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Solid State Sciences 12 (2010) 1146e1151

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Solid State Sciences

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Inter-conversion of Tb3þ and Tb4þ states and its fluorescence properties inMOeAl2O3: Tb (M ¼ Mg, Ca, Sr, Ba) phosphor materials

R.K. Verma, K. Kumar, S.B. Rai*

Laser Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India

a r t i c l e i n f o

Article history:Received 13 January 2010Received in revised form3 April 2010Accepted 7 April 2010Available online 14 April 2010

Keywords:Optical propertiesLuminescenceValence state conversion

* Corresponding author. Tel.: þ91 542 230 7308; faE-mail address: sbrai49@yahoo.co.in (S.B. Rai).

1293-2558/$ e see front matter � 2010 Elsevier Masdoi:10.1016/j.solidstatesciences.2010.04.004

a b s t r a c t

Terbium ion doped MOeAl2O3 (M ¼ Mg, Ca, Sr and Ba) series phosphors have been synthesized throughcombustion technique and their luminescence properties have been studied and compared. Terbium ionin different phosphors has shown different fluorescence properties due to the presence of different ratiosof Tb3þ and Tb4þ states in different samples. The UV/Visible absorption and XPS techniques have beenused to probe the existence of Tb3þ and Tb4þ states. The host sensitive 4fe5d and the charge transfertransitions enabled the use of terbium ion as an indicator of the structure.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

Though a number of optical devices already employ rare earth(RE) ions as emitting centers with good quantum efficiency, thenew generation of RE doped nano-phosphors has attracted theattention due to their various prospective applications. Thanks tothe chemistry and the chemical engineers, for the development ofnew sample preparation routes with vast capability to producesamples with better properties. One of the new routes proposed isthe combustion technique which has been shown to produce rareearth doped highly homogeneous, nano-crystallized fine phosphormaterials [1e3]. This technique is now widely in use for the prep-aration of phosphor materials with interesting properties andapplications [4,5].

Terbium is one of the most widely used RE ion which emitsstrongly in green region on X-ray, electron beam as well as UVexcitations when doped in solid hosts and have attracted greatattention due to their significant applications in display devices,X-ray detectors, C. T. Scan, lasers, TV monitors etc. [6,7]. In natureterbiumexists in twovalence states viz Tb3þ and Tb4þ. Tb4þdoes notgive any emission and its presence in any host acts as a quenchingcenter. Simultaneous presence of the two valence states in case ofterbium ion which is unusual, affects the optical properties ofmaterial. The electron donation ability of the lower valence state ion

x: þ91 542 236 9889.

son SAS. All rights reserved.

facilitates the excitation of electron from the 4f to 5d shell. On theother hand, the electron acceptance ability of the higher valencestate ion favors charge transfer (CT) transition from ligands to rareearth ions. The 4fne 4fn�15d and CT transitions of rare earth ions areused in a wide range of applications [8]. Since 4fe5d and CTabsorptions are allowed electric dipole transitions, they have highintensities and are also highly sensitive to the host matrix. The hostsensitive nature of 4fe5d and CT transitions enables the use ofterbium ion as an indicator for the study of structure.

Several studies have been performed in recent years on Tb3þ

doped phosphor materials prepared through new chemical routesand fluorescence properties of Tb3þ ion in this state have beendiscussed [9]. But, simultaneous presence of Tb3þ and Tb4þ states ina single host has not been studied in any host and also their inter-conversion mechanism has not been fully resolved.

The present work reports a comparative study of Stokes fluo-rescence of Tb3þ ions in MOeAl2O3 (M ¼ Mg, Ca, Sr, Ba) seriesphosphors and also the effect of calcination on the emissionproperties. Correlation between the FTIR and XRD spectra of thesamples has also been established. The main conclusion is thatterbium has been found in Tb3þ and Tb4þ states with differentratios in different phosphors at different calcinations temperaturesand thus affects the luminescence intensity.

2. Experimental

The phosphor samples have been prepared through combustionmethod using urea as an organic fuel as described in our earlier

R.K. Verma et al. / Solid State Sciences 12 (2010) 1146e1151 1147

report [10]. Following compositions of the phosphor has beenselected for study:

50 MO þ (50 � x) Al2O3 þ xTb4O7where M ¼Mg, Ca, Sr and Ba and x ¼ 0.5, 0.75, 1.0, 1.5, 2.0 mol%.To produce/increase crystallization in the as-synthesized

samples, the samples were calcinated at three different tempera-tures viz. 1073 K, 1273 K and 1473 K each for 3 h. The existence ofcrystallinity has been checked by powder X-ray diffraction (XRD)patterns using CuKa radiation (1.5406 �A).

The absorption spectra of the samples (both as-synthesized andcalcinated) were taken in reflectance mode using Perkin Elmer,Lambda-35 spectrophotometer. The down conversion fluorescencewas recorded using 266 nm radiation from an Nd: YAG laser and aniHR320,Horiba JobinYuon, spectrometer. PerkinElmer SpectrumRX1spectrophotometerwasused torecordtheFTIRspectraof thesamples.

3. Results and discussion

Samples with different concentrations of Tb4O7 were prepared.The down conversion emission intensity was found maximum for1.0 mol% Tb4O7 and all further studies therefore, have been madewith samples having 1.0 mol% Tb4O7 concentration.

3.1. X-ray diffraction studies

The XRD patterns of the as-synthesized as well as samples cal-cinated at different temperatures were monitored to know thegrowth of crystallinity. From the XRD studies it was noted that allthe alkaline earths samples possess crystallinity (except calciumaluminate (CA), which is amorphous in as-synthesized case) evenin as-synthesized form. The crystallinity increaseswith the increasein calcination temperature. Fig. 1 compares the XRD patterns of CAphosphor samples calcinated at 1073 K and 1473 K for 3 h with as-synthesized sample. The as-synthesized sample of CA is totally

Fig. 1. XRD spectra of CA phosphor as-synthesized heated at 1073 K and 1473 K. The1473 K calcinated sample shows a mixture of two phases.

amorphous and the detectable crystallinity occurs only above 973 Kwhich is due to large thermal stability of the CA sample. At calci-nation temperatures >973 K the single Ca5Al6O14 phase begins togrow. The Ca5Al6O14 structure is cubic with cell parameterw11.95�A. On the other hand the sample calcinated at 1473 K for 3 hshow mixed phase of Ca5Al6O14 and Ca3Al2O6. The other new cubicphase Ca3Al2O6 grows with cell parameter 15.26 �A and belongs tospace group Pa3. Doping of Tb3þ ions has no effect on the crystalphase of the sample. The crystallite size was calculated using theScherer equation:

t ¼ l� 0:9b� cos q

Where, t is the crystallite size for (h k l) planes, l is the wavelengthof the incident X-ray [CuKa (0.154056)], b is the full width at halfmaximum (FWHM) and q is the diffraction angle for (h k l) plane.Three most significant peaks were selected for the calculations indifferent samples and the average crystallite size was found to be inthe range w40e50 nm. The XRD patterns of other samples havealso been found to change with temperature. The crystallite size inthese cases also is found in the same range.

3.2. Fourier Transform Infrared (FTIR) study

The FTIR spectra of the prepared samples were recorded in the4000e400 cm�1 region. Fig. 2 shows the spectra of calcinatedsamples of CA phosphor along with that of as-synthesized one. TheFTIR spectra show the presence of conventional impurities (NO3�,OH�) in the materials which decrease with calcination. The inset ofFig. 2 shows the presence of NO3� impurity at 1390 cm�1 whichdecreases with calcination temperature. The splitting in the bandsdue to host matrices (900e450 cm�1) at higher temperatures is dueto the presence of two phases in the lattice. The broad band at

Fig. 2. FTIR absorption spectra of terbium doped CA phosphor as-synthesized and ofthe samples heated at different temperatures.

Fig. 4. UV/visible absorption and corresponding emission spectra of MA phosphor as-synthesized and heat treated to different temperatures.

R.K. Verma et al. / Solid State Sciences 12 (2010) 1146e11511148

844 cm�1 in as-synthesized sample is ascribed to the AleOeAlstretching mode while the band in the range 520e600 cm�1 isrelated to CaeO and the bending modes of Al2O3. Normallya decrease in quenching centers on calcination enhances the fluo-rescence intensity of rare earth. However, it is interesting to notethat the results on MOeAl2O3 (M ¼ Mg, Ca, Sr and Ba) seriesphosphors are not according to as expected. In case of CA and BA(barium aluminates) phosphors the fluorescence intensity of theTb3þ decreases with the increase in calcination temperature, but inSA (strontium aluminates) the intensity increases with the increasein calcination temperature. However in the case of MA (magnesiumaluminate) the fluorescence intensity increases up to 1273 K but at1473 K, the fluorescence intensity suddenly drops. This unusualbehavior is due to the inter-conversion of Tb3þ and Tb4þ valencestates on heating.

3.3. Absorption and fluorescence studies

The Tb3þ ion exhibits two types of transitions viz 4fn / 4fn and4fn/ 4fn�15d in the 200e800 nm regions. The former one arises atlower energies (10 000e20 000 cm�1) due to transition in between4fn levels where as the later one at higher energies(30 000e40 000 cm�1) due to 4fn / 4fn�15d transitions. The 4fe4ftransitions are weakly host sensitive and relatively sharper. On theother hand 4fn / 4fn�15d transitions are weak, broad and not yetwell understood because they are easily perturbed by crystal field,spineorbit interaction, electrostatic interaction etc. which are hostdependent. In the 4fn / 4fn�15d transition crystal field dominatesto other interactions [11] and so its luminescence properties arestrongly host dependent. The absorption spectra of terbium ions inreflection mode in different hosts are shown in Figs. 3e6. In case ofas-synthesized samples, few sharp peaks are seen in UV region. Theintensity of these peaks is decrease with increase in calcinationtemperature. These peaks are due to the Tb3þ absorption (4fe5dtransitions). The as-synthesized sample also shows decreasing

Fig. 3. UV/visible absorption and corresponding emission spectra of CA phosphor. As-synthesized and heat treated to different temperatures.

reflectance towards lower wavelength side and appears as a broadband. It occurs due to the band gap (valence to conduction bandtransition) of the host material and is not related to the Tb4þ

absorption. This is also verified by XPS studies. As the samples are

Fig. 5. UV/visible absorption and corresponding emission spectra of BA phosphor. As-synthesized and heat treated to different temperature.

Fig. 6. UV/visible absorption and corresponding emission spectra of SA phosphor. As-synthesized and heat treated to different temperature.

Fig. 7. Emission spectra of CA phosphor as-synthesized and heat treated at differenttemperatures corresponding to 5D3 / 7Fj transition.

Fig. 8. Energy level diagram of Tb3þ and emissions in different hosts.

R.K. Verma et al. / Solid State Sciences 12 (2010) 1146e1151 1149

calcinated there absorption shows broadening in the absorptionspectrum. The broadening increases with the increase in calcina-tion temperature. This broadening extends through out visibleregion and is definitely unexpected for Tb3þ ion as Tb3þ gives sharpbands. Even the fed transition of Tb3þ should not be so broad. Inorder to know the origin of these bands we recorded the XPSspectra of these calcinated samples. Two bands could be marked inXPS spectra at 146 eV and 148.9 eV which match well with Tb4þ

bands earlier reported by Long et al. [12]. The broad absorption ofTb4þ in UVeVis region have also reported by Zych et al.[13].Therefore, we suppose that when terbium doped phosphors arecalcinated at higher temperatures, a part of Tb3þ is converted intoTb4þ (initially in as-synthesized sample only Tb3þ state is present).The Tb3þ ion in aluminate host fluoresces very efficiently [14]. Inour case samples show emission in a wide excitation range(355e230 nm) available in the laboratory. The 266 nm excitedemission spectra of all the four samples in 380e700 nm regions areshown in Figs. 3e6 along with their absorption spectra. The fluo-rescence bands arise due to 5D3 / 7Fj (j ¼ 0e6) transitions in therange 380e500 nm [only in CA phosphor] and from 5D4/

7FJ (j¼ 0to 6) transitions in the 480e700 nm range due to Tb3þ. Fig. 7compares the emission spectra obtained for calcinated samples ofCA series corresponding to the 5D3 / 7Fj transitions. The increasein emission intensity with calcination temperature as observed inFig. 7 is related to the decrease in effective phonon frequency of theCA sample due to the removal of the impurities from the samples(Fig. 2). Since the energy difference between 5D3 and 5D4 levels isapproximately equal to the energy difference between 7F0 and 7F6levels, the excited ions in 5D3 level populate 5D4 level through theprocess called cross-relaxation as represented in Fig. 8. This cross-relaxation increases the population of the 5D4 level at the expanseof 5D3 level so that intensity of emission bands originating from the5D3 level decreases. This may be the reason for non-appearance ofbands from 5D3 in other cases. The 5D4 /

7F5 transition in all cases

is the most intense one. The intensity for this transition amongstdifferent phosphors calcinated at the same temperature follow thetrend CA >MA > SA> BA but the total intensity emitted is greatestfor MA phosphor. From emission spectra it is found that thequantum efficiency decreases as the alkaline earth is replaced bynext higher atomic number member i.e. (MA > CA > SA > BA). Twotypes of quenching centers are present in these samples. The first

Table 1Absorption peak positions of Tb3þ, Tb4þ, and host in MOeAl2O3: Tb (M ¼ Mg, Ca, Sr,Ba) phosphor materials.

MA CA SA BA

Host 212 212 216 216Tb3þ 265 250 273 264Tb3þ 297 297 e e

Tb4þ 360 350 357 364

Fig. 9. XPS spectra of as-synthesized and heat treated (at 1273 K) CA samples.

R.K. Verma et al. / Solid State Sciences 12 (2010) 1146e11511150

types of quenching centers are the conventional one such as NO3�,OH� etc. that decrease with temperature. The second type ofquenching center is Tb4þ (due to conversion of Tb3þ to Tb4þ) whichnot only reduced the Tb3þ emitting centers but also absorbs radi-ations arising due to 5D3,4 / 7FJ transitions. A more detaileddescription about the same is given below.

3.4. Variation in emission intensity on calcination: Tb3þ 4 Tb4þ

inter-conversion

From the emission spectra of the samples (Figs. 3e6) it is clearthat there is no systematic change in intensity with calcinationtemperature. The variation in emission intensity of Tb3þ ion withcalcination can be understood with the co-existence of Tb3þ andTb4þ ion states. The Tb4þ is known to be non-fluorescing one andshows broad absorption band due to the 4fn�1 / 4fn�25d transi-tion. The lowest lying 4f5d state of Tb3þ and electrostatic interac-tion between 4f and 5d produces two excited states, the lower lying9DJ and the next higher lying 7DJ [15]. The 7FJ / 7DJ transition isallowed and thus has high intensity and appears in 215e300 nmregions in absorption spectrum (depending on the host). The lowerlying 7FJ / 9DJ transition appears with weak intensity and lies inhigher wavelength side of the 7FJ / 7DJ band. No absorption banddue to the 4fe4f transition from Tb3þ has been observed because ofits weaker absorbance and also due to the use of reflectance modeused in recording the spectra.

The emission spectra shown in Figs. 3(b)e6(b) show an irregularvariation in emission intensities as we go fromMA to SA hosts. Thefluorescence intensity of Tb doped magnesium aluminate (MA)series samples (see Fig. 4b) attain optimum intensity for the samplecalcinated at 1073 K and then reduces to negligibly small intensityat 1473 K. At 1473 K calcination, the sample shows a broad emissioncentered at 720 nm. This has been assigned to arise from the MgOemission. Though a band appears at this wavelength even in as-synthesized sample but it becomes very intense at 1473 K. In case ofcalcium aluminates (CA) series the emission intensity decreasescontinuously from as-synthesized to samples calcinated at 1273 K.However, a slight increase in intensity is observed for sample cal-cinated at 1473 K. Stark components are also resolved in emissionbands in this case. SA samples again show a decrease in intensityfrom as-synthesized to sample heated at 1273 K while the samplecalcinated at 1473 K shows almost 5 times enhancement in inten-sity. Here also Stark components are well resolved. On the otherhand, the BA series samples show a different behavior. The as-synthesized sample shows fluorescence very bright [almost 5 timesstronger than that of calcinated samples]. The fluorescence inten-sity was found to decrease rapidly for calcinated samples.

To understand the above emission behavior, the UV/Visibleabsorption and X-ray photoelectron spectra (XPS) of differentsamples have been studied. The as-synthesized Tb3þ phosphor in allthe hosts is white powder and terbium in them is in Tb3þ state. Nor-mally, Tb4þ dopedmaterials showbrowncolor due to the low-energycharge transfer absorption of Tb4þ ion inUVvisible region [16].Whenthe Tb3þ containing samples [all the sample of series] are heated allthe powders become brown indicating a probable conversion of Tb3þ

to Tb4þ. A gradual change in color from white to brown is shown ininsetofFig. 3 forCAseries. Thefluorescence spectraof thesealsoshowa broadening and red shift. The luminescence spectrumand the colorof the sample both show a transformation of Tb3þ / Tb4þ has takenplace at temperature�1273K.However,when the temperature tendsto 1473 K (different for different samples) slowly a back conversion ofTb4þ into Tb3þ has taken place. This resulted an increase in lumi-nescence intensity of Tb3þ bands.

The most prominent feature observed in absorption spectra isthe change in intensities of the bands and appearance of newer

bands on calcination. In case of absorption spectra of MA, as-synthesized sample shows three additional bands at 265, 297 and360 nm. The first two bands are sharp and are due to the Tb3þ ions.The third band at 360 nm is broad. Its FWHM is nearly four timesthe FWHM of earlier two peaks and is due to Tb4þ ions. Uponcalcination at 1073 K the host absorption show a red shift and theintensity Tb3þ bands is reduced. The sample calcinated at 1473 Kshows very broad absorption in the 200e750 nm range. Sucha broad absorption extending over UV and almost whole visiblepart of spectrum is surprising, since it is definitely unexpected forTb3þ ion, for which the fed absorption should be located wellwithin the UV range [14,15], and in any case should not be so broad.On the other hand it is common for materials containing Tb4þ ionsto be off-white in color [17e19], which results from low-energycharge transfer absorption of Tb4þ ion [17]. Therefore, we supposethat a part of the Tb3þ has been converted into Tb4þ on heating inopen atmosphere. In the case of CA, the content of Tb4þ graduallyincreases up to 1273 K. After this temperature a back conversion ofTb4þ to Tb3þ has been inferred from the fluorescence spectrum. Inthe case of SA, the samples calcinated up to 1273 K contains Tb4þ asmajor part but in sample calcinated at 1473 K, a substantial part ofTb4þ is transferred into Tb3þ and resulted an increase in emissionintensity. We can take ratio (R)¼ absorption band intensity of Tb3þ/absorption band intensity of Tb4þ as ameasure of ratio of Tb3þ/Tb4þ

in the samples at a particular temperature. For SA sample calci-nated at 1473 K this ratio (273 nm/357 nm) is largest indicatinghighest percentage of Tb3þ. In BA phosphor this value is largest foras-synthesized sample [264 nm/364 nm] and therefore possesseshighest fluorescence among different BA samples. Symbolically, wecan write this ratio for all samples as:

MA : Ras�syn > R1073K > R1473KCA : Ras�syn > R1073K > R1473K > R1273KSA : R1473K > R1073K > Ras�synBA : Ras�syn > R1073K > R1473K

(A)

This trend occurs due to the variation in activation energy ofTb3þ in different hosts and needs to be studied in detail. The

R.K. Verma et al. / Solid State Sciences 12 (2010) 1146e1151 1151

observed band positions and their assignments in the case of as-synthesized samples are given in Table 1. We have identified thepresence of Tb4þ by Tb4d XPS spectra. Tb3þ exhibits single photo-electron line at 146 eV and Tb4þ exhibits two photoelectron lines at146 and 148.9 eV, as shown in Fig. 9.

Terbium is a medium rare earth ion with electron configuration[Xe]4f96s2. It tends to reach a relatively stable state when equal-energy orbits are in all-full, semi-full or all-empty. So it is easy forTb3þ to lose an electron and to be oxidized into Tb4þ, which is inaccordance with the fact that the standard oxidation reductionpotential of Tb4þ/Tb3þ is 3.1 � 0.2 V. Blasse et al. [20] in their studyon the luminescence properties of Tb dopedmaterial have reportedthat Tb3þ is unstable and tends to oxidize into Tb4þ especially incubic phase materials. Therefore, during the calcination process ofthe materials, it converts into Tb4þ and releases an electron. Theemitted electron is captured by vacancies in the lattice. At largercalcination temperatures when redox barrier is overcome, eitherelectron captured is released from the vacancies or oxygen ionreduces the Tb4þ into Tb3þ. However, if we change the alkalineearth element the energy required to convert Tb3þ 4 Tb4þ variesand hence the temperature effect is observed. A similar explanationhas also been given by Jian et al. [21] in the case of multi-phaseceramics.

In case of MA sample containing magnesium oxide (band gap7.9 eV), magnesium band emission has been detected at 4.5 eV atvery low laser power with wavelength l ¼ 266 nm. Chao [22] hasreported red band at about 720 nm in thermo luminescencespectrum of bulk MgO associated with certain impurity. Thestructure observed near 720 nm in the photoluminescence of MgOnano-phosphors is expected to be the luminescence of MgO asso-ciated with defect centers [23,24].

4. Conclusions

Terbium doped MOeAl2O3 (M ¼ Mg, Ca, Sr, Ba) nanocrystallinephosphors have been prepared through combustion technique. Thesamples were calcinated at three different temperatures and theirfluorescence properties have been studied and compared. Allsamples show presence of Tb4þ state along with Tb3þ and theirpercentage in the material has been found to be dependent onparticular alkaline earth ion and the calcination temperature. The

XPS spectra support the presence of Tb4þ state in the samples. Acomparison of emission spectra reveals that MgO and CaO are thegood additives in order to obtain high luminescence intensity ofTb3þ ions. TheCAphosphorhas shownemission from5D3/

7FJ levelwhichhas notbeenobserved in theother series phosphors. The Tb4þ

state has been found to act as fluorescence killer of Tb3þ emissiondue to its very broad and strong absorption in visible region.

Acknowledgement

Authors are grateful to Alexander von Humboldt foundation,Germany for the donation of Nd: YAG laser and also to Dr. H. Mis-hra, MMV, BHU, Varanasi for providing his fluorescencespectrometer.

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