Materials Science and Engineering B 156 (2009) 73–78

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Materials Science and Engineering B

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Polyspectral white light emission from Eu3+, Tb3+, Dy3+, Tm3+ co-doped

GdAl3(BO3)4 phosphors obtained by combustion synthesis

E.C. Fuchsa,∗,1, C. Sommerb, F.P. Wenzlb, B. Bitschnaua, A.H. Paulitschc,1, A. Mühlangera, K. Gatterera

a Institute of Physical and Theoretical Chemistry, Graz University of Technology, Rechbauerstrasse 12, 8010 Graz, Austriab Institute of Nanostructured Materials and Photonics, Joanneum Research Forschungsges.mbH, Franz-Pichler-Strasse 30, 8160 Weiz, Austriac Institute of Hygiene, Medical University of Graz, Universitätsplatz 4, 8010 Graz, Austria

a r t i c l e i n f o

Article history:

Received 11 July 2008

Received in revised form 4 November 2008

Accepted 21 November 2008

Keywords:

GAB

Gadolinium aluminium borate

White light emission

White phosphor

a b s t r a c t

Polycrystalline sub-micron-sized GdAl3(BO3)4 phosphors co-doped with Eu3+, Tb3+, Dy3+ and Tm3+ have

been prepared by combustion synthesis with urea. The phosphors have been characterised by X-ray

diffraction, scanning electron microscopy, excitation and emission spectroscopy. The chromaticity co-

ordinates and the colour temperatures of the fluorescence of the materials presented have been calculated

and analysed with Commission Internationale l’Eclairage (CIE) programs and diagrams. Depending on

the excitation wavelength, different colour temperatures of the light emitted can be achieved. Due to its

polyspectral nature, the emitted light reveals a high colour rendering index.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Gadolinium aluminium borate (GAB), which is isostructural

and very similar in its physical properties to yttrium aluminum

borate (YAB), is a well-known host for rare-earth ions. Single crys-

tals of such materials exhibit interesting optical properties due

to their high non-linear optical coefficient, strong absorption in

the vacuum-UV, transparency in the near UV, high radiation dam-

age threshold, environment durability and chemical stability [1–6].

Trivalent rare earth ions replace gadolinium or yttrium in the crystal

structure. Recently, polycrystalline powders of Ce,Dy:GAB produced

by combustion synthesis were investigated for their white-light

emitting properties upon UV illumination [7,8]; the lumines-

cence properties of Eu:GAB/YAB, Tb:GAB/YAB and Eu,Tb:GAB/YAB

micropowders have been of interest as well [17–19]. In this paper,

we report a detailed study on the luminescence properties of

Dy,Tm:GAB, Dy,Tm, Eu:GAB, Dy,Tm,Tb:GAB, and Eu,Tb,Dy,Tm:GAB

and compare them to one of the materials presented in the litera-

ture [7,8].

∗ Corresponding author. Tel.: +31 58 2846398.

E-mail address: elmar.fuchs@wetsus.nl (E.C. Fuchs).1 Present address: Wetsus Center of Excellence for Sustainable Water Technology,

Agora 1, 8900 CC Leeuwarden, The Netherlands.

2. Experimental

It has been shown by Yang et al. [8] that rare earth doped GAB

powders can be successfully prepared by the simple method of

combustion synthesis. Therefore, this method has been used to

prepare the materials listed in Table 1, starting from aqueous solu-

tion of the nitrates with their respective stoichiometry. An excess of

urea and the stoichiometric amount of boric acid were added under

vigorous stirring and boiling the solution until all components dis-

solved completely. Then, the beakers containing the solutions were

covered with finely woven steel mesh to avoid the spilling of prod-

uct in the course of the combustion process. The beakers were

put into an oven for 15 min at 500 ◦C. The generated precursor

powders were ground carefully in a TiC mortar and then sintered

at 1100 ◦C for 24 h. After cooling to room temperature, rare earth

doped GdAl3(BO3)4 sub-micrometer-sized powder phosphors were

obtained.

For comparison and in order to verify spectral assignments, sin-

gle crystals of Tm:YAB (1%), Tb:YAB (8%), Eu:YAB (10%) and Dy:YAB

(8%) (all mol%) were grown according to the procedure described

in Ref. [9].

The crystallinity as well as the microscopic and the macroscopic

homogeneity of the phosphors were monitored by X-ray diffraction

and by scanning electron microscopy (SEM), respectively. For SEM,

a sample of each powder was applied on a disc with a pipette were

it was fixed on double-faced adhesive tape. The investigation of the

samples was performed with a Philips XL30 SEM using the high

vacuum mode (emission electrons detection, acceleration voltage

0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.mseb.2008.11.024

74 E.C. Fuchs et al. / Materials Science and Engineering B 156 (2009) 73–78

Table 1Phosphors prepared with combustion synthesis, all contents are in mol%.

Short-cut notation Ce content Eu content Tb content Dy content Tm content

Ce,Dy:GAB(2,3) 2% – – 3% –

Ce,Dy:GAB(4,3) 4% – – 3% –

Ce,Dy,Tm:GAB(2,3,2) 2% – – 3% 2%

Dy,Tm:GAB(3,1) – – – 3% 1%

Dy,Tm:GAB(3,2) – – – 3% 2%

Dy,Tm:GAB(3,4) – – – 3% 4%

Eu,Dy,Tm:GAB(2,3,2) – 2% – 3% 2%

Tb,Dy,Tm:GAB(2,3,2) – – 2% 3% 2%

Eu,Tb,Dy,Tm:GAB (1,1,3,2) – 1% 1% 3% 2%

Eu,Tb,Dy,Tm:GAB (2,2,3,2) – 2% 2% 3% 2%

20 kV, operating distance 10 mm), which offered the best resolu-

tion. Pictures were taken using the software of the microscope.

The X-ray powder diffraction data were collected with a labo-

ratory powder diffractometer (Philips X-pert) from a flat sample

(Bragg–Brentano geometry) using Cu K� radiation. The measure-

ments were carried out with 15–95◦ 2�, stepwidth 0.02◦, constant

counting time of 15 s per step, and a K�2/K�1 ratio of 0.5. Rietveld

refinement was performed using FULLPROF [10,11]. The space group

is, according to YAB, hexagonal R32, the starting model for struc-

tural parameters were taken from Refs. [12,13]. A pseudo-Voigt

profile shape function was used. The background was refined with

a polynomial function. An amount of preferred orientation was cor-

rected by the March–Dollase function. For some of the samples, in

particular for those which were sintered for longer times, a small

fraction of a monoclinic modification was observable [14]. How-

ever, in the case of the most prominent phosphor presented in this

work, Eu,Tb,Dy,Tm:GAB(2,3,2,2), this amount is negligible, and the

crystallographic data for this phosphor is given in Table 2. The emis-

sion spectra were recorded with a Spectrofluorometer (Shimadzu

RF5301-PC with a Xe light source and an R3788 photomultiplier),

which was also used to record the excitation spectra; or alterna-

tively with an Omega-10 spectrometer (Bruins instruments) which

has been modified to record emission spectra using a Hönle Blue-

point 4 UV source (internal BP4 320–390 nm UV filter) with an

additional Schott UG 1 UV filter having a maximum transparency

at 390 nm, which is referred to as “390 nm excitation” hereafter.

In order to compare emission intensities, 150 mg of each phosphor

were applied as layers with constant diameter inside a horizontally

mounted quartz cuvette on top of the UG 1 UV filter which was

placed 2 cm above the end of the excitation light guide and cooled

by external ventilation. The emission was fed through a light guide

(Lumatec, 5 mm diameter), which was mounted 1 cm above the

phosphor layer on top of the cuvette, and recorded in the range from

400 to 700 nm with a slit width of 1.5 mm at the spectrometer. Each

spectrum was cumulated 50 times to increase the signal-to-noise

ratio.

Table 2Crystallographic data of Eu,Tb,Dy,Tm:GAB (2,2,3,2).

Atom (position) Lattice parameters (Å)

a 9.314(1) c 7.263(2)

x/a y/b z/c

Gd, Eu, Tb, Dy, Tm (3a) 0 0 0

Al (9d) 0.547(3) 0 0

B(1) (3b) 0 0 0.5

B(2) (9e) 0.43(1) 0 0.5

O(1) (9e) 0.850(8) 0 0.5

O(2) (9e) 0.589(7) 0 0.5

O(3) (18f) 0.449(3) 0.148(10) 0.515(3)

Occupation factor: Gd 0.91, Eu 0.02, Tb 0.02, Dy 0.03 and Tm 0.02.

The absorption spectra of the crystals were recorded with the

Omega-10 spectrometer above mentioned in absorption mode with

0.5 mm slit width and 400 cummulations.

The colour rendering index (Ra), the colour temperatures and

the chromaticity co-ordinates were calculated according to the

CIE Technical Report CIE 13.3-1995 with the software that comes

with the report. For the colour rendering index, the chromatic-

ity co-ordinates of 8 specified test colours were measured in the

CIE1978u‘v’ chromaticity plane under illumination by the emission

of the phosphors, and then compared with the chromaticity co-

ordinates of the same colours under daylight illumination, if their

colour temperature was above 5000 K, or with a black body radiator

if their colour temperature was below that. After inverting and nor-

malising these differences, eight separate colour rendering indices

(Ri) were obtained. A value of 100 means that there is no change in

colour rendering, whereas a value of 0 stands for the total inability

of the light source to render the specified colour. The general colour

rendering index (Ra) was obtained as the average of the 8 Ri values

[15,16].

3. Results and discussion

When comparing the emission properties of the phosphors

listed in Table 1, Ce,Dy:GAB(4,3) showed significantly higher total

emission than Ce,Dy:GAB(2,3); the lowest emission was seen

from the Ce,Dy,Tm:GAB(2,3,2) phosphor which is possibly due to

quenching by cross-relaxation between Ce3+ and Tm3+. In the case

of Eu,Tb,Dy,Tm:GAB, the increase of the Eu3+ and Tb3+ concentra-

tion from 1 to 2% lead to an increase of their emission while the Tm3+

and Dy3+ emissions were slightly reduced, resulting in an net emis-

Fig. 1. Observed, calculated and difference profiles for the Eu,Tb,Dy,Tm:GAB(2,2,3,2)

compound (Rietveld refinement).

E.C. Fuchs et al. / Materials Science and Engineering B 156 (2009) 73–78 75

sion increase, however. Therefore, the Eu,Tb,Dy,Tm:GAB(2,2,3,2)

phosphor was chosen to be investigated more thoroughly in this

work using Rietveld refinement, SEM, and wavelength-depended

excitation spectra.

The Rietveld refinement shown for the sample Eu,Tb,Dy,

Tm:GAB(2,3,2,2) resulted in a reasonable agreement between mea-

sured and calculated patterns in hexagonal space group R32 (Fig. 1).

Positional parameters and isotropic thermal parameters for the

atoms were allowed to vary while the occupancy parameters for

the atoms were held constant during refinement. No crystal distor-

tion – within limits of errors – was observed compared to GAB (see

Table 2). A constraint refinement was chosen because of the pre-

dominant scattering of Gd and the other rare earths over the lighter

atoms. Lattice parameters show an increase of about 1% compared

to GAB.

Fig. 2 shows SEM images of the Eu,Tb,Dy,Tm:GAB phosphor

as a representative example. The powder consisted of (sub)-

micrometer-sized particles (see Fig. 2b) that agglomerate to larger

lumps of material (see Fig. 2a). The average particle size was

between 0.5 and 2 �m, the average cluster size about 100 �m. The

morphology of the phosphor is typical for this type of synthesis

[7,8].

The excitation spectra for Eu,Tb,Dy,Tm:GAB(2,2,3,2) are given in

Fig. 3a–d and are compared to the absorption spectra of the singly

doped YAB crystals. The Russell–Saunders terms of the target states

Fig. 2. SEM image of the Eu,Tb,Dy,Tm:GAB(2,2,3,2) phosphor, magnification 500 (a)

and 2000 (b).

Fig. 3. Excitation spectra of Eu,Tb,Dy,Tm:GAB(2,2,3,2), detection at 613 nm (Eu3+:5D0 → 7F2) (a), 541 nm (Tb3+: 5D4 → 7F5) (b), 574 nm (Dy3+: 4F9/2 → 6H13/2) (c) and

452 nm (Tm3+: 1D2 → 3F4) (d) compared to the absorption spectra of the singly doped

YAB crystals (10, 8, 8 and 1% for Eu3+, Tb3+, Dy3+ and Tm3+, respectively).

of the transitions were assigned according to Ref. [20] for Eu3+, Tb3+

and Tm3+, and [27] for Dy3+, respectively. A more detailed analysis

of the singly doped materials is given in the literature (EuAB [21],

Eu:YAB/GAB [2,17], Tb:YAB [18,22], Dy:YAB [23–30] and Tm:YAB

[31]).

In the case of Tm3+ and Dy3+, the spectra of the single crys-

tal and the powder sample show a reasonably good agreement. In

the case of Eu3+ there are two additional features in the excitation

76 E.C. Fuchs et al. / Materials Science and Engineering B 156 (2009) 73–78

spectra of the powder: one broad band from 450 to 550 nm, and a

narrow one around 350 nm. The broad feature resembles a d-d tran-

sition and is probably due to an energy transfer from Fe3+ impurities

introduced by the steel mesh in course of the preparation. Similar

broad features were reported in Fe3+ and Eu3+ co-doped ceramic

materials [32]. The additional narrow band, however, has the typ-

ical half width of a rare-earth transition. More additional narrow

bands can be seen in the excitation spectra of the Tb3+ doped pow-

der sample (Fig. 3b). The position of these bands in the Eu3+ and

Tb3+ doped powder samples corroborates the following interpre-

tation. Low-lying excited states of Eu3+ and Tb3+ are populated

by emissions from the 5D0 and 4D4 states, respectively. In con-

trast to the bulk material, where the lifetimes of these low-lying

excited states is extremely short because of efficient radiationless

de-excitation pathways, their lifetimes are considerably longer in

the nano-structured powder materials, because the non-radiative

relaxation rates are modified in the case of small particles [33]. The

prolonged lifetimes allow for excited state absorption (ESA) from

the 7F2 level (ca. 1000 cm−1 above the ground state) in the case of

Eu3+, and from the 7F5 and 7F4 levels (ca. 2200 and 3400 cm−1 above

the ground state) in the case of Tb3+, respectively. In Fig. 3 these ESA

are identified by asterisks and the target states of the transitions are

indicated.

As can be seen, the four excitation spectra show a considerable

overlap between 350 and 370 nm. Therefore, by broad band exci-

tation in this region, all four ions can be excited simultaneously,

resulting in a multi-band emission which produces a white colour.

White light has already been obtained with similar materi-

als like Ce,Dy:GAB [7,8]. Exchanging Ce3+ with Tm3+ increases the

colour rendering of the emitted light, and a further addition of Tb3+

results in a greenish white, whereas an addition of Eu3+ leads to

a phosphor with a reddish white emission. When both Tb3+ and

Eu3+ are added, a polychromatic white phosphor can be obtained,

whose chromaticity co-ordinates are closest to the white point

whilst intensity and colour rendering index are highest (next to

Eu,Dy,Tm:GAB(2,3,2)) when compared to the other phosphors pre-

sented. The emission spectra of the phosphors described above are

given in Fig. 4, where the Russell–Saunders terms of the emission

bands were assigned according to Ref. [8] for Ce3+, Ref. [17] for Eu3+,

Ref. [18] for Tb3+, Refs. [8,27] for Dy3+ and Ref. [20] for Tm3+, respec-

tively. The CIE1931 xy chromaticity co-ordinates of the phosphors

under 390 nm excitation are depicted in Fig. 5, and a comparison of

their relative emission intensity and their colour rendering index is

given in Fig. 6.

A series of emission spectra with excitation from 350 to 370 nm

is given in Fig. 7. In order to simulate a technical application like

Fig. 4. Emission spectra of Ce,Dy:GAB(4,2) (a), Dy,Tm:GAB(3,2) (b),

Eu,Dy,Tm:GAB(2,3,2) (c), Tb,Dy,Tm:GAB(2,3,2) (d) and Eu,Tb,Dy,Tm:GAB(2,2,3,2)

(e) upon 390 nm excitation. For this figure, the spectra were smoothed using the

method of Savitzky and Golay [34] (width 4.1 nm, second order polynomial).

Fig. 5. CIE1931 xy chromaticity co-ordinates of Ce,Dy:GAB(4,3) (a), Dy,Tm:GAB(3,2)

(b), Eu,Dy,Tm,GAB(2,3,2) (c), Tb,Dy,Tm:GAB(2,3,2) (d) and Eu,Tb,Dy,Tm:GAB(2,2,3,2)

(e) upon 390 nm excitation (all marked with an ×) as well as Gd0.97Al3(BO3)4:Dy0.03

(328 nm excitation) (1), Gd0.97Al3(BO3)4:Dy0.03 (351 nm excitation) (2),

Gd0.9Al3(BO3)4:Dy0.03,Ce0.07 (328 nm excitation) (3) and a commercial white

LED (4) (all marked with a +): (1)–(4) are taken from Ref. [8]. The asterisk marks the

position of the white point.

down-conversion for UV LEDs, the excitation was relatively broad

(15 nm spectral band width).

A comparison of the colour temperature, the relative emission

intensities and the colour rendering indices of these spectra is given

in Fig. 8. It should be noted here that the spectra with 350 and

355 nm excitation were cut off before at 670 and 680 nm, respec-

tively, because of the second order peak of the spectrometer grating.

Therefore, the Eu3+ emission at 700 nm (5D0→ 7F4) is not included

in these spectra. Hence, the presented values for Ra and the inten-

sity might be slightly too low and the colour temperature slightly

too high, however, this lies presumably within an estimated error

of 5%.

Fig. 9 shows the CIE1931 xy chromaticity co-ordinates of the

emission of Eu,Tb,Dy,Tm:GAB(2,2,3,2) as a function of the excitation

Fig. 6. Comparison of the emission intensities and the colour rendering

indices of Ce,Dy:GAB(4,3) (a) Dy,Tm:GAB(3,2) (b), Eu,Dy,Tm:GAB(2,3,2) (c),

Tb,Dy,Tm:GAB(2,3,2) (d) and Eu,Tb,Dy,Tm:GAB(2,2,3,2) (e) upon 390 nm excitation.

E.C. Fuchs et al. / Materials Science and Engineering B 156 (2009) 73–78 77

Table 3Comparison of the colour rendering index Ra and the correlated colour temperature of Ce,Dy:GAB(4,3), Dy,Tm:GAB(3,2), Eu,Tb,Dy,Tm:GAB(2,2,3,2) upon 390 nm excitation,

Eu,Tb,Dy,Tm:GAB(2,2,3,2) under 370 nm excitation (15 nm spectral band width), with a fluorescent lamp (type 32), a tungsten lamp, a daylight distribution (CIE D5500K).

Furthermore, the colour temperatures of similar phosphors and of a commercial white LED are given for comparison [8].

Light source Colour temperature (K) Colour rendering index (Ra)

Ce,Dy:GAB 4,914 3

Tm,Dy:GAB 10,038 30

Eu,Tb,Dy,Tm:GAB (390 nm excitation) 6,609 48

Eu,Tb,Dy,Tm:GAB (370 nm excitation) 3,356 53

Fluorescent tube 32 3,937 57

Tungsten lamp 2,836 99

Daylight CIE D5500K 5,500 100

Gd0.97Al3(BO3)4:Dy0.03 (328 nm excitation) [8] 4,676 –

Gd0.97Al3(BO3)4:Dy0.03 (351 nm excitation) [8] 4,774 –

Gd0.9Al3(BO3)4:Dy0.03,Ce0.07 [8] 6,483 –

Commercial white light LED [8] 6,400 –

Fig. 7. Emission spectra of Eu,Tb,Dy,Tm:GAB(2,2,3,2), excitation from 350 to 370 nm,

15 nm excitation band width.

wavelength. A comparison regarding the colour rendering indices

Ra and the correlated colour temperatures of Ce,Dy:GAB(4,3),

Dy,Tm:GAB(3,2), Eu,Tb,Dy,Tm:GAB(2,2,3,2) upon 390 nm excitation

and Eu,Tb,Dy,Tm:GAB excited with 370 nm (15 nm spectral band

width), with a fluorescent lamp (type 32), a tungsten lamp and a

daylight distribution (D5500K, obtained from the GRAMS32® v. 5.3

colour module) is given in Table 3.

Fig. 8. Comparison of the relative emission intensities, colour temperatures and the

colour rendering indices of Eu,Tb,Dy,Tm:GAB(2,2,3,2), excitation from 350 to 370 nm,

15 nm excitation band width.

Fig. 9. CIE1931 xy chromaticity co-ordinates of Eu,Tb,Dy,Tm:GAB(2,2,3,2), excitation

from 350 to 370 nm, 15 nm excitation band width. The asterisk marks the position

of the white point.

4. Conclusions

Polycrystalline sub-micron-sized GdAl3(BO3)4 phosphors

doped with rare-earth ions have been prepared by combustion

synthesis with urea. Whereas Dy,Tm:GAB(3,2) exhibits a higher

colour rendering index (Ra = 30) than Ce,Dy:GAB(4,3) (Ra = 3),

Eu,Tb,Dy,Tm:GAB(2,2,3,2) reveals even more advantages. In addi-

tion to Dy3+ and Tm3+, this material shows the highest emission

intensities of the phosphors investigated herein. A total of seven

emission bands occurs, resulting in a 40% higher total emission

compared to Ce,Dy:GAB(4,3) and a higher colour rendering index

(Ra = 48). The colour temperature is stable from 350 to 360 nm

excitation at ∼3900 K, and can be gradually lowered ∼3300 K by

increasing the excitation wavelength up to 370 nm. According to

a recent work by Krames et al. [35], such low colour tempera-

tures close to 3200 K (incandescent tungsten halogen lamp) are

a common target for white LEDs in indoor illumination. These

beneficial optical properties in combination with the chemical

inertness, the temperature stability and the high radiation damage

threshold, discloses Eu,Tb,Dy,Tm:GAB(2,2,3,2) as a promising

78 E.C. Fuchs et al. / Materials Science and Engineering B 156 (2009) 73–78

phosphor material for the application with UV sources of high

junction temperatures like high-power LEDs.

Acknowledgment

The authors would like to thank Dr. Paul Hartman of TRI-

DONIC.ATCO, Tridonic Optoelectronics GmBH, Technologiepark 10,

A-8380 Jennersdorf, Austria, for his kind support.

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