnO4:2% Sm3+ sample. The photoluminescence inten-sity of Ca2SnO4:Sm3+ and Sr2SnO4:Sm3+ phosphors increases withincreasing Sm3+ content up to 1 mol%, and then significantlydecreases due to the concentration quenching, i.e. energy migration.In contrast, concentration quenching was not observed for Ba2-

SnO4:Sm3+ with increasing concentration of samarium from 1 to2 mol%. Moreover, the integral emission intensity for 2 and 4 mol%Sm-doped Ba2SnO4 (kex = 283 nm) phosphors are very similar. Basedon the obtained results we conclude that Sm3+ ions occupy Ca and Srsites in Ca2SnO4 and Sr2SnO4 ortho-stannates, respectively. Incontrast to that, Sm3+ replaces Sn4+ ions in barium ortho-stannateBa2SnO4, which is also the cause for the observed afterglow.

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