Optical Materials 36 (2014) 633–638

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Optical Materials

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Luminescent GeO2–Pb–Bi2O3 glasses co-doped with Tb3+1 and Eu3+:Excitation energy transfer and color chromaticity

0925-3467/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.optmat.2013.10.042

⇑ Corresponding authors. Address: Faculty of Tech. Phys. & Appl. Mathem.,Gdansk University of Technology, G. Narutowicza 11/12, 80-233 Gdansk, Poland(A.M. Kłonkowski). Tel.: +48 58 348 66 05.

E-mail addresses: fizmgr@univ.gda.pl (M. Grinberg), aklonkowski@mif.pg.gda.pl(A.M. Kłonkowski).

B. Kuklinski a, D. Wilenska b, S. Mahlik a, K. Szczodrowski a, M. Grinberg a,⇑, A.M. Kłonkowski b,c,⇑a Institute of Experimental Physics, University of Gdansk, Wita Stwosza 57, 80-952 Gdansk, Polandb Faculty of Chemistry, University of Gdansk, Sobieskiego 18/19, 80-952 Gdansk, Polandc Faculty of Tech. Phys. & Appl. Mathem., Gdansk University of Technology, Narutowicza 11, 80-233 Gdansk, Poland

a r t i c l e i n f o

Article history:Received 6 June 2013Received in revised form 29 September 2013Accepted 31 October 2013Available online 23 November 2013

Keywords:Luminescent materialsLanthanide ionsGlassesGlass–ceramicsEnergy transferCross relaxation

a b s t r a c t

Heavy metal oxide glass of the type GeO2–PbO–Bi2O3 was applied as a matrix for luminescent materialsco-doped with Tb3+ and Eu3+ ions of different molar ratio. The undoped glass matrix with the composition80GeO2–19PbO–1Bi2O3 was characterized by XRD and DSC techniques. Its structure was studied by FTIRspectroscopy. The FTIR spectra suggested that GeO4, GeO6, BiO3, BiO6, PbO3 and PbO4 structural units arepresent in the glass. In the case of the 79GeO2–19PbO–Bi2O3–xEu2O3–(1 � x)Tb2O3 luminescent glasses,reflection, luminescence and excitation spectra were recorded as well as time resolved luminescencetechnique was used. Two luminescence effects were analyzed, viz. cross relaxation and Tb3+ ? Eu3+

energy transfer.� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Heavy metal oxide glasses are of particular interest in optoelec-tronic and photonic applications. It is due to their optical propertiessuch as high refractive index, optical non-linearity and infraredtransmittance [1,2]. Thus, they can be applied in more efficient la-sers and fiber optic amplifiers at longer wavelength than availablefrom other oxide glasses [3]. GeO2–PbO–Bi2O3 glasses present abroad transmission window (from 400 nm to 4.5 lm) and highrefractive index (equal to 2.0) which are consequences of the smallfield strengths and relatively large masses of the metal componentsin such oxides as PbO and Bi2O3 [4–6]. Although conventional oxideglasses are characterized by high-phonon energy, presence of heavymetal oxides such as PbO, Bi2O3, WO3 or TeO2 in an oxide glass leadsto decrease of its phonon energy [7]. Then the host glasses with lowphonon energy provide less non-radiative relaxation rates and highquantum efficiencies of dopant ions [8,9].

GeO2 is a typical glass former, while Bi2O3 and PbO are condi-tional glass formers [10]. It means that GeO2 alone can form a glassnetwork consisting of tetrahedral and octahedral structural unitsof Ge bonded with each other by oxygen bridges. Despite high

polarizability of the Bi3+ and Pb2+ ions, Bi2O3 and PbO as non-clas-sical network formers may build glass networks in the presence ofconventional glass former oxides such as SiO2, GeO2, B2O3 and P2O5

[11–13]. In the case of the glass consisting of PbO and Bi2O3 in thenetwork are present [BiO3], [BiO6], [PbO3] and [PbO4] structuralunits [14].

Energy transfer between Tb3+ and Eu3+ has been investigatedsince years, [15–20]. Typically in several materials the excitationenergy transfer from Tb3+ to Eu3+ was reported [19-21]. Specificallythe energy transfer from Tb3+ to Eu3+ has been recognized as theeffect that enhances the red emission of Eu3+ in Li2O–LiF–B2O3–CdO glasses [20].

On the other hand, sometimes the energy transfer between Tb3+

ions and from Eu3+ to Tb3+ has been reported. Tb3+ and Eu3+ acti-vated Y2O3 crystals were investigated both by static and kineticanalysis of luminescence by Anh et al. [17]. In particular theTb3+ ? Tb3+ and Tb3+ ? Eu3+ energy transfer processes were dom-inant. Energy transfer processes between Tb ions and from Eu3+ toTb3+ in Cs2NaTb1�xEuxCl6 has been also observed by Bettinelli andFlint [18].

The aim of the study is to provide more information about theenergy transfers that occur between co-doping Eu3+ and Tb3+ ionsin a glassy matrix of the type GeO2–PbO–Bi2O3. We present thespectral properties of this glass system codoped with both the lan-thanide ions such as absorption in UV–vis and infrared regions,luminescence, luminescence excitation and time resolved lumines-

634 B. Kuklinski et al. / Optical Materials 36 (2014) 633–638

cence spectra. The luminescence measurements confirmed theexistence of two types of excitation energy transfer, viz. betweenTb3+ ? Tb3+ and Tb3+ ? Eu3+.

2. Experimental procedures

2.1. Sample preparation

Glasses with the nominal composition 79GeO2–19PbO–1Bi2O3–xEu2O3–(1 � x)Tb2O3, concisely written as GeO2–PbO–Bi2O3:-xEu,(1 � x)Tb, (where x = 0, 0.1, 0.2, 0.3, 05 and 1 mol.%) and glassmatrix 80GeO2–19PbO–1Bi2O3 (shortly 80GeO2–19PbO–1Bi2O3)for comparison were prepared by the melt quenching techniqueusing reagent grade purity GeO2, Bi5O(OH)9(NO3)4, Pb(NO3)2,Eu(NO3)3�5H2O and Tb(NO3)3�5H2O (all from Aldrich Co.) in suit-able proportions.

The mechanically homogenized mixtures in corundum cruci-bles were heated at a rate 10 K min�1 and melted in air at1100 �C for 1 h. The melts were poured onto a brass plate andimmediately annealed at 420 �C for 3 h to relieve residual stress.

The glass samples were analysed by means of X-ray diffraction.The patterns obtained did not reveal any crystalline phase in thesamples.

0 200 400 600 800 1000

-30

-25

-20

-15

-10

-5

Tm

989

759

Tc

Tg

479

Hea

t Flo

w /

mW

g-1

Temperature t, oC

Fig. 1. DSC curves of the 80GeO2–19PbO–1Bi2O3 matrix glass at heating process.

2.2. Apparatus

Powder X-ray diffraction (XRD) analysis of the as-quenchedmelt was made using a Bruker diffractometer of the D2PHASERmodel employing Cu Ka radiation operated at 30 kV and 10 mA.The XRD patterns were collected using scanning step of 0.02�and counting time 0.4 s per step.

The glass transition (Tg), crystallization (Tc) and melting (Tm)temperatures were determined by TA Instruments differentialscanning calorimeter (DSC) of the SDT Q600 model using a heatingrate 10 K min�1.

The FTIR spectra were recorded on a Bruker spectrometer(IFS66) using the the KBr pellet technique in the range of 400–1200 cm�1. For good quality spectra the glass samples werecrushed to a fine powder in an agate mortar.

The optical absorption spectra at room temperature, were ob-tained in a Perkin–Elmer UV–vis spectrophotometer of the Lambda35 model equipped with integration sphere.

Steady state emission and excitation spectra were measuredupon front an excitation and observation of the sample fluores-cence using the spectrofluorometer built in our laboratory. Theexcitation source was a 450 W xenon lamp (OSRAM). The wave-lengths required for excitation and observation were selectedusing two prismatic monochromators SPM-2 (Carl Zeiss Jena).The emission was detected with a R928 photomultiplier (Hamama-tsu). Emission spectra were corrected for the sensitivity of the pho-tomultiplier tube and monochromator sensitivity. The excitationspectra were additionally corrected for the intensity of the xenonlamp.

The experimental setup for the luminescence kinetics and timeresolved spectroscopy consists of a PL 2143 A SS laser pumpingthat generates 30 ps pulses with a frequency of 10 Hz. In our exper-iments we have used 355 nm excitation wavelengths. The detec-tion part consists of a 2501S (Bruker Optics) spectrograph and amodel C4334-01 streak camera (Hamamatsu). The time resolvedemission spectra and the luminescence decays were obtained fromthe streak camera images by integrating over selected time andwavelength intervals, respectively.

3. Results and discussion

3.1. XRD and DSC

The glass state was confirmed by the absence of XRD peaks.Fig. 1 shows DSC curve of the studied GeO2–PbO–Bi2O3 glass ma-trix. The endothermic signal at 479 �C indicates the glass transitiontemperature Tg. The exothermic peak at 759 �C (Tc) is due to crys-tallization of hexagonal GeO2, while the endothermic peak at989 �C (Tm) is attributed to partial melting of the crystalline phase.

3.2. FTIR

The FTIR spectrum of the undoped GeO2–PbO–Bi2O3 glass ma-trix is presented in Fig. 2. The spectrum consists of two broadand intense absorption bands placed at 830 and 560 cm�1. Thereare also three shoulders located at ca. 1080, 960 and 760 cm�1.The band peaked at about 560 cm�1 is a resultant band consistingof overlapped bands attributed to Ge–O–Ge bending and symmet-ric stretching vibration in [GeO4] tetrahedral units [22], Bi–O�

stretching vibration of [BiO6] units [23–25] and Pb–O symmetricalbending vibration [26]. While under the 760 nm shoulder are pres-ent two bands. The first is assigned to stretching vibration mode ofthe Ge–O–Ge bond in [GeO6] octahedral units [27] and the secondis related to antisymmetrical stretching vibration mode of the Ge–O–Ge in [GeO4] tetrahedral units [22]. The dominated band at830 cm�1 is due to vibrations in [BiO3] groups [28,29]. Presenceof this band indicates that in the glass structure not only [BiO6]but also [BiO3] units participate. Under the next shoulder at ca.960 cm�1 can be probably identified bands peaked at 850 and890 cm�1 corresponding to Bi–O symmetrical stretching vibrationin [BiO3] and [BiO6] structural units [30] and a band related toPb–O symmetrical stretching vibrations in [PbO3] and [PbO4] units[31]. While Pb–O asymmetrical bending oscillation was identifiedunder shoulder at 1080 cm�1 [26].

3.3. UV–vis absorption

The absorption spectra of the glasses doped with Tb3+ (x = 0.0,curve a), Eu3+ (x = 1.0, curve b) as well as Tb3+ and Eu3+ (x = 0.5,curve c) are shown in Fig. 3. The absorption edge of the glass ma-trix exists at ca. 340 nm. The weak absorption bands of Tb3+ ionsare observed at 368, 378 and 485 nm and are attributed to 7F6 ?5D2, 5D3 and 7F6 ?

5D4 transitions, respectively (curve a). Whiledistinct bands peaked at 378, 394, 414, 465 and 529 nm are as-

1200 1000 800 600 4000.1

0.2

0.3

0.4

0.5

0.6

0.7Tr

ansm

ittan

ce T

Wavenumber ν / cm-1

960

830760

5601080

Fig. 2. FTIR spectra of the 80GeO2–19PbO–1Bi2O3 matrix glass.

350 400 450 500 550

0,1

0,2

0,3

0,4

0,5

Abs

orba

nce

A

Wavelength λ / nm

a

b

c

5D4

7F0-5L6

7F0-5D3

7F0-5D2

7F0-5D1

7F6-5D2,

5D3

7F6-5D4

d

7F0-

Fig. 3. Optical absorption spectra of (a) the pure 79GeO2–19PbO–1Bi2O3 glass andthe doped 79GeO2–19PbO–1Bi2O3–xEu3+–(1 � x)Tb3+ glasses, where x: (b) 0.0, (c)1.0 and (d) 0.5 in mol.%.

14000 16000 18000 20000 22000 24000

Emis

sion

inte

nsity

I / a

rb. u

nits

Wavenumber ν / cm-1

λexc = 490 nm

λexc = 490 nm

λexc = 468 nm

a

b

c

5D0-7F4

7F3

700 600 500

5D4-7F0,1,2

Wavelength λ / nm

7F2

7F1,0

7F3

7F4

7F5

Fig. 4. Steady state luminescence spectra of the 79GeO2–19PbO–1Bi2O3–xEu3+–(1 � x)Tb3+ glasses: (a) x = 0.0, (b) x = 1.0 and (c) x = 0.5.

B. Kuklinski et al. / Optical Materials 36 (2014) 633–638 635

signed to 7F0 ?5D4, 5L6, 5D3, 5D2, 5D1 transitions in Eu3+ ions

(curves b and c), respectively.

3.4. Luminescence (emission, excitation, lifetime)

The emission spectrum of the GeO2–PbO–Bi2O3 glass dopedwith Tb3+ ions excited with 490 nm (that is attributed to the tran-sition from the ground 7F6 state to the 5D4 state of Tb3+) is pre-sented in Fig. 4 (curve a). The spectrum consists of three linespeaked at 18500, 17230 and 16140 cm�1 related to transitionsfrom the 5D4 state to the 7F5, 7F4 and 7F3 states of Tb3+, respectively.

The transitions from the 5D4 to the 7F2, 7F1 and 7F0 are representedby weak bands extended between 14200 and 15800 cm�1.

Luminescence spectrum of the glass doped with Eu3+ excitedwith 468 nm (that corresponds to the transition from the ground7F0 state to the 5D2 state of Eu3+) is shown in Fig. 4 (curve b). Thesharp lines located at 16,950, 16,275, 15,300 and 14,140 cm�1 cor-respond to transitions from the 5D0 states to the 7F0,1,2, 7F3, 7F4 and7F5 states, respectively.

While the luminescence of the GeO2–PbO–Bi2O3:0.5Eu,0.5 Tbglass system under excitation wavelength 490 nm (that is relatedto the excitation 7F6 ?

5D4 transition in Tb3+ ion) is demonstratedin Fig. 4 (curve c). This spectrum is obviously a superposition of theEu3+ and Tb3+ luminescence, although the excitation of 490 nmdoes not fit the transition in the Eu3+ ions.

For reasons of an energy transfer between dopants, lumines-cence excitation spectra are presented in Fig. 5 (curves a–c). Curvea represents the luminescence excitation spectrum of the GeO2–PbO–Bi2O3 glass doped with Tb3+; the emission monitored at540 nm (18,500 cm�1). The spectrum consists of a band peakedat 490 nm (20,400 cm�1) and attributed to 7F6 ?

5D4 as well as sev-eral bands placed between 26,200 and 32,000 cm�1 correspondingto the 7F6 ? 5DJ<3,5LJ transitions. The excitation spectrum of theglass with Eu3+, monitored at 707 nm (14140 cm�1) is presentedby curve b. The spectrum consists of lines peaked at 16800,18,520, 21,350, 24,200, 25,450, 26,250 and 27,670 cm�1 that areattributed to transitions from the ground state 7F0 and the 7F1 stateto the excited states 5D0, 5D1, 5D2, 5D3, 5L6, 7GJ and 5D4, respectively[32]. In addition, weak lines of energy greater than 28000 cm�1 areobserved.

The luminescence excitation spectrum of the GeO2–PbO–Bi2O3:0.5Eu,0.5 Tb system monitored at 707 nm (14,140 cm�1) thatcorresponds to the 5D0 ? 7F4 transition in the Eu3+ ion is presentedas curve c in Fig. 5. The spectrum is rather similar to the excitationspectrum of the Eu3+ doped glass (curve b). There is only oneevident difference between the excitation spectrum of the Eu3+

emission in the GeO2�PbO�Bi2O3:0.5Eu,0.5 Tb and GeO2

�PbO�Bi2O3:Eu seen at the spectral region between 19,000 and21,000 cm�1, where additional band at 490 nm (20,400 cm�1)

16000 20000 24000 28000 32000

7F6-

λem = 707 nm

Inte

nsity

I / a

rb. u

nits

λem = 540 nm

λem = 707 nm

Wavenumber ν / cm-1

a

b

c

7F6-5D4

5DJ<3,5LJ

7F0-5D0

5D1

5D25D3

5LJ

7GJ 5D4

5D4

600 550 500 450 400 350Wavelength λ / nm

Fig. 5. Luminescence excitation spectra of the 79GeO2–19PbO–1Bi2O3–xEu3+–(1 � x)Tb3+ glasses: (a) x = 0.0, (b) x = 1.0, and (c) x = 0.5.

0 2 4 6 8 10 12 1410-2

10-1

100

101

102

103

c

b

5D4-7F5

5D0-7F4

5D0-7F4

5D4-7F5

Inte

nsity

I / a

rb. u

nits

Time τ / ms

a

Fig. 6. Luminescence decay profiles of the 79GeO2–19PbO–1Bi2O3–xEu3+–(1 � x)Tb3+ glasses: (a) x = 0, (b) x = 1, and (c) x = 0.5, monitored at maximum ofluminescence Tb3+ (5D4 ?

7F5) and/or Eu3+ (5D0 ?7F4). Luminescence was excited

with pulse laser with wavelength 355 nm.

0,0 0,2 0,4 0,6 0,8 1,01,3

1,4

1,5

1,6

1,7

1,8

1,9

Tb3+

Life

time

τ / m

s

Eu concentration x / mol.%

Eu3+

5D0-7F4

5D4-7F5

Fig. 7. Luminescence of Tb3+ (rectangles) and Eu3+ (balls) in the 79GeO2–19PbO–1Bi2O3–xEu3+–(1 � x)Tb3+ glasses as a function of x.

636 B. Kuklinski et al. / Optical Materials 36 (2014) 633–638

appears in the excitation spectrum of the GeO2–PbO–Bi2O3:0.5-Eu,0.5 Tb sample. This additional band corresponds to the 7F6 ?5D4 transition in Tb3+ ion.

It is observed that intensity of the line related to the 7F6 ? 5D4

(Tb3+) transition in the spectrum c is smaller in comparison to theline representing direct excitation of the Eu3+ ion. Surprisingly, thelines corresponding to the 7F6 ? 5DJ<3, 5LJ transitions of Tb3+ withenergies between 26,200 and 32,000 cm�1 (see spectrum a) arenot seen in the excitation spectrum c. The reason for these effectsmay be partly overlapping of 7F6 ?

5DJ<3,5LJ transitions in Tb3+ withthe internal transitions of Eu3+. Moreover, under excitation of theGeO2–PbO–Bi2O3:0.5Eu,0.5 Tb system in the region between24,000 and 30,000 cm�1 effectively and directly the Eu3+ is excited.The energy transfer from Tb3+ to Eu3+ cannot take place when theEu3+ is in excited state. This is the second reason why we the linesrelated to the 7F6 ? 5DJ<3, 5LJ transitions in Tb3+ in the excitationspectrum c are not seen.

Luminescence kinetics of the all samples was measured. The de-cay profiles of the 5D4 ?

7F5 luminescence of the GeO2–PbO–Bi2-

O3:Tb (a), 5D0 ?7F4 luminescence of GeO2–PbO–Bi2O3:Eu (b) and

5D4 ?7F5,

5D0 ?7F4 luminescence of GeO2–PbO–Bi2O3:0.5Eu,0.5 -

Tb (c) are presented in Fig. 6. It was found that all considered emis-sion decay profiles are mono-exponential. The emission line5D0 ?

7F4 of Eu3+ (peaked at 707 nm) have been chosen due to factthat in this region there is lack of Tb3+ luminescence.

The plot of decay time of the 5D0 ?7F4 luminescence of Eu3+

and the 5D4 ? 7F5 luminescence Tb3+ vs. Eu content x are exhibitedin Fig. 7. Estimated emission decay time values (related to the5D0 ?

7F4 transition in Eu3+) do not depend on Eu3+ concentrationwithin experimental error for x = 0.1, 0.2, 0.3 and 0.5 and are equalto �1.8 ms, whereas the 5D4 ? 7F5 luminescence decay time de-creases from 1.67 to 1.35 ms with decreasing of the Tb3+ content,i.e. increasing of the Eu3+ concentration x from x = 0 to 0.5 mol.%.In our opinion this lifetime shortening is due to appearance of anew pathway for depopulation of the 5D4 state of Tb3+ by non-radi-ative energy transfer to Eu3+.

To certify presence of the possible pathways of the Tb3+ ? Eu3+

energy transfer, the luminescence spectrum of Tb3+ and excitation

spectrum of the Eu3+ should be compared. It was noticed that theTb3+ emission line related to the 5D4 ? 7F5 transition at18,500 cm�1 is in energetic resonance with excitation band of theEu3+ luminescence assigned to the 7F0, 7F1 ? 5D1 transition at18,520 cm�1.

The resonance is very effective since both the bands have thehalf-width equal to 600 cm�1. Following the suggestion presentedin Ref. [33] we can consider also the resonance between the 5D4 ?7F4 transition in Tb3+ at 17230 cm�1 and 7F0, 7F1 ? 5D0 in Eu3+ at16,950 cm�1.

It was observed that luminescence lifetime of Eu3+ in the GeO2–PbO–Bi2O3 glass doped only with Eu3+ content of 1 mol% is shorterthan lifetime of the Eu3+ for the glass samples co-doped with Eu3+

and Tb3+ ions. The co-doped samples contain less than 1 mol.% ofEu3+ ions (i.e. 0.1–0.3 and 0.5 mol%). Therefore the effect of theEu3+ lifetime shortening in the the GeO2–PbO–Bi2O3:Eu (1 mol%of Eu3+) is related to concentration quenching of the Eu3+ emission.The Eu3+ emission decay time is longer for the co-doped glasses,although in all the samples the total concentration of lanthanidedopants was 1 mol.%. Therefore may be concluded that energytransfer from Eu3+ to Tb3+ does not exist.

Emission spectra of the synthesized series of glasses with thecomposition 79GeO2–19PbO–1Bi2O3–xEu2O3–(1 � x)Tb2O3 (where

14000 16000 18000 20000 22000

Emis

sion

inte

nsity

I / a

rb. u

nits

Wavenumber ν / cm-1

λexc = 376 nm

b

c

d

a

750 700 650 600 550 500Wavelength λ / nm

Fig. 8. Emission spectra of the 79GeO2–19PbO–1Bi2O3–xEu3+–(1 � x)Tb3+ glasses,where x = 0.1 (a), 0.2 (b), 0.3 (c) and 0.5 (d) in mol.%. Excitation wavelengthkexc = 376 nm.

Fig. 9. CIE chromatic coordinates diagram of the 79GeO2–19PbO–1Bi2O3–xEu3+–(1 � x)Tb3+ glasses, where x = 0.1 (A), 0.2 (B), 0.3 (C) and 0.5 (D) in mol.%.

Table 1The CIE chromatic coordinates of the 79GeO2-19PbO-1Bi2O3-xEu3+-(1-x)Tb3+ glasses.

Glass Lanthanide molar ratio (x) X Y

A 0.1 0.41 0.47B 0.2 0.48 0.48C 0.3 0.52 0.43D 0.5 0.57 0.38

B. Kuklinski et al. / Optical Materials 36 (2014) 633–638 637

x = 0.1, 0.2, 0.3 and 0.5) are shown in Fig. 8. It is well known thatunder 376 nm excitation, the emission spectra of these glasses dis-play green and red emission coming from Tb3+ and Eu3+,respectively.

It is interesting to noticed that in the luminescence spectrameasured under UV excitation, emission from higher excited states(5D3 in the case of Tb3+ and 5D1–3 for Eu3+) was not observed for any

concentration of Eu3+ (Fig. 8). This is a result of non-radiative relax-ation of the higher excited states. In the case of Tb3+ ion, theabsorption line related to the 5D3 state appears at the energy cor-responding the fundamental absorption edge (Fig. 3). However, it isnot the case of the 5D1 state of Eu3+. This absorption line is placedat the beginning of the broad band absorption. Thus, the lumines-cence excitation spectra (Fig. 5) do not reproduce the absorptionspectra as far as the broad band absorption or fundamental absorp-tion edge are considered. The states seen in absorption just belowthe fundamental absorption edge can be related to the chargetransfer effect (CT) in Eu3+ and impurity trapped exciton states(ITE) in the case of Tb3+. These can be responsible for non-radiativedepopulation of higher excited states. Influence of ITE on efficiencyof Tb3+ luminescence is discussed in Ref. [34]. The second possiblereason of depopulation of the 5D1 state of Eu3+ and the 5D3 of Tb3+

can be clustering of the Tb3+ and Eu3+ ions in the GeO2–PbO–Bi2O3

matrix.

3.5. Chromaticity

The corresponding chromatic coordinates of these glasses (forx = 0.1, 0.2, 0.3, 0.5 and kexc = 376 nm) are demonstrated in Fig. 9.The result shows that the color tones change from near cool white(at point A, X = 0.41, Y = 0.47) to red (at point D, X = 057, Y = 0.38)with increasing the concentration of Eu3+ from x = 0.1 to 0.5 inthe glasses, respectively (Table 1).

4. Conclusions

The FTIR studies show that structure of the three-componentglass matrix of the type GeO2–PbO–Bi2O3 consists of such struc-tural units as tetrahedral [GeO4] as well as octahedral [GeO6] and[BiO6].

In the studied 79GeO2–19PbO–1Bi2O3:xEu3+,(1�x)Tb3+ glasses(where x = 0.1, 0.2, 0.3, 0.5 mol.%) appeared excitation energytransfer from Tb3+ to Eu3+ and it is related to cross relaxation, asfollows: Tb3+(5D4 ? 7F5) ? Eu3+(7F0,7F1 ? 5D1) and Tb3+(5D4 ?7F4) ? Eu3+(7F0, 7F1 ? 5D0), what is in accordance with results inRef. [34].

The concentration quenching of Eu3+ emission for concentration1 mol.% in the glass system is observed. In contrary, in the case ofTb3+ no concentration quenching effect in the Tb3+ luminescence ispresent or this effect is much weaker than non-radiative energytransfer from Tb3+ to Eu3+.

Under UV excitation, the emission from the higher excitedstates (5D3 for Tb3+ and 5D1�3 for Eu3+) was not observed, althoughthese states are seen in the respective excitation spectra. It sug-gests that for the higher excited states depopulation can be a resultof existence of ITE states in the case of Tb3+ and CT effect for Eu3+

and/or a possible clustering of the Tb3+ and Eu3+ ions.Energy transfer from Eu3+ to Tb3+ was not evidenced in our co-

doped samples if the Tb3+ ions appeared in higher concentrationthan Eu3+.

If the Eu3+ content x increases from 0.1 to 0.5, the emission colorof the GeO2–PbO–Bi2O3–xEu3+,(1�x)Tb3+ glasses changed forkexc = 376 nm from cool white to red.

638 B. Kuklinski et al. / Optical Materials 36 (2014) 633–638

Acknowledgements

This work is supported by UDA-POIG.01.01.02-02-006/09-00.We would like to thank Dr. Konrad Trzebiatowski and Witold Lizak,M.Sc. (Gdansk University of Technology) for information regardingthe preparation procedure.

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