Study of anomalous emission and irradiation effecton the thermoluminescence properties of barium aluminate

Bhushan P. Kore a, N.S. Dhoble b, S.J. Dhoble a,n

a Department of Physics, RTM Nagpur University, Nagpur 440033, Indiab Department of Chemistry, Sevadal Mahila Mahavidyalaya, Nagpur 440009, India

a r t i c l e i n f o

Article history:Received 22 October 2013Received in revised form12 January 2014Accepted 20 January 2014Available online 28 January 2014

Keywords:Anomalous emissionAluminatesCombustionTrapsTracksDeconvolution

a b s t r a c t

The photoluminescence (PL) and thermoluminescence (TL) properties of BaAl2O4:Eu2þ phosphor

synthesized by the combustion method is studied. Electron beam and gamma rays are used forirradiating the phosphor while performing TL measurements. The structural and morphological analysisof the resulting phosphor was carried out by X-ray diffraction study and scanning electron microscopy,whereas their optical properties were examined by PL spectroscopy and TL glow curve measurements.The intense bluish green broad band emission near 494 nm that originates from 5d to 4f transition inEu2þ ions was observed in the BaAl2O4:Eu

2þ phosphor. The broad band emission was further examined,showing that emission consist of two broad peaks originating due to occupation of two different latticesites by Eu in BaAl2O4 host. The occurrence of two broad bands with large stokes shift and henceanomalous emission is explained by proposing energy level model. Calculations based on Dorenbosformulations were carried out which support the presence of anomalous emission in BaAl2O4:Eu

2þ

phosphor. The TL response of BaAl2O4:Eu2þ phosphor for two different radiations was compared andstudied in detail. Detailed process and possible mechanisms for PL and TL are studied and discussed withthe help of energy level models.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Rare earth activated aluminate based phosphors have beenextensively investigated because of their high chemical stability,long life and bright emission characteristics in the visible region ofelectromagnetic spectrum [1–6]. Several aluminate based compo-sitions are investigated and used in PL, cathodoluminescence andplasma display panel applications due to their high quantumefficiency in the visible region [7,8]. The synthesis and spectro-scopic study of BaAl2O4:Eu2þ phosphor has attracted a tremen-dous amount of attention because of their potential use as longpersistent phosphor [9–12]. BaAl2O4 which belongs to the familyof stuffed tridymites is a high melting point material with gooddielectric, pyroelectric, and mechanical properties. The structureof BaAl2O4 is derived from the structure of SiO2 β-tridymite [13].In this Al3þ replaces Si4þ in the tetrahedra of SiO2 tridymite, Ba willoccupy sites in channels parallel to the c-axis. It is observed fromliterature that at room temperature, BaAl2O4 is hexagonal and has asuperstructure with unit cell parameters 2A, C; where, A and C arelattice parameters of hexagonal tridymite [14].

There is growing interest in developing oxide nanophosphorsdoped with transition metal or rare earth ions due to their ease ofsynthesis methods and chemical stability [5]. Lou et al. preparedthin films of barium aluminate (BaAl2O4) deposited by a spraypyrolysis method [15]. They investigated the effects of preparationconditions on the structural and luminescence properties of thefilms. Divalent europium ions (Eu2þ) have widely been used toobtain blue color emitting phosphors [16]. In rare earth activatedphosphors Eu2þ doped phosphors usually show strong broadbandPL emission in blue region and sharp peaks in red region. Theemission of Eu2þ is strongly affected by host lattice and can occurfrom the blue to the red region of the visible region [17]. This isdue to change in electric dipole of the 5d24f transition and thevariation of 5d excited state by crystal field effects. Recently,Rezende et al. studied X-ray excited optical luminescence ofCe-doped BaAl2O4 phosphor and found that decrease in theluminescence when excited at Ce LIII-edge. This is due to theenergy transfer via X-ray fluorescence from Ce to Ba in the X-rayphoton energy range of the Ce LIII-edge absorption [18]. Katsumaet al. studied the long afterglow phosphorescent characteristic ofBaAl2O4:Eu,Dy films prepared by laser ablation. They correlatedthe poor afterglow with TL measurements [12].

Preparation of nano-sized phosphor is very promising in thefield of photonics, luminescent displays, fluorescent lamps, TLdosimetry etc. Different methods such as spray pyrolysis, co-

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

Journal of Luminescence

0022-2313/$ - see front matter & 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jlumin.2014.01.057

n Corresponding author.E-mail address: sjdhoble@rediffmail.com (S.J. Dhoble).

Journal of Luminescence 150 (2014) 59–67

precipitation, force hydrolysis [19] and sol–gel, have been used tofabricate nanophosphors. Amongst these methods co-precipi-tation and sol–gel are complicated and also consumes lot of time.The advantage of choosing the combustion method is the facilepreparation of materials within short time. Furthermore nosintering is needed to change the ionizing state of Eu3þ to Eu2þ

since the reducing atmosphere produced during synthesis issufficient to accomplish this change [12].

BaAl2O4:Eu2þ is good long persistent phosphor, to understandthe mechanism of long persistence the thermoluminescence studyof this phosphor has been done by many researchers [1,3,7,9].Recently Bos et al. presented a new versatile facility to studyphotoionization processes in impurity doped compounds. In thisfacility they coupled additional monochromatic light with a TLreader, enabling a fully automated recording of glow curves as afunction of photon excitation wavelength which provides detailedinformation on the mechanism of trap filling preceding persistentluminescence [20]. Therefore, we also took a small step to under-stand the variation in TL behavior and trapping parameters for twodifferent types of radiations. In the present work, PL and TL studyof this phosphor is carried out. The main interest of this study is toexamine anomalous emission in BaAl2O4:Eu2þ and variation of TLglow curves and trapping parameters for two different radiations.

2. Experimental section (chemical synthesis)

Recent investigations have focused extensively on variousaluminates as host for luminescent materials doped with transi-tion metal or rare earth ions [14,21]. It was noted that thesealuminates have been prepared traditionally by solid state reac-tions, which demand generally long times and high annealingtemperatures, which results in wastage of power [1,4,22]. Whereassome other preparation methods require, repeated heating, specialequipments and expensive raw materials [9,23,24]. Indeed thereis a growing demand for economically viable synthesis methodsfor manufacturing the aluminates. Combustion synthesis is veryuseful from this point of view [25]. In this work, we have success-fully employed relatively a low temperature, fast and simplemethod for the preparation of BaAl2O4 material. BaAl2O4 phosphorwas prepared by combustion synthesis route. The combustionsynthesis is carried out at 550 1C, the starting materials used wereBa(NO3)2 �4H2O, Al(NO3)3 �9H2O, Eu2O3 and urea (NH2CONH2) ofanalytical grade. Eu2O3 was converted into the nitrate form bymixing it into appropriate amount of nitric acid. All the startingmaterials were mixed and milled using a mortar and pestle, and athick white paste was formed fromwater of crystallization presentin the metal nitrates. The resulting paste was then transferred tosilica crucible and kept inside a vertical muffle furnace, which ismaintained at a constant temperature 550 1C. The paste melted,underwent dehydration and finally decomposed with the evolu-tion of gases (oxides of nitrogen and ammonia). The mixtureswelled, forming the foam that ruptured with a flame and glowedto incandescence. The combustion process was completed within4 min (approximately). The ash formed after combustion results inBaAl2O4:Eu material which was milled to powder, using a mortarand pestle. Note that the synthesized samples were not sinteredbecause the reducing atmosphere created during the reaction isenough to reduce Eu3þ to Eu2þ , which is confirmed from photo-luminescence studies and detailed mechanism for reductionprocess is discussed in Section 3.3.5. To the best of our knowledge,the thermoluminescence properties of Eu2þ doped BaAl2O4 host,for different irradiation have not been studied yet. Therefore, inthis study we focused on thermoluminescence properties of thisphosphor in addition to photoluminescence properties.

SEM micrographs were obtained using a HITACHI S-4800scanning electron microscope. The SEM micrographs were takenat 5000 V accelerating voltage, 8300 μm working distance,7800 nA emission current, at high lens mode with fast scan speedand gray scale color mode. The prepared host lattice was char-acterized for their phase purity and crystallinity by X-ray powderdiffraction (XRD) using a X'pert-PRO PANalytical diffractometer(Cu-Kα radiation) at a scanning step of 0.001, in the 2θ rangefrom 10 to 801.The photoluminescence (PL) emission spectra ofthe samples were recorded using a Fluorescence spectrometer(Shimadzu, RF 5301 PC). Excitation and emission spectra wererecorded using a spectral slit width of 1.5 nm. For TL studies,samples were exposed to gamma rays from a 60Co source at roomtemperature at the rate of 0.58 kGy/h. After the desired exposure,TL glow curves were recorded with the help of Nucleonix 1009I TLreader, at a heating rate of 5 1Cs�1. All the measurements werecarried out in an open atmosphere.

3. Results and discussion

3.1. XRD and crystal structure

Fig. 1 shows the XRD pattern for BaAl2O4. The XRD patternmatched perfectly with standard data available in ICDD file no.82-2001. No separate phases of constituents and other probablephases were observed which suggest formation of homogeneousBaAl2O4 phase. Although the Eu ions occupied the Ba ions, thesmall amount of the Eu dopant had no significant effect on thehost structure of the BaAl2O4, as observed from XRD pattern andcalculated lattice parameters. Aluminates are known for their stiffmechanical properties and BaAl2O4 has a very high melting point(1815 1C) and holds a stuffed tridymite (hexagonal) structure. TheBaAl2O4 belongs to C6

6 � P63 space group. In BaAl2O4 host there aretwo different sites of Ba2þ , one site (site-1) is twice as abundant asthe other (site-2) [26]. Ba2þ has fairly large ionic radius r¼1.47 Åwhich is much bigger than most of the divalent/trivalent rare earthions. Due to the bigger size of Ba the substitution of rare earth ions(Eu2þ¼1.3 Å) is easy at Ba2þ sites.

The crystal structure of BaAl2O4 phosphor projecting along[0 1 0] directions is shown in Fig. 2. In AlO4 tetrahedra each oxygenatom is shared with two aluminum ions so that each tetrahedronhas one net negative charge. Charge balance is accomplished bythe large divalent cations (Ba2þ) which occupy interstitial sites

Fig. 1. XRD pattern of the as-prepared BaAl2O4:Eu(0.5 mol%) phosphor.

B.P. Kore et al. / Journal of Luminescence 150 (2014) 59–6760

within the tetrahedral framework as shown in Fig. 2. The crystalstructure of BaAl2O4 consists of AlO4 tetrahedra alternately con-nected to the 3 downward AlO4 tetrahedra by corner sharing thusforming a ring around Ba which consists of six tetrahedra, asshown in Fig. 2(b). Six AlO4 tetrahedra in a ring attached to eachother by a common oxygen of upward or downward tetrahedrawhich form channels parallel to c-axis. These channels of AlO4

further connected to form a 3-D structure. In BaAl2O4 structure Bahas two different sites Ba(I) and Ba(II), Ba(I) site occurs 3 timesmore than Ba(II). According to the crystal structure shown in Fig. 2,the Ba(I) site has multiplicity of two and site symmetry of c3 whilethe site Ba(II) has multiplicity of six and site symmetry c1. Thelattice parameters 2A and C were calculated to be 10.45 and8.791 Å respectively. The calculated parameters are consistentwith ICDD file no. 82-2001 and lattice parameters reported inRefs. [1,17].

3.2. Scanning electron microscopy

A SEM study was carried out to investigate the surfacemorphology and crystallite sizes of the synthesized phosphor,which is in powder form. Fig. 3 reveals the grains having rod likemorphology with some arbitrary shaped grains. The higher mag-nification image shows that the particles were interconnected,which reflects the inherent nature of the combustion process. It isevidently observed that the rods have width below 100 nm. Theshape of the phosphor particles is influenced by the synthesismethod and temperature. The surfaces of the phosphor showedmany pores formed by the escaped gases during the combustionreaction. Moreover, the phosphor surface is formed from highlyagglomerated crystallites.

3.3.1. Photoluminescence study of BaAl2O4:Eu2þ phosphor

It is well recognized that europium ions have divalent andtrivalent states. In trivalent state Eu3þ ion emits a characteristicred light with a number of narrow lines due to forbidden 4f-4f(5D0 -7FJ) transitions, while in divalent state emission from Eu2þ

is broad and structure less in the blue region is due to electricdipole allowed 4f65d1-4f7 transitions. The luminescence of Eu2þ

depends upon the type of ligands surrounding Eu2þ ion [15].In case of Eu3þ luminescence, emission is slightly influenced bysurrounding ligands because electronic transitions of Eu3þ involveonly a redistribution of well shielded electrons within the inner 4fsubshell.

The excitation spectra of phosphor consist of broad bandpeaking at 353 and 328 nm for emission of Eu2þ at 494 and430 nm respectively, as shown in Fig. 4(A). The emission spectra ofBaAl2O4:Eu phosphor with varying Eu concentration from 0.05, 0.1,0.2, 0.5 to 1 mol% is shown in Fig. 4(B). The PL emission wasobserved at 494 nm in the bluish green region of the spectrum at353 nm excitation. No characteristic emission peaks arising fromEu3þ are observed in the PL spectra, indicating that Eu3þ havebeen completely reduced to Eu2þ , a detailed mechanism for Eu3þ

reduction is discussed in Section 3.3.5. Eu2þ emission spectra ishighly sensitive to the surrounding ligands because the nearestexcited state of Eu2þ lies in the outer d-shell and emission getsaffected due to the complex interactions amongst host structures,activators and defects. The charge imbalance and lattice distortioncould induce defects in the host lattice, which would enhance thenon-radiative transition rate which results in reduction of lumi-nescence intensity of phosphor. In BaAl2O4:Eu2þ phosphors, theEu2þ ions entered in the host crystal lattice and preferentiallyoccupy Ba2þ sites, which gives rise to the charge balance and thusreduces the lattice distortion and enhances the luminescence

Fig. 2. Crystal structure of BaAl2O4, (a) 3-D crystal structure along (0 1 0) plane, (b) AlO4 ring connected by six AlO4 tetrahedra, (c) crystal structure of BaAl2O4 viewed alonga axis, and (d) crystal structure of BaAl2O4 viewed along b axis.

B.P. Kore et al. / Journal of Luminescence 150 (2014) 59–67 61

intensity. The maximum PL intensity is observed at 494 nm for1 mol% of Eu.

The emission spectra consist of a broad band ranging from400 nm to 600 nm peaking at 494 nm (20242.91 cm�1) and ahump at 430 nm (23255.81 cm�1), as shown in Fig. 5. Theasymmetric broad band emission was fitted to 2 Gaussian peaks(Fig. 5), which is due to occurrence of Eu2þ ions in two differentsites, a detail of this discussed later. Gaussian fitted curve resolvesthis broad band which consists of two overlapping peaks of which

the maximum intensity was observed at 494 nm. Observing theGaussian fitting result, we deduce that Eu(I) should emit 430 nmlight and Eu(II) should emit 494 nm light, as shown in Fig. 5. Thereare two different Ba2þ sites, one site is twice as abundant as theother and Ba2þ has an ionic radius r ¼1.47 Å which is much biggerthan most of the divalent/trivalent rare earth ions so that the rareearth ions can easily substitute for the Ba2þ sites, in present caseEu2þ ions. The two peaks are due to the two types of lumines-cence centers occupying two different Ba2þ sites [7,10]. Further-more, in the case of Eu2þ emission, the emission wavelengthdepends strongly on the crystal field strength of site. Hence, theemission of Eu2þ often gives different wavelengths, if Eu2þ

occupies different crystallographic sites. Nakamura et al. has alsodrawn the same conclusion of having two sites of Eu, theysuggested that one quarter of europium (II) ions with a |D|valueof 0.0345 cm�1 are located in site-1 and the remaining europium(II) ions with a |D|value of 0.108 cm�1 are in site-2, where D-zerofield splitting parameter [26]. The intensity of 430 nm peak is veryless as compared to emission at 494 nm peak. This indicates thatone of the Ba2þ sites is less preferentially occupied by the Eu2þ

ions and that the second site is filled with higher concentrationof Eu2þ . Stefani et al. explained that the first Ba2þ site has themultiplicity of two while the second one (6c) has six [7]. Boththe Ba2þ sites have nine-coordination and the sites are similarin average distance between Ba and O. The lower symmetry sitehas shorter Ba–O distances compared to those of Eu2þ–O there-fore, it seems reasonable that this 6c site is filled preferentially.These results are in good agreement with the results obtained byStefani et al. [7].

Fig. 3. SEM images of the BaAl2O4:Eu phosphor prepared with the combustion method.

Fig. 4. Excitation (Fig. (A)) and emission (Fig. (B)) spectra of BaAl2O4:Eu phosphor.

Fig. 5. Gaussian fitted emission spectra of BaAl2O4:Eu phosphor. (- - - - fitted, ���experimental).

B.P. Kore et al. / Journal of Luminescence 150 (2014) 59–6762

3.3.2. TheoryThe energy difference between the lowest 4fn and the first

4fn�15d1 level in divalent (Q¼2þ) and trivalent (Q¼3þ) lanthanideions always shows the same characteristic variation with the numberof electrons n in the 4f shell, as formulated by Dorenbos [27]. Whenapplied to the case of Eu2þ , one can write for the energy of fdabsorption in a host crystal X as,

EabsðEu2þ ;XÞ ¼ EX f reeðEu2þ Þ�Dð2þXÞ ð1Þwhere EX free (Eu2þ) is a constant for Eu2þ ion with a value equalor close to the fd-transition energies for the free/gaseous ions,D (2þ , X) redshift, The value EX free Eu2þ is the fd transition energyfor the free/gaseous ion and is rounded to 34,000 cm�1 [28]. Theredshift D (2þ , X) can be determined from any divalent lanthanide inthe compound X. Shifting Eabs (Eu2þ , X) down with the Stokes shiftΔS (2þ , X) leads to the df emission energy

EemðEu2þ ;XÞ ¼ EX f reeðEu2þ Þ�Dð2þ ;XÞ�ΔSð2þ ;XÞ ð2Þwhen the first 5d level energy is known for a specific lanthanide ionin compound X; one can determine the redshift and from that 5dlevel energies for all other lanthanides in the same compound can bepredicted on the basis of above two equations. In compounds whereEem (Eu2þ , X) is lower than the 4f7(6P7/2)–4f7(8S7/2) transition energy,the df broadband emission is observed instead of line emission [28],this includes most of the compounds except some fluorides andoxides Table 1.

Following three different types of Eu related emissions can bedistinguished:

(1) Normal broad band df-emission.(2) ff Narrow line emission.(3) Anomalous Eu2þ emission.

In each case the emission can be excited via the lowest fd levelof Eu2þ; and in each case the 4f7[8S7/2] (ground state of Eu2þ) isthe final state of the radiative transition. The normal emission isthe dipole and spin allowed df-emission starting from the relaxed4f6 [7F0] 5d1 level. For compounds having normal emission ofEu2þ , the most common Stokes shift is 1350 cm�1 and theemission bandwidth is about 1600 cm�1 [27,28]. In our case theStokes shift and emission bandwidths are found to be 7232 cm�1

and 1465 cm�1 for (EuI) and 8086 cm�1 and 3168 cm�1 for (EuII)respectively, see Table 2. The two criteria for the anomalousemission are observed in present sample. These are evidencesfor the presence of anomalous emission of Eu2þ in the presenthost. To avoid errors, the approximation proposed by Dorenbos forcalculation of λabs was used. The emission parameters such asredshift, stoke shift, λabs and λemi are calculated from this approx-imations and comparison between experimentally observed andcalculated parameters is given in Table 2.

As reported by Dorenbos [27], on the basis of configurationcoordinate diagrams, the anomalous emission is categorized bythe following observations such as,

(1) A much larger Stokes shift ΔS(2þ , X) 4000 cm�1

(2) A much larger bandwidth 3000 cm�1

(3) A deviant temperature quenching and(4) Luminescence decay profile

In anomalous emission, the auto-ionization of the Eu2þ 5delectron to the conduction band takes place, which can befollowed by trapping of the electron at impurities including theionized Eu ion. The anomalous emission occurs when electronreturns to the ground state of Eu2þ through the radiative transi-tion. Out of the above discussed criteria for anomalous emissionthe first and second was observed in present sample indicatingpresence of anomalous emission.

3.3.3. Energy level diagramEu2þ ion has been recognized as an excellent activator for

long-persistent phosphors because its 5d electron state is usuallyclose to the CB of the host, which makes trapping of electronspossible [29]. Fig. 6 illustrates the mechanism responsible for twodifferent colored emissions. Under 353 nm UV-light excitation,ground state electrons of Eu2þ ions are promoted to the 5d level[excited level 4f65d band] (step 1). The electrons from the excitedlevel make transitions to exciton level (step 2) and then to the

Table 1Crystallographic data and structural parameters of BaAl2O4 host.

Formula BaAl2O4 (this work) BaAl2O4 (ICDD-82-2001)

Crystal structure Hexagonal HexagonalSpace group P63 P63Cell parameters a¼b¼10.45 Å, c¼8.791 Å α¼β¼90.00, γ¼120.00 a¼b¼10.44 Å, c¼8.793 Å α¼β¼90.00, γ¼120.00Volume 831.57 Å3 831.41 Å3

Table 2Comparison between observed and calculated luminescence parameters, thecalculated parameters are based on Dorenbos formulations [27].

Parameters Observed Calculated (Dorenbos formulation)

λabs 328, 353 nm 383, 434 nmλemi (EuI) 430 nm 530 nmλemi (EuII) 494 nm 666 nmStoke shift (EuI) 7232 cm�1 7232 cm�1

Stoke shift (EuII) 8086 cm�1 8086 cm�1

Redshift (EuI) 7894 cm�1 12276 cm�1

Redshift (EuII) 10907 cm�1 16143 cm�1

Bandwidth (EuI) 1465 cm�1 –

Bandwidth (EuII) 3168 cm�1 –

Note: The Stoke shift was calculated with respect to the peak wavelength.

Fig. 6. Energy level scheme for anomalous emission in BaAl2O4:Eu2þ with theground and excited state for Eu2þ , the conduction band (CB) and valence band(VB), the exciton level.

B.P. Kore et al. / Journal of Luminescence 150 (2014) 59–67 63

ground state 8S7/2 of Eu2þ ions (step 4). This results in emissionat 494 nm, as shown in Fig. 6. The similar type of emission at430 nm could be observed when the phosphor is excited with328 nm wavelength. In present case of BaAl2O4:Eu2þ phosphorthe difference between two centroid bands of 4f65d of Eu2þ

was found to be 2159 cm�1, which results in two emissionwavelengths. The electrons making transition from Eu(II) site areabundant in comparison with that of present in Eu(I) site, due towhich we get different emission intensities for emission from twodifferent sites under the same excitation wavelength. Furthermore,the higher emission intensity of Eu(II) than Eu(I) illustrates thatEu2þ ions prefer to occupy Ba(II) sites.

3.3.4. Anomalous photoluminescence in BaAl2O4:Eu2þ

As per discussion in previous Section 3.3.3, the PL in BaAl2O4:Eu2þ powders can be called an anomalous emission as out of fourrequirements for anomalous emission proposed by Dorenbos, twocriteria come across in BaAl2O4:Eu2þ phosphor. BaAl2O4:Eu2þ

phosphor shows a very large Stokes shift [7232 cm�1 (EuI) and8086 cm�1 (EuII)] and a large FWHM of the emission band(1465.1 cm�1 and 3168.09 cm�1), which are basis for anomalousemission to occur. This anomalous emission can be explained withthe help of energy level diagram, shown in Fig. 6. The anomalousemission in BaAl2O4:Eu2þ phosphor is due to the presence of 5dexcitation band near the host material's conduction band, TL studyalso shows presence of traps near conduction band (shallow traps)for γ-irradiated BaAl2O4:Eu2þ phosphor. Whereas for normalemission Eu2þ excited states lies well below the conduction bandand there is no low-energy crossing possible between the Eu2þ

excited state and the conduction band or exciton levels. In case ofanomalous emission, during excitation of a Eu2þ ion [transition(1)], it becomes auto-ionized and an impurity trapped exciton iscreated [28,30,31] shown by transition (2) and (3). From this webelieve that the anomalous emission band is the emission fromdefect centered exciton, as shown in Fig. 6, which is the lowestexcited state of the host-activator system. When any of thelocalized levels of the impurity ion excited, the excitation leadseither to photoionization or to nonradiative decay into lowerenergy levels i.e., ground state (here 8S7/2). Normally the lowestexcited localized level would give emission when electron fromthis state makes transition to ground state. On the other hand,if this level lies above the exciton energy then it may first decayupto exciton level and then delocalized exciton may emit light,the possible mechanism is shown in Fig. 6 by transition (2).A subsequent return transition to the Eu2þ ground state [8S7/2]results in large redshift in the emission, shown by transition (4).Hence, we can conclude that the bottom of the 5d band lies wellabove the exciton level, and possibly situated in the conductionband or situated between the exciton level and the bottom of theconduction band. Exact position of the 5d levels cannot bedetermined from this study. There are several trends for anom-alous emission as discussed by different researchers [27–29,32,33].In Eu2þ doped compounds, anomalous emission occurs frequentlywhen the Eu2þ ion is substituted for a larger cation site [28,30] inpresent case Ba2þ . Hence, anomalous emission is more common inbarium Ba2þ (r¼1.47 Å which is much larger than radii of rareearth elements) compounds than in other divalent compounds.Smet et al. found the anomalous emission in BaS:Eu2þ , whereasnormal emission in case of CaS:Eu2þ and SaS:Eu2þ [28] which isin accordance with the result presented here.

3.3.5. Reduction of Eu3þ to Eu2þ

The BaAl2O4:Eu2þ materials have a special property thateuropium can be reduced without the use of a reducing atmo-sphere [8]. Following four conditions are necessary to realize the

reduction of Eu3þ to Eu2þ , in solid-state compounds whenprepared in open atmosphere at high temperature.

(1) Host compounds should not contain oxidizing ions/agents.(2) Trivalent dopant ion (Eu3þ) substitutes for the divalent cation

in the hosts.(3) Substituted cation has similar radius to that of the divalent

Eu2þ ion.(4) Host compound has an appropriate structure, which is com-

posed of the tetrahedral MO4 anion groups (M¼ B, S, P, Sior Al).

In BaAl2O4, Ba2þ and Al3þ are not oxidizing ions, which meetthe first condition. An acceptable percentage difference in ion radiibetween doped and substituted ions must not exceed 30% [34] inpresent case it is 17%. The crystal structure of BaAl2O4 composed ofAlO4 tetrahedra, which is relatively enclosed and stiff 3-D networkstructure of BaAl2O4 matches condition (4). Therefore, all the fourconditions for the reduction of Eu3þ to Eu2þ in BaAl2O4 preparedin air/open atmosphere are satisfied. When trivalent Eu3þ ions aredoped into BaAl2O4 host, they incompatibly replace the Ba2þ ions,as there is a charge imbalance. In order to keep the charge balance,two Eu3þ ions are needed to substitute for three Ba2þ ions. Whilesubstituting this one vacancy defect of V″(Ba) with two negativecharges, and two positive defects of [Eu3þ]�(Ba) will be created byeach substitution of every two Eu3þ ions in the host lattice. Thevacancy of V0 0(Ba) then acts as a donor of electrons, while the two[Eu3þ]�(Ba) defects will become acceptors of the electrons.In combustion reaction very high temperature is generated dueto exothermic reaction, at such higher temperature the electronsin the vacancy defects of V0 0(Ba) will be released in the host. At thesame time, migration of [Eu3þ]�(Ba) defects in host will becomestronger. This will increase the meeting probability between therelatively free electrons and [Eu3þ]�(Ba) defects. As one electronmeets one [Eu3þ]�(Ba) defect, it will be captured by [Eu3þ]�(Ba),and then filled into the 4f orbit of europium ion. As a result, part ofEu3þ ions is reduced to Eu2þ . The AlO4 tetrahedral group probablyplayed an electron transferring role in the reduction process.The whole process might be expressed as shown by Eqs. (3)–(5).In following reactions, the Kröger–Vink notations are used for easyunderstanding of the reduction process.

3Ba2þ þ2Eu3þ-V″ðBaÞþ2½Eu3þ ��ðBaÞ ð3Þ

V″ðBaÞ-V�ðBaÞþ2e ð4Þ

2½Eu3þ ��ðBaÞþ2e-2½Eu2þ ��ðBaÞ ð5Þhere, all the symbols have their usual meanings except for therelative charges marked as follows: [�] positive, ['] negative and[� ] zero charge.

3.4.1. Thermoluminescence (TL) studyThe TL glow curves of the BaAl2O4:Eu2þ phosphors were

recorded with Nucleonix TL Reader 1009I with a linear heatingrate of 5 1C s�1 in the temperature range between room tempera-ture to 400 1C. Fig. 7 shows TL glow curves of BaAl2O4:Eu crystal-line phosphor, irradiated with electron beam for the dose of 5 Gy.The samples from the same batch were also irradiated withgamma rays from a 60Co source for comparative studies of twodifferent ionizing radiations, as shown in Fig. 8. As it can be seenfrom the Figs. 7 and 8, the glow curves of BaAl2O4:Eu exposed toelectron beams are entirely different to that of the γ irradiatedsample. The TL glow curve structures of the samples irradiatedwith gamma rays consists of a single peak at around 126 1C, whilethat exposed to electron beams has prominent peak at 178 1C andan additional deep lying peak at 260 1C. He et al. also observed the

B.P. Kore et al. / Journal of Luminescence 150 (2014) 59–6764

similar results for MgAl2O4 ceramics, in which two absorptionbands are produced for electron irradiated sample and singleabsorption band is produced in γ-irradiated ceramic [35]. Thereis a significant variation in the glow curve structure and thenumber and position of the peaks for the two irradiations. It iswell known that the interactions of two different types of radia-tions with matter are based on different principles and this canlead to the alteration in the luminescence and trapping centers.The appearance of low temperature peak in case of gamma rayexcited BaAl2O4:Eu phosphor and high temperature peaks forelectron irradiated BaAl2O4:Eu phosphor is due to the differentenergies of two radiations. The response of BaAl2O4:Eu phosphor

towards electrons and photons is different this can be attributedto different linear energy transfer (LET) or disorganization ofthe initial trap centers (TC)/luminescent centers (LC) [36]. Com-parison between the TL glow curve of γ-rays irradiation and thatinduced by electron beam illustrates the change in the peakpositions in conjunction with the glow curve structure whichcan also be observed by the modifications in the trapping para-meters (Table 3).

3.4.2. Models for thermoluminescenceThe presence of defects in material can upset the periodicity of

the lattice and disturb the normal band structure, results in theformation of localized energy levels within the forbidden gap.Moving electrons (holes) through the conduction (valence) bandcan become trapped at these defect sites. While irradiating thematerial with ionizing radiations, some electrons gain sufficientenergy to be raised from the valence band to the conduction bandfrom which they may subsequently be trapped at available defectcenters. When the irradiated material is heated at sufficientlyhigher temperatures, these trapped electrons can gain enoughthermal energy to escape from the traps back to the conductionband. From here they may make direct transitions back to thevalence band or alternatively, they may become re-trapped or maycombine with trapped holes. If the electron trap energy levels areclose to the conduction band, then this thermal de-trapping mayoccur at lower temperatures and recombination with holes leadsto emission of light. Deeper laying traps will require heating atelevated temperature in order to release electrons from traps. Inthis study we tried some models to explain the TL response ofBaAl2O4:Eu2þ phosphor for two different radiations.

The occurrence of two peaks in case of electron irradiatedBaAl2O4:Eu2þ and single peak in case of γ-irradiated BaAl2O4:Eu2þ

phosphor is explained here using the model shown schematicallyin Figs. 9 and 10 respectively. According to the given models,irradiation of sample by electron beam creates two types of traps(one type are of shallower traps and other of deeper traps) in theforbidden band gap of material, as shown in Fig. 9. In electronirradiated samples shallow as well as deep taps are produced dueto irradiation as evident from the deconvolution analysis (dis-cussed in Section 3.4.4) whereas for gamma ray irradiated sam-ples, only shallow traps are produced after irradiation, as shown inFig. 10. The higher intensity of low temperature peak in electronirradiated BaAl2O4:Eu2þ phosphor is due to the contribution fromshallow traps, which are getting emptied earlier and the deeptraps (acting as a reservoir) may replace them successively (step 2)or also they can go directly to the conduction band (step1) andrecombine with the trapped holes/luminescence centers duringtheir back journey giving rise to a high temperature peak (secondpeak) [37], as shown in Fig. 9. For low doses of radiation, either theshallow traps get emptied at relatively low temperatures during TLreadout and the deep traps may replace some of them or the deeptraps go directly (step1) to the conduction band and recombinesubsequently giving rise to the high temperature peak.

Irradiating the materials with radiations create free electronhole pairs which moves freely through the lattice and becometrapped at defect center. The lifetime of these traps vary from few

Fig. 7. TL glow curves of BaAl2O4:Eu phosphor exposed to 7 MeV electron beam.

Fig. 8. TL glow curves of BaAl2O4:Eu samples exposed to gamma rays.

Table 3Kinetic parameters using the GCD function.

Sample Peak no Tm (1C) Geometrical form factor (μg) Activation Energy (eV) Frequency factor s (s�1)

BaAl2O4:Eu2þ (gamma rays) 1 126 0.5070.164 0.6770.162 5.94�107

BaAl2O4:Eu2þ (electron beam) 1 178 0.5070.083 0.8070.104 1.82�108

2 260 0.5170.073 1.1170.127 6.611�109

B.P. Kore et al. / Journal of Luminescence 150 (2014) 59–67 65

seconds to years. Usually the shallow/low temperature traps mayget emptied during storage of material at room temperature.

Upon heating the gamma ray irradiated material the chargecarriers trapped during irradiation are released from the trap andmakes transition to the conduction band (CB) through (step 1), asshown in Fig. 10. Electrons while making transition from CB mayeither get re-trapped at trap level/defect center (step 2) or go tohole trap/luminescence center (step 3) where they recombinewith hole giving rise to luminescence/glow peak. The occurrenceof two unlike forms of glow curves for two different radiations isdue to the different interacting properties of these radiations. Dueto change in type of radiation corresponding change in the trapdepth (position of trap level in the forbidden band gap) of thetrapped charge carriers inside the forbidden band gap is observed,calculated trap parameters also support this observation.

3.4.3. Dose response comparisonTL glow response curves of BaAl2O4:Eu phosphor irradiated

with different doses of both 60Co gamma rays and electrons in therange of 1 Gy–100 kGy are presented in Fig. 11. The response of theglow peak of BaAl2O4:Eu in a very high dose range of exposures ofelectron as well as gamma radiations (Fig. 11) is one of the remar-kable results obtained in this phosphor. The TL dose response ofelectron and gamma ray irradiated BaAl2O4:Eu phosphor shows asublinear response in the range of 1–100 Gy, beyond this dose the

response becomes supralinear upto 1000 Gy after this dose rangethe response gets saturate. The electron irradiated BaAl2O4:Euphosphor saturates earlier at a dose of 10 kGy whereas phosphorirradiated with gamma rays saturates at relatively higher dose(50 kGy). The sublinear response for low doses of radiations andsupralinear behavior for higher doses can be explain on the basisof track interaction model (TIM) [38,39]. According to this model,dense ionized state are created through inelastic collisions nearbyan ion track in the TL material. When samples are irradiated withlow doses of radiations the distance between neighboring tracksis large and recombination occurs entirely within the track.The low sensitivity (sublinear response) of TL material is dueto the capturing of electrons which are escape from tracks bynon radiative centers in the inert track region. Now, as the doseincreases the distance between the neighboring tracks decreasesand the electrons escaping from parent track reach the lumines-cent center in neighboring tracks, which results in supralinearityin TL. This suggests that the present material having one of theparticle dimensions in nano range will have more linearitytowards higher doses as compared to lower doses.

3.4.4. Trapping analysisKinetic parameters determination has always been an active

area of research for better understanding of the TL mechanismand long afterglow. The complex glow curves in case of electronbeam irradiated BaAl2O4:Eu2þ have been fitted by the computer-ized glow curve fitting method, as shown in Fig. 12. The trap depthand the frequency factor of isolated peaks were evaluated usingthe Chen's peak shape method [40]. The trap depth was evaluatedby following equation:

E¼ cγkT2

m

γ

!�bγð2kTmÞ ð6Þ

where, cγ, bγ and γ are the parameters which were defined asconstants [40] and Tm – the peak temperature at maximumintensity. As it was found that the shape of peak is symmetricwith geometrical form factor 0.50 for gamma irradiated BaAl2O4:Eu2þ and 0.50 (peak 1) and 0.51 (peak 2) for electron irradiatedBaAl2O4:Eu2þ . This suggests the kinetics close to second orderkinetics. The frequency factor was calculated by,

βE

kT2m

¼ s exp�EkTm

� �½1þðb�1ÞΔm� ð7Þ

Fig. 9. Schematic representation of TL glow process for electron beam irradiatedBaAl2O4:Eu phosphor.

Fig. 10. Schematic representation of TL glow process for gamma ray irradiatedBaAl2O4:Eu phosphor.

Fig. 11. TL dose response of BaAl2O4:Eu phosphor irradiated with gamma andelectron beam.

B.P. Kore et al. / Journal of Luminescence 150 (2014) 59–6766

where, Δm¼2kTm/E, β¼5 1C/s was the heating rate, b – the order ofkinetics and E – the trap depth.

Trapping parameters of all the peaks are calculated and aresummarized in Table 3. From the analysis it is clear that all thepeaks follow second order kinetics and hence probably there isretrapping takes place. The retrapping is necessary and essentialcriteria for long persistence. Hence, the results obtain fromtrapping analysis could be qualitatively associated with the longpersistence characteristics.

4. Conclusion

The crystalline BaAl2O4:Eu2þ phosphor was successfully pre-pared by the combustion method. A rod shape crystallites wereobserved by SEM analysis. The reduction of Eu3þ–Eu2þ has beeneffectively observed in this compound which was prepared in air,as the atmosphere created during combustion reaction is sufficientfor reducing Eu3þ to Eu2þ . In this work, the emission propertiesBaAl2O4:Eu2þ phosphor was explored and compared to the scarceand often contradictory literature data. At room temperature, thephotoluminescence is characterized by a broad emission bandpeaking at 430 and 494 nm and by a large Stokes shift. The largestokes shift and bandwidth confirms anomalous emission inBaAl2O4:Eu2þ phosphor. This anomalous emission is related tothe position of the 5d excited level of Eu2þ relative to the conduc-tion band levels of the host material. The models presented heresuggests this anomalous emission is related to the position of the5d excited level of Eu2þ relative to the conduction band levels ofthe host material.

There is a significant variation in the glow curve structure andposition of the peaks for the two irradiations which is attributed tothe change in the population of the LCs/TCs and different interac-tion mechanism of the radiations. The TL dose responses havebeen studied over wide range of doses and explained on the basisof the track interaction model, which successfully explains thesublinear and supralinear behavior of BaAl2O4:Eu2þ phosphor. Thecomplex glow curve in case of electron irradiated BaAl2O4:Eu2þ

phosphor is deconvoluted using glow curve deconvolution pro-gram. Trapping parameters study is carried out for two types of

radiations and is in agreement with glow curve's position and shape.This type of study will be helpful in understanding the traps genera-tion mechanism and their behavior with respect to temperature.

Acknowledgment

Authors are grateful to Board Of Research in Nuclear Sciences(BRNS), Department of atomic Energy, Government of India, forproviding financial assistance to carry out this work under researchproject (sanctioned Letter no. 2011/37P/10/BRNS/144).

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Fig. 12. Deconvoluted glow curves of BaAl2O4:Eu2þ irradiated with electron beam.

B.P. Kore et al. / Journal of Luminescence 150 (2014) 59–67 67