Journal of Luminescence 167 (2015) 45–50

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Journal of Luminescence

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

Photoluminescence properties of U in SrBPO5 host: Effect of con-centration and annealing temperature

Annapurna Rout a, M. Mohapatra b, N. Suriyamurthy a, B.S. Panigrahi a,n

a Reactor Operation and Maintenance Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Indiab Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e i n f o

Article history:Received 24 February 2015Received in revised form8 June 2015Accepted 9 June 2015Available online 18 June 2015

Keywords:SrBPO5

UraniumPhotoluminescenceColor coordinates

x.doi.org/10.1016/j.jlumin.2015.06.01513/& 2015 Published by Elsevier B.V.

esponding author.ail address: bsp@igcar.gov.in (B.S. Panigrahi).

a b s t r a c t

Strontium borophosphate (SrBPO5) with and without uranium doped samples were synthesized throughsolid state reaction route in air atmosphere at different annealing temperatures. The dopant ion con-centration was varied between 1 and 8 mol%. X-ray diffraction studies confirmed the formation of singlephase compound in all the samples without any impurity phase up to 8 mol% of the dopant ion. Theoverall morphology and particle size was confirmed by scanning electron microscopic studies whichsuggested the presence of uneven, agglomerated particles with less than 1 mm individual size. It wasobserved that upon annealing at higher temperatures, the particles get agglomerated more and morewith smooth boundaries. Photoluminescence (PL) studies confirmed stabilization of uranium as uranylion (UO2

2+) in the system. Based on the PL emission data concentration quenching was observed beyond7 mol% of the dopant ion concentration. The critical distance (Lc) was estimated to be 3 Å suggestingDexter type of energy transfer mechanism responsible for the quenching. The life time decay studiesindicated the presence of two different types of environment around Uranyl ion. On annealing at tem-peratures beyond 900 °C the PL emission and decay time reduced drastically. It was concluded that onannealing at temperatures beyond 900 °C, defect centres get agglomerated around the metal ion pro-viding non radiative pathways for the energy to get dissipated thereby reducing the PL emission anddecay time. Color coordinates were evaluated for the 7 mol% uranium doped sample annealed at 900 °C.The values suggested that the system can be used as a potential green emitting phosphor material.

& 2015 Published by Elsevier B.V.

1. Introduction

Actinide ions with their partly filled 5f orbitals exhibit interestingphoto luminescence properties which can throw light on the elec-tronic structure and nature of metal–ligand bonding for the ions [1].The later actinides with their closely spaced energy levels do not showefficient PL and decay via non-radiative mode. However, amongst thefirst half of the actinide series, U, Am and Cm are known to have goodPL yield, when excited by a suitable wavelength [2]. Unlike their 4fseries counter parts, the early actinide series members show variablevalence states which make them as an interesting class of compounds.In case of most of the actinide ions, electronic transitions within the5fn levels occur in visible and near UV region that are Laporte for-bidden thereby making them very less intense. The forbiddenness ofthe Laporte selection rule can be made less rigorous by stabilising theactinide ions in different crystalline matrices [3,4]. Many efforts havebeen made to explore suitable matrices, in order to achieve better

luminescence from actinide elements. Among the actinide elementsphoto luminescence studies of uranium activated solid matrices havegained much attention owing to its application in the nuclear indus-tries, as the requirement of inert solid matrix for incorporation ofnuclear waste is a challenging area of investigation [5]. Uraniumactivated matrices are well known and promising catalyst for thermaldegradation of pollutant molecules [6]. Moreover many uraniumdoped solid matrices are known to be efficient green light emittingphosphor [7]. Uranium being a strategic element for nuclear industry(fissile element) has got limited scope of investigation, as it is asso-ciated with safety and security concerns. At the same time the uniquephoto physical and photo electronic properties of uranium has fasci-nated and restored the research interest [8].

Plenty of literature reports are available dealing with the PLproperties of various oxidation states of U such as U(VI), U(V), U(IV) and U(III) in different matrices [9–12]. Out of these, the mostinvestigated and the most commonly encountered species is thehexavalent uranium ion in uranyl form (UO2

2+). The PL emissionspectra of these species gives unique vibrationally resolved ligandto-metal charge transfer (LMCT) emission in green region [13].

A. Rout et al. / Journal of Luminescence 167 (2015) 45–5046

Unlike lanthanides, the 5f electrons (in 5fn�1 6d configuration) ofuranium show high oscillator strength and more sensitivity towardsligand and crystal field effects. Consequently, the PL properties likeexcitation, emission and life time data are greatly influenced by thesymmetry and coupling to external vibrational modes of closely lyingligands and crystal fields that in turn makes it an useful probe forinvestigating the co-ordination environment [14]. In solids the crystalstructure of host matrix and synthesis conditions are responsiblefactors for stabilization of Uranium in different forms such as octa-hedral/tetrahedral uranium groups (UO6

6−, UO42− and U6þ) other than

the usual uranyl species [15]. Thus stabilization of uranium indirectlygives information about the lattice chemistry as well as the crystalfield experienced in that particular matrix. The UO2

2+, UO66−, and U6þ

centres are well documented for green emissionwhere as UO42− centre

is known for red emission [16]. Among the above forms of uranium,only uranyl (UO2

2+) is stable both in aqueous solution and in solid stateand also different from other species with respect to its characteristicemission consisting of equidistant vibronic progressions. The nearlinear O–U–O structural geometry with individual U–O bond length of�1.8 Å is a constraint on stabilizing the whole moiety in a crystallattice. The stabilization of uranyl varies with respect to host crystallattice. It has been reported that uranium can either form a secondphase with the host matrix to stabilize its uranyl form, or in somecases the inversion centre present in the crystal lattice appears to bethe stabilization centre [17]. Suitable lattice sites for stabilizing Uranylion is a continuing matter of investigation.

The nature of uranium PL can also be influenced by con-centration of the dopant ion and synthesis conditions. In thisregard, the nature of uranium emission spectra has been studied inmany matrices. In the series of matrices, phosphate based hostlattices are well investigated and it has been observed thatmatrices like Sr3(PO4)2, SrP2O7 offer the regular Sr2þ lattice sites tostabilize uranium as U(VI) without affecting the crystal structure[18,19]. Recently Kumar et al. have reported the stabilization ofuranium as uranyl in one of the borophosphate based host i.e.SrBPO5 (SBP) prepared through solid state reaction route [20].However, the dopant ion concentration in the work was arbitrarilyfixed at 1 mol%. There has not been any effort to explore theluminescence properties of this actinide ion as a function of con-centration or annealing temperature. In the present investigationwe report an extensive PL study of uranium ion in the boropho-sphate host and its effect on the dopant ion concentration andannealing temperature. In addition, color rendering index of thesystem was also evaluated.

2. Experimental

The undoped and uranium doped samples were prepared viasolid state reaction route. All the chemicals used here were ofAnalytical Reagent (AR) grade. Stoichiometric proportions ofSrCO3, (NH4)2HPO4, H3BO3 (5 mol% excess), and UO2(NO3)2 �6H2Owere ground together thoroughly to prepare a homogenizedmixture followed by pelletization. In the first phase, all the sam-ples were fired at 600 °C in air atmosphere for three hours in amuffle furnace. The samples were cooled to room temperature andthoroughly ground again before pelletization. In the second phase,pre fired samples were annealed at 900 °C for 3 h in air atmo-sphere and cooled to room temperature. The reactants react witheach other by the following equation to form the final product.

SrCO3þH3BO3þNH4H2PO4-SrBPO5þCO2↑þNH3↑þ3H2O↑ (1)

To investigate the effect of annealing temperature on the sys-tem, the samples were further heated at 1000 and 1100 °C. Thedopant ion concentration was varied between 1 and 8 mol%. The

crystal structure and phase purity of the prepared powders wereexamined through powder X-ray diffraction using a Philips dif-fractometer (model PW 1071) operating with monochromaticCuKα (λ¼1.5418 Å) radiation. The diffraction pattern was recordedat a scan rate of 0.05°/s in the scattering angle range (2θ) of10°–80°. The morphology of the samples was recorded by scanningelectron microscopy (SEM) model-AIS-2100, Merero Inc. PL exci-tation and emission measurements were made using a ShimadzuRF 5301pc spectrofluorimeter. The excitation source was a 150 WCW (continuous wave) xenon lamp. The bandwidths for theexcitation and emission monochromator were set at 3 nm. A long-wavelength-pass filter (UV-35, Shimadzu), with a maximum anduniform transmittance (more than 85%) above 350 nm, was placedin front of the emission monochromator in order to reduce thescatter of the incident beam into the emission monochromator. Inthe RF 5301pc spectrofluorimeter, the gain of the photometricphotomultiplier tube is dynamically adjusted to correct for thechanges in the Xe lamp output intensity using a monitoring pho-tomultiplier. The PL decay time measurements were carried outusing an Edinburgh FLS-900 time resolved fluorescence spectro-meter at room temperature.

3. Results and discussion

3.1. Crystal structure and morphology

SrBPO5 belongs to the stillwellite (LnBSiO5) type compoundwith trigonal system with hexagonal setting [21]. (BO4)3� and(SiO4)4� tetrahedra anions are two primary constituents of theborophosphate host. The (SiO4)4� tetrahedral form edge sharingvertical columns parallel to the c-axis. Each (BO4)

3� tetrahedron islinked to two (SiO4)4� tetrahedra and has two common edgeswith the lanthanide polyhedral of adjacent columns resulting in ahelical chain like structure [22]. The anionic network forms tor-tuous vertical channels that surround the lanthanide polyhedral,the lanthanide ion being in a 9-coordinated site with oxygens.However in case of SrBPO5 crystal structure (that mimics still-wellite (LnBSiO5)), the incorporation of Sr2þ in to a nine-coordi-nated polyhedron found to be difficult because of its smallercharge to size ratio compared to Ln3þ . In order to stabilize thesystem the bond length of the anion network gets in to readjust-ment with formation of some kind of oxygen defects and finally aneight coordinated Sr2þ site is formed. It is the internal crystallattice arrangement of SrBPO5 that stabilizes different types ofdopants (rare earths, transition metals and actinides) in it. Aschematic of the crystal structure is shown in Fig. 1A and B.

Figs. 2 and 3 show the XRD patterns of the SBP:U system as afunction of dopant ion concentration and annealing temperaturerespectively. The observed patterns indicated that the synthesizedsamples are single hexagonal phase with a space group of P3221matching with ICDD file no. 18-1270. There was no detectableimpurity peak observed indicating successful synthesis of materialin pure and single phase. The Uranyl being a heavy molecule andhaving higher charge to size ratio, solubility in SrBPO5 matrix wasstudied with different Uranium concentration (1 mol%, 4 mol% and8 mol%) by recording XRD.

It can be seen from Fig. 2 that with dopant ion concentration upto 8 mol%, there was no change in the patterns suggesting noimpurity phase formation in the system.

The detailed XRD data for some of the major peaks of the SBPsystem are presented in Table 1. The average cell parameters cal-culated from the observed ‘d’ values for these samples werea¼6.869(2) Å, c¼6.7969(7) Å which are in close agreement withthe reported values.

Fig. 1. (A) Shows the crystal structure of SrBPO5 showing the 8 coordinated ‘Sr’ site; (B) shows the crystal structure with the two planes (2 0 0) and (1 0 4) (the shadedregion) that show the highest intensity in XRD data.

Fig. 2. XRD patterns for SrBPO5 doped with uranium as a function of dopant ionconcentration (in mol%) (A): 1 mol%, (B): 4 mol% and (C): 8 mol%; all samples aresynthesized at 900 °C annealing temp. The figure (D) indicates the standard patternof the ICDD file no.: 18-1270.

Fig. 3. XRD patterns for SrBPO5 doped with uranium as a function of annealingtemperature.

Table 1X-ray diffraction data of SBP:U system.

Sr. no. d observed (Å) d calculated (Å) hkl I

1. 4.480 4.476 102 202. 3.439 3.426 110 803. 2.966 2.967 200 1004. 2.957 2.956 104 1005. 2.416 2.416 114 106. 2.272 2.273 006 107. 2.242 2.243 210 408. 2.239 2.238 204 409. 2.129 2.129 106 50

10. 1.894 1.894 116 1011. 1.873 1.874 214 6012. 1.713 1.713 220 1013. 1.711 1.711 304 1014. 1.531 1.531 224 1015. 1.526 1.526 118 10

A. Rout et al. / Journal of Luminescence 167 (2015) 45–50 47

This observation could be due to the fact that the latticeparameters with higher Uranium concentration may be below thedetection limit of the XRD technique. Another possibility is thatthe Uranium ion before reaching the lattice sites got stabilized inthe interstitial sites without affecting the cell parameters. Recentlyit has been strongly emphasized that in strontium pyrophosphate(Sr2P2O7) matrix uranium got stabilized at regular Sr2þ site [19]. Inpresent case uranium may be stabilized in the same manner. Asdiscussed earlier, the present host lattice is a stillwellite type ofcompound: a trigonal system with hexagonal settings as observedin rare earth borosilicate, LnBO (SiO4) with three formula units percell. The charge to size ratio of Uranium (U6þ) is greater than thecharge to size ratio of Sr2þ and Ln3þ , so unlike Sr2þ it can suitablyrest at nine oxygen co-ordinated sites [21].

Fig. 4. SEM micrographs of the SBP samples as a function of annelaing temperature; (A): 900, (B): 1000 and (C): 1100 °C.

Fig. 5. PL emission spectra of the SBP:U system as a function of dopant ion con-centration; the inset figure shows the emission intensity values for the 520 nmpeak as a function of Uranium concentration.

A. Rout et al. / Journal of Luminescence 167 (2015) 45–5048

From the XRD data of the samples with varying annealing tem-perature (Fig. 3), it can be noticed that between 900 °C and 1100 °Cthere was no change in the XRD pattern. However, the intensity of thediffraction peaks got enhanced at higher temperatures which areprobably due to better crystalinity at higher annealing temperaturesindicating a better thermal stability of the material.

Fig. 4 shows the SEM data for the SBP system as a function ofannealing temperature. The figure shows an inhomogeneous sizedistribution and agglomeration of the particles in the SBP system.From the figure it is clear that, on annealing beyond 900 °C, theoverall roughness of the edges of the samples gets improvedowing to the improved crystallinity. The overall particle size forthe system was found to be less than 1μm based on the statisticalanalysis from several micrographs. The uneven shape of the par-ticles observed in this case was due to the synthesis procedure.

3.2. PL investigations

Fig. 5 shows the emission spectra of SBP:U system as a functionof the dopant ion concentration with 434 nm excitation.

The spectra is similar to the data reported by Kumar et al. [20] thatshowed a five band structure for the samples with peaks at 481 nm(20,790 cm�1), 498 nm (20,080 cm�1), 519 nm (19,268 cm�1),539 nm (18,518 cm�1) and 560 nm (17,875 cm�1). Another weak butprominent hump in the spectra was observable at 595 nm(16,806 cm�1). These bands are known to be associated with Uranylemissions [23].

Uranyl with D1h symmetry makes the ligand to metal chargetransfer (LMCT) process parity forbidden. It is the local co-ordinationenvironment that lowers the symmetry and results in relaxation ofparity allowing the transition [24]. One of the significant electronictransitions, the zero phonon (zp) transition is a result of electronicexcitation from the 3su ground state, a highly hybridized configura-tion of the O (2s, 2p) and U (5p, 5f, 6d, 7s) atomic orbitals to the fδ,φorbitals of uranyl. In the present case, the emission band centred at481 nm is ascribed as the zp line while other bands are assigned asradiative transition from electronic excited state to various highervibronic levels in ground state [25]. The position of the zp line can varyfrom 470 to 520 nm depending on the equatorial coordination

environment of Uranium in the system. The five emission bands athigher wavelengths are known to be vibration progressions resultingfrom strong coupling of the ground state Raman active symmetricvibrational (ν1) mode with the 3Δg or 3Φg electronic excited state [26].The average spacing between the five lowest energy bands is798 cm�1 that corresponds to the vibrational spacing in the groundstate indicating a high force constant of the electronic state [27].

From the figure it is clear that, with increase in the dopant ionconcentration, the PL emission intensity increases up to 7 mol% of theuranium ion. The decrease in PL intensity beyond 7mol% could beascribed to concentration quenching, a cross relaxation betweenUranyl ions with loss of excited state energy through a non-radiativepath. There are two types of mechanism to investigate the non-radiative transfer of energy. One is Dexter type, a collisional energytransfer and other one is Forster resonance energy transfer (FRET), amultipole–multipole interaction mechanism [28]. Generally theenergy transfer depends on the donor–acceptor spectral overlap (theemission of donor and excitation of the acceptor should have an

Fig. 7. PL emission spectra for the U:SBP system as a function of annealingtemperature.

Fig. 8. PL decay time data for the U:SBP system as a function of annealingtemperature.

A. Rout et al. / Journal of Luminescence 167 (2015) 45–50 49

overlap) and the relative orientation of the dipole moments. In addi-tion to these in case of FRET, the distance (L) between the donor andacceptor ion is important as the probability of the energy transferdecreases at a rate of 1/L6. The typical distance (critical distance, Lc) forFRET is maximum 100 Å. In case of Dexter mechanism the probabilityof energy transfer decreases as (exp)�L and here maximum typicaldistance is r10 Å. In present case an approximate Lc value was cal-culated using the following relation [24].

L V NX2 3 /4 2c c1 3/( )π= ( )

Where V is the unit cell volume, N the number of sites and Xc is thecritical concentration. The value of N and V are obtained from XRDdata. Considering 7 mol% as the critical concentration value, Lc wascalculated to be 3 Å. This shows that the Dexter type energytransfer process is the reason leading to Uranyl luminescencequenching in the present matrix for UO2

2+ concentration 47 mol%.Fig. 6 shows the corresponding excitation spectra for the

7 mol% uranium doped sample at 520 nm emission wavelength.The sample was annealed at 900 °C. It can be seen from the figurethat the broad excitation spectra has peak maxima at 335, 343,352, 407, 417, 434, and 440 nm. For Uranyl, the optical excitation ispurely a ligand-to-metal charge transfer (LMCT) process, where anelectron transition occurs from a bonding oxygen orbital (su, sg,πu, πg) to a non-bonding Uranium 5f orbital.

Earlier the XRD studies had indicated a higher thermal struc-tural stability of the annealed samples. Fig. 7 shows the PL emis-sion data of the 7 mol% U doped SBP samples as a function ofannealing temperature. From the figure it can be observed that,with increase in the annealing temperature, the PL yield decreasedso much so that at 1100 °C, the spectrum is barely visible. Asdiscussed earlier, in this matrix oxygen vacancies can act as defectcentres for the charge neutrality purpose. At higher annealedtemperatures, defects can arise which can migrate or diffuse andcombined to form defects of larger size. These defect centres canenhance the non-radiative energy loss of Uranyl ion and therebydecreasing the PL yield [29].

To further confirm this fact, PL decay time investigations werecarried out on the U:SBP samples heated at different temperaturesas shown in Fig. 8. The decay time curves were recorded withλex¼434 nm and λem¼520 nm. The decay curves could be fitted into bi-exponential decay using the following iterative formula.

300 350 400 450

50

100

150

200

250

300

350

400

450

500

550

600

650

Int.

(a.u

.)

wavelength(nm)

434nm

343nm352nm

417nm

em = 520 nm

SBP:U (7 mol%)

λ

Fig. 6. Excitation spectrum for the 7 mol% uranium doped SBP system annealed at900 °C.

I t A t A t yexp exp/ / 31 1 2 2 0( ) ( )τ τ( ) = − + − + ( )

Here, A1 and A2 are pre-exponential factors that are scalarquantities, ‘t’ is the time of measurement and τ1 and τ2 are thedecay time values.

The PL decay time values for the 900 °C annealed sample wereobserved to be τ1¼165 ms (65%) and τ2¼54 ms (35%) withχ2¼1.489. The data suggests the presence of Uranyl ions at siteswith two different types of surrounding environment. The crystalstructure of SrBPO5 informs the presence of single type of Sr2þ siteand the system is accompanied with oxygen defects. The defectcentres are responsible for energy loss through non-radiativedecay, thus a luminescent species surrounded by defect centresmay lose its energy non-radiatively resulting in a reduced lifetimewith respect to the same type of species present at a site wherethe concentration of defect centres is less. As observed in case ofthe PL emission data, the decay time values were also decreasingwith increasing temperature. Thus corroborating the fact that, onannealing, more and more number of defect centres are agglom-erating in the system to form large defect centre volumes whichprovides a non radiative path there by reducing the PL emissionintensity and PL decay time.

Fig. 9. CIE chromaticity diagram for the SBP:U (7 mol%) system annealed at 900 °C;the coordinates are demarked by an asterisk.

A. Rout et al. / Journal of Luminescence 167 (2015) 45–5050

3.3. Evaluation of color coordinates

Color coordinates for the 900 °C annealed and 7 mol% uraniumdoped sample were evaluated adopting standard procedure [30].Recently Taikar et al. have reported Uranium doped SrO as a greenlight emitting phosphor and compared it with another greenphosphor namely ZnS:Cu, Al [7]. The CIE (Commission Inter-nationale de I’éclairage) chromaticity coordinates are useful todetermine a light emitting materials performance on color lumi-nescent emission. A (x, y) co-ordinate in the color space deter-mines the color of any light source. The spectral power distribu-tion (SPD) of the light source and the CIE color matching functionsare the required parameters to determine the (x, y) coordinate. Thechromaticity coordinates x, y, z can be obtained from tristimulusvalue X, Y and Z as follows:

x X X Y Z y Y X Y Z z Z X Y Zand/ , / / 4= + + = + + = + + ( )

The predominating wavelength from a light source is evaluatedusing CIE chromaticity diagram. This wavelength is an interceptionpoint of a straight line drawn from one of the CIE white illumi-nants, through the (x, y) coordinates up to the extent, where ittouches the outer locus of points along the spectral edge of the1931 CIE chromatic diagram. In SrBPO5:UO2

2+ the calculated valuesof x and y coordinates were found to be 0.226 and 0.715 respec-tively indicated as an asterisk mark (*) in Fig. 9. The CIE indexvalues show that the present compound can be used as a potential‘Green’ emitting phosphor.

4. Conclusion

Uranium doped SrBPO5 samples were prepared via solid statereaction route in air atmosphere. XRD studies confirmed the for-mation of single phase and better solubility of uranium at higherconcentration. It was observed that up to 8 mol% of the dopant ion

there was no phase separation in the system. SEM studies indi-cated the presence of uneven, agglomerated particles with lessthan 1 mm individual particle size. PL studies confirmed the sta-bilization of uranium as UO2

2+ in the system. Based on the PLemission data, concentration quenching was observed beyond7 mol% of the dopant ion. The estimated critical distance (Lc) of 3 Åsuggests Dexter type energy transfer mechanism to be the pre-dominant one in the system. The life time decay studies indicatedthe presence of two different types of environment around Uranylion. On annealing at temperatures beyond 900 °C the PL emissionand decay time values reduced drastically. It was concluded thaton annealing at temperatures beyond 900 °C, defect centres getagglomerated around the metal ion providing non radiativepathways for the energy to get dissipated thereby reducing the PLemission and decay time. The CIE indices for the 7 mol% uraniumdoped sample annealed at 900 °C suggested that the material canbe used as a potential green emitting phosphor.

References

[1] (a) W.M.A. Smit, G. Blasse, J. Lumin. 29 (1984) 367;(b) J.T.W.D. Hair, G. Blasse, J. Lumin. 14 (1976) 307;(c) R.U.E. ‘t Lam, G. Blasse, J. Chem. Phys. 72 (1980) 1803.

[2] (a) L.S. Natrajan, Co-ord. Chem. Rev. 256 (2012) 1583;S. Hubert, N.M. Edelstein, Encyclopaedia of Materials: Science and Technology,Elsevier, Amsterdam (2001), p. 18–22;(c) J.C. Krupa, Inorg. Chim. Acta 139 (1987) 223.

[3] J.V. Beitz, J. Alloys Compd. 207–208 (1994) 41.[4] D.C. Harris, M.D. Bertolucci, Symmetry and Spectroscopy: An Introduction to

Vibrational and Electronic Spectroscopy, Oxford University Press, New York,1978.

[5] A. Singh, J.G. Catalano, K.U. Ulrich, D.E. Giammar, Environ. Sci. Technol. 46(2012) 6594.

[6] S.D. Pollington, A.F. Lee, T.L. Overtone, P.J. Sears, P.B. Wells, S.E. Hawley, I.D. Hudson, D.F. Lee, V. Ruddock, Chem. Commun. (1999) 725.

[7] D.R. Taikar, C.P. Joshi, S.V. Moharil, J. Lumin. 153 (2014) 304.[8] G. Liu, N.P. Deifel, C.L. Cahill, V.V. Zhurov, A.A. Pinkerton, J. Phys. Chem. A 116

(2012) 855.[9] M. Sobczyk, J. Drozdzynsk, R. Lisiecki, W.R. Romanowski, Opt. Mater. 29 (2007)

1029.[10] R. Parrot, C. Naud, C.J. Delbecq, P.H. Yuster, Phys. Rev. B 15 (1976) 137.[11] M. Louis, E. Simoni, S. Hubert, J.Y. Gesland, Opt. Mater. 4 (1995) 657.[12] S.V. Godbole, A.G. Page, S.C. Sabharwal, J.Y. Sangeeta, M.D. Gesland, J. Sastry, J.

Lumin. 93 (2001) 213.[13] T.K. Seshagiri, M. Mohapatra, T.K. Gundu Rao, R.M. Kadam, V. Natarajan, S.

V. Godbole, J. Phys. Chem. Solids 70 (2009) 1261.[14] G. Blasse, K.C. Bleijenberg, D.M. Krol, J. Lumin. 19 (1979) 57 , p. 18.[15] G. Blasse, J. Electrochem. Soc. 115 (1968) 738.[16] M. Mohapatra, V. Natarajan, J. Radioanal. Nucl. Chem. 302 (3) (2014) 1327.[17] (a) G. Blasse, J.P.M. vanden Dungen, J. Inorg. Nucl. Chem. 40 (1978) 2037;

L. Schwarz, E. Brecht, E. Birckner, B. Reimer, in: Proceedings of the 2ndInternational Meeting on Luminescence, E.M. Amdt Universitàt, Griefs-wald, 1989, p. 153.

[18] M.E. Azenha, G. Blasse, J. Alloys Compd. 196 (1993) 81.[19] M. Mohapatra, A.K. Yadav, S.N. Jha, D. Bhattacharyya, S.V. Godbole,

V. Natarajan, Chem. Phys. Lett. 601 (2014) 81.[20] Mithlesh Kumar, M. Mohapatra, V. Natarajan, Mater. Res. Bull. 60 (2014) 858.[21] A. Karthikeyania, R. Jagannathan, J. Lumin. 86 (2000) 79.[22] Z. Qinghua, K. Nathan, R. Mark, J. Lumin. 101 (2003) 167.[23] M. Mohapatra, B. Rajeswari, R.M. Kadam, M. Kumar, T.K. Seshagiri, N.K. Porwal,

S.V. Godbole, V. Natarajan, J. Alloys Compd. 611 (2014) 74.[24] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin,

Heidelberg (1994), p. 91–107 , ISBN: 978-3-540-58019-5.[25] (a) J. Su, Z. Wang, D. Pan, J. Li, Inorg. Chem. 53 (2014) 7340;

(b) R.G. Denning, J. Phys. Chem., A 111 (2007) 4125.[26] G. Blasse, Spectra and Chemical Interactions, Structure and Bonding: The

Influence of Charge-Transfer and Rydberg States on the Luminescence Prop-erties of Lanthanides and Actinides, (1976) 43–79.

[27] G. Blasse, J. Electrochem., Soc. 124 (1977) 1280.[28] Th. Főrster, Discuss. Faraday Soc. 27 (1959) 7.[29] S.K. Gupta, M. Mohapatra, S. Kaity, V. Natarajan, S.V. Godbole, J. Lumin. 132

(2012) 1329.[30] T. Smith, J. Guild, Trans. Opt. Soc. 33 (3) (1931) 73.