Structure and optical properties under VUV/UV excitation of Eu2+ doped alkaline earth aluminate phosphors

Original

Paper

phys. stat. sol. (a) 203, No. 5, 919–929 (2006) / DOI 10.1002/pssa.200521208

Structure and optical properties under VUV/UV excitation

of Eu²⁺doped Alkaline Earth Aluminate Phosphors

*

D. Nötzold , H. Wulff, S. Jilg, L. Kantz, and L. Schwarz

Institute of Chemistry and Biochemistry, Ernst-Moritz-Arndt-University Greifswald, Soldmannstraße 16,

17487 Greifswald, Germany

Received 6 August 2005, revised 28 August 2005, accepted 7 February 2006

Published online 24 March 2006

PACS 61.66.Fn, 78.55.Hx

Polycrystalline hexagonal alkaline earth aluminates M Al O doped by divalent europium were prepared

16 27

3

by solid state reaction. The crystal structure and optical properties at room temperature were investigated.

The structure parameters were refined using Rietveld refinement procedures. The space group of the ma-

2+

terials is P6 /mmc. Eu ions occupy the 2d cation position of the large cation in the unit cell. The blue

3

6

7

2+

emission of the phosphors due to transitions between 4f 5d and 4f configurations of the Eu ion can be

excited by UV radiation and very efficient also by VUV radiation. Spectral position and band width at

half height of the emission band as well as the shape of the excitation spectrum and the luminous flux of

the investigated aluminate phosphors depend on their composition.

Polykristalline hexagonale Erdalkalialuminate M Al O , dotiert mit zweiwertigem Europium, wurden

16 27

3

durch Festkörperreaktion präpariert. Die Kristallstruktur und optische Eigenschaften bei Raumtemperatur

wurden untersucht. Die Verfeinerung der Strukturparameter erfolgte mit Hilfe der Rietveld-Methode. Die

2+

Präparate kristallisieren in der Raumgruppe P6 /mmc. Eu -Ionen besetzen in der Elementarzelle die 2d-

3

Punktlage des großen Kations. Die Lumineszenz der Mischkristalle liegt im blauen Spektralbereich. Sie

6

7

2+

resultiert aus Übergängen von der 4f 5d- in die 4f -Konfiguration der Eu -Ionen. Die Emission kann so-

wohl durch UV-Strahlung als auch sehr effektiv durch VUV-Strahlung angeregt werden. Die spektrale

Position der Emissionsbande und ihre Halbwertsbreite, die Gestalt des Anregungsspektrums und die

Lichtstrom-Werte werden von der Zusammensetzung des Aluminat-Leuchtstoffes beeinflusst.

1 Introduction

Synthetic hexagonal alkaline earth aluminates doped by divalent europium are efficient photolumines-

cent materials. They show a blue emission which is characterized by a high quantum efficiency under

254 nm and VUV excitation [1]. In the past we have investigated these materials to study the influence

of the preparation conditions and of substitutions of the alkaline earth ions in Ba Eu Mg Al O by

0.9

0.1

2

16 27

other cations on the optical properties of these phosphors under 254 nm excitation [2–4].

Recently we dealt especially with the optical properties of these aluminate phosphors under VUV

excitation. We have investigated these materials to study the relations between structural and optical

properties in dependence on the chemical composition of the polycrystalline materials.

The results of these investigations are given in the present paper.

*

Corresponding author: e-mail: KDDNoetzold@t-online.de, Phone: +493834 840531, Fax: +493834 864413

920

D. Nötzold et al.: Structure and optical properties under VUV/UV excitation

2 Experimental

Alkaline earth carbonates, ZnO, γ-Al O , and Eu(NO ) were mixed to prepare polycrystalline materials

2 3 3 3

of the compositions

(Ba Sr ) Eu Mg Al O

16 27

with 0 ≤ y ≤ 1 and 0 ≤ z ≤ 0.2 ,

1–y y 1–z

z

2

(Ba Eu )Mg Zn Al O

1–z z 2–u u 16 27

with 0 ≤ u ≤ 2 and 0 ≤ z ≤ 0.2 , and

with 0 < x ≤ 1 , 0 ≤ y ≤ 1 and 0 ≤ z ≤ 0.2

(Ba Sr ) Eu Mg Al O

1–y y 1+x–z z 2–x 16 27

by solid state reaction. The starting mixture was fired at 1703 K in a water vapour saturated reducing

atmosphere (N /H ). 3 to 5% of the aluminium were used as Na AlF which acts as a flux. NH Cl was

2

3

6

4

added as a second flux in case of the Zn-substituted samples.

Powder diffraction data were recorded using an X-ray powder diffractometer (HZG 4, Seifert-FPM)

with Cu K radiation and a graphite secondary monochromator GSM6. The samples were step-scanned

α

in the range 5° ≤ 2θ ≤ 100° with 10 s fixed time for each 0.02° per 2θ step. The structure parameters

were refined from the X-ray powder diffraction data using the Rietveld method (Program FullProf_2K,

version 2.80, July 2004). Bond lengths were estimated with the help of the program PowderCell (ver-

sion 2.3, December 1999).

The emission spectra were investigated with a monochromator (SPM2, Carl Zeiss Jena) and a photo-

multiplier (M11FVC520, WF Berlin) in connection with a LCD-Digital-Multimeter (M-4660A, Conrad

Elektronik) as detector. The excitation source was a low-pressure mercury arc lamp (HNU6, NARVA) in

connection with an interference filter IF 254. In addition emission spectra after 147-nm excitation and

moreover the excitation spectra were measured at the experimental station SUPERLUMI of HASYLAB

at DESY Hamburg using synchrotron radiation for excitation of the phosphors. Sodium salicylate was

standard for investigations of the excitation spectra. Luminous flux measurements were carried out in a

high-vacuum chamber using alternative a low-pressure xenon arc lamp (INP Greifswald) or a xenon

excimer lamp (Radium Xeradex 20 Excimer radiator, Radium Lampenwerk Wipperfürth) for excitation

of the samples. A V photomultiplier (M10FS29V , WF Berlin) in connection with a LCD-Digital-

λ

Multimeter (M-4660A, Conrad Elektronik) was used as detector.

3 Results and discussion

3.1 Structure

Beside the BaMgAl O with “ideal” β-Al O -structure also the starting material BaMg Al O with

10 17 2 3 2 16 27

smaller content of the large cation crystallizes in the hexagonal crystal system with β-Al O -structure

2 3

[5, 6]. The unit cell consists of blocks with spinel structure containing Al³⁺, Mg²⁺and O^2–ions which are

separated of each other by layers containing one Ba²⁺and one O^2–ion in a relative wide range. The large

cation can occupy different lattice positions in the layer between the spinel blocks. The space group is

P6 /mmc [7]. The first coordination sphere of the Ba²⁺ions consists of eight oxygen ions. Six of them are

situated in the two spinel blocks including the layer with the Ba²⁺ion and two are in this layer [8]. Eu²⁺

ions occupy a lattice position of the large cation in Eu²⁺doped hexagonal aluminates.

3

The X-ray patterns of the polycrystalline materials were compared with those of BaMgAl O [9],

10 17

Ba Mg Al O [10] and SrMgAl O [11]. The investigated aluminates show X-ray patterns

0.956 0.912 10.088 17 10 17

which are typical for the crystal structure of β-Al O .

2 3

Small amounts of crystalline MgAl O [12] and Al O [13] (in BaMg Al O ), MgAl O (in

2 4 16 27 2 4

2

3

2

SrMg Al O ), Al O (in BaZn Al O ), and SrAl O [14] (in Sr Mg Al O ) could be found as addi-

2 4 1.5 1.5 16 27

2

16 27

2

3

2

16 27

tional phases. In BaMg Al O samples the amount of Al O and MgAl O diminishes with growing

2 16 27 2 3 2

4

partial substitution of Mg²⁺ions by Ba²⁺ions [15]. In agreement with this observation neither Al O nor

2

3

MgAl O could be found in Ba Mg Al O . Only a trace of BaAl O [16] was identified as second

2 4 1.5 1.5 16 27

2

4

phase in this sample.

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phys. stat. sol. (a) 203, No. 5 (2006)

921

Table 1 Lattice constants, the ratio c /a , and the volume of the unit cell of aluminate samples M Al O

16 27

0

3

with M : 1. BaMg ; 2. SrMg ; 3. BaZn ; 4. Ba Mg ; 5. Sr Mg .

1.5

3

2

1.5

sample

a [pm]

0

c [pm]

0

c /a

0 0

V [Å³]

1

2

3

4

5

562.29(3)

562.86(4)

561.27(2)

562.21(2)

562.54(2)

2263.43(12)

2234.84(14)

2262.32(7)

2265.46(7)

2236.74(11)

4.0254

3.9705

4.0307

4.0296

3.9761

619.75(6)

613.17(7)

617.21(3)

620.13(3)

613.00(4)

A structure model for the Rietveld refinement of the aluminates M Al O was taken from the data

3 16 27

given by Iyi et al. for Ba Mg Al

0.956 0.912

O

10.088 17

[17] using also the International Tables for Crystallo-

graphy for the space group P6 /mmc [18]. The final refinement parameters R-WP and Gof of the samples

3

range from 0.11 to 0.26 and from 1.5 to 2.4.

The lattice constants a and c of the aluminate BaMg Al O (Table 1) are very similar to those given

0 0 2 16 27

for BaMgAl O :Eu²⁺in Ref. [7, 19].

Substitution of the Ba²⁺ions by the smaller Sr²⁺ions (ionic radii for coordination number eight are 156

10 17

and 140 pm respectively [20]) causes a diminution of the volume of the unit cell and of the ratio c /a in

0 0

both cases investigated due to a decrease of the lattice constant c . The lattice constant a increases in a

0 0

very small extent (Table 1). Its value seems to depend preferentially on the size of the spinel blocks in

the unit cell and this size should hardly be influenced by the substitution of the Ba²⁺ions by Sr²⁺ions.

2+

Substitution of the Mg ions by the somewhat larger Zn ions (ionic radii for the fourfold coordi-

2+

nated ions are 71 pm (Mg ) and 74 pm (Zn ) [20]) does not cause the expected dilatation of the unit

cell. A small diminution of the lattice constants and the volume of the unit cell were observed, the ratio

c₀/a₀somewhat increases (Table 1). Possibly there are covalent parts in the bond between zinc and oxy-

2+ 2+

gen due to the distinct smaller electropositive character of the Zn ion in comparison with the Mg ion.

Table 2 Atomic coordinates of samples M Al O with M : 1. BaMg ; 2. SrMg ; 3. BaZn ; 4. Ba Mg ;

3

16 27

3

2

1.5

5. Sr Mg .

1.5

component

multiplicity

Wyckoff

letters

sample atomic coordinates

x

y

z

Ba

Sr

6h

2d

6h

2d

1

2

3

4

5

0.6802(48)

0.6667

0.3604(96)

0.3333

0.2500

Ba

Sr

0.6882(29)

0.6826(25)

0.6667

0.3763(58)

0.3651(49)

0.3333

Al(1)

12k

1

2

3

4

5

1

2

3

4

5

0.8323(16)

0.8319(12)

0.8332(9)

0.8330(7)

0.8358(8)

0.3333

0.6646(33)

0.6639(25)

0.6664(18)

0.6660(13)

0.6716(15)

0.6667

0.1072(2)

0.1084(2)

0.1065(2)

0.1064(1)

0.1089(1)

0.0271(4)

0.0236(3)

0.0244(2)

0.0252(2)

0.0285(3)

Al(2) /Mg

0.47

4f

0.53

Al(2) /Mg

0.47

0.3333

0.6667

0.53

Al(2) /Zn

0.47

0.3333

0.6667

0.53

Al(2) /Mg

0.53

0.3333

0.6667

0.47

Al(2) /Mg

0.53

0.3333

0.6667

0.47

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922

D. Nötzold et al.: Structure and optical properties under VUV/UV excitation

Table 2 (continued)

component

multiplicity

sample atomic coordinates

Wyckoff

letters

x

y

z

Al(3)

4f

1

2

3

4

5

1

2

3

4

5

0.3333

0.0000

0.6667

0.0000

0.1733(4)

0.1773(4)

0.1739(3)

0.1745(2)

0.1742(3)

0.0000

Al(4)

2a

0.0000

O(1)

O(2)

O(3)

O(4)

O(5)

12k

4f

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

0.1498(22)

0.1437(17)

0.1520(13)

0.1534(10)

0.1508(10)

0.5069(22)

0.5089(16)

0.5001(14)

0.5022(10)

0.5018(12)

0.6667

0.2996(45)

0.2874(35)

0.3041(26)

0.3069(20)

0.3016(21)

0.0139(44)

0.0178(35)

0.0002(28)

0.0044(19)

0.0038(24)

0.3333

0.0530(3)

0.0539(4)

0.0506(4)

0.0521(1)

0.0523(2)

0.1447(4)

0.1543(4)

0.1497(4)

0.1480(2)

0.1495(3)

0.0598(7)

0.0535(7)

0.0644(6)

0.0603(3)

0.0606(4)

0.1422(9)

0.1623(8)

0.1443(6)

0.1449(4)

0.1555(6)

0.2500

0.6667

0.3333

0.6667

0.3333

0.6667

0.3333

0.6667

0.3333

4e

0.0000

2c

0.3333

0.6667

0.3333

0.6667

0.2500

0.3333

0.6667

0.2500

0.3333

0.6667

0.2500

0.3333

0.6667

0.2500

Substitution of 25% of the Mg²⁺ions by Ba²⁺ions in BaMg Al O and by Sr²⁺ions in SrMg Al O

16 27

2

16 27

2

causes a very small increase of the lattice constant c and the ratio c /a . The value of a becomes a little

0 0 0 0

bit smaller. The volume of the unit cell increases somewhat when Mg²⁺ions are substituted by Ba²⁺ions.

It decreases a little bit in case of substitution by Sr²⁺ions (Table 1). As described in the literature oxygen

ions occupy a part of the lattice positions of the large cation in the layer between the spinel blocks in the

unit cell of the aluminate BaMg Al O :Eu²⁺[6]. With rising amount of Ba²⁺or Sr²⁺ions in the alumi-

2

16 27

nates the Ba²⁺or Sr²⁺ions should occupy increasingly a position of the large cation in the layer between

the spinel blocks causing a diminution of the number of oxygen ions on these lattice positions. More-

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phys. stat. sol. (a) 203, No. 5 (2006)

923

over, also additional Ba²⁺or Sr²⁺ions can be incorporated into the layer between the spinel blocks [6].

Changes in this layer influence hardly the lattice parameters due to the relative wide range of the layer.

So the merely small changes of the lattice parameters after the substitutions can be explained.

It was found as a result of our structure refinement procedures that Ba²⁺ions occupy in the lattice a 6h

cation position, but Sr²⁺ions are on a 2d cation position (Table 2) due to the different size of both

cations. In the Eu²⁺doped aluminates M Al O the Eu²⁺ions – like Sr²⁺ions – should occupy the 2d

3

16 27

cation position because of their almost equal size. Therefore, Eu²⁺ions replace Sr²⁺ions but not Ba²⁺ions

as described also by Stephan et al. [7]. (That Eu²⁺ions can occupy the 2d lattice position is assumed also

in case of the blue emitting phosphor 0.9BaO·0.1EuO·MgO·8Al O [21] and for a solid solution of

2

3

BaMgAl O :Eu²⁺and Ba Al O :Eu²⁺[22].) The 2d position is a Beevers–Ross site [7]. The

10 17

0.75

11 17.25

atomic coordinates of the Ba²⁺ions on a 6h cation position are very similar to those of the Sr²⁺ions

(Table 2) although the two possible 6h cation positions in this lattice which could be a mid-oxygen site

or an anti-Beevers-Ross site are characterized by distinctly other x- and y-coordinates [7]. The atomic

coordinates of the small cations and the oxygen ions in M Al O change scarcely in dependence on the

3

16 27

composition of M . There are distinct differences only in the y-coordinate of O(2) (Table 2). The oxygen

3

ions O(1) to O(4) are situated mirror-symmetrically in the two spinel blocks including the layer with the

large cation and only the oxygen ion O(5) is in this layer (Table 2). The point symmetry of the large

cation is C .

s

The coordination number of the Ba²⁺ions is eight [8] and six O(2) ions and two O(5) ions are the

ligands of the first coordination sphere. The bond lengths Ba–O(2) are smaller than the distance between

Ba and O(5) (Table 3). Sr²⁺and Eu²⁺ions have only the coordination number six due to their smaller size

in comparison with the Ba²⁺ions. (The ionic radii of the Sr²⁺and Eu²⁺ions for the coordination number

six are 132 and 131 pm respectively [20].) Only the six O(2) ions are the ligands of the first coordination

Table 3 Bond lengths of the cation on the 6h/2d position (X) to the oxygen ions of the first coordination

sphere for samples M Al O with M : 1. BaMg ; 2. SrMg ; 3. BaZn ; 4. Ba Mg ; 5. Sr Mg . (in addi-

1.5

3

16 27

3

2

1.5

tion values are given for M Eu )

0.1

2.9

interatomic

distance

bond lengths [pm]

sample 1

X: Ba

sample 2

X: Sr

sample 3

X: Ba

sample 4

X: Ba

sample 5

X: Sr

X–O(2)

292.0

281.3

263.5

263.4

291.4

273.4

290.2

277.2

277.1

276.3

276.2

av. 284.9

av. 279.4

av. 281.5

317.2

X–O(5)

318.3

(325.0)

314.2

(324.8)

317.2

X: Eu

X–O(2)

284.6

263.5

263.4

278.8

281.2

276.3

276.2

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924

D. Nötzold et al.: Structure and optical properties under VUV/UV excitation

sphere and the two O(5) ions are at a longer distance from the Sr²⁺ions than from the Ba²⁺ions (Table 3).

The single bond lengths X–O(2) differ of each other only when the large cation is Ba²⁺. In case of the

Sr-aluminates the single values are almost equal (Table 3). Obviously, the six O(2) ions are situated

more symmetrically around the 2d cation position than around the 6h position. The average distances

Ba–O(2) are longer than the average bond lengths Sr–O(2) (Table 3) which is in agreement with the

values of the lattice constant c and the volume of the unit cell (Table 1). The average bond lengths Ba–

0

O(2) and Ba–O(5) decrease when Mg²⁺ions are completely substituted by Zn²⁺ions. A such decrease

also show the lattice parameters (Table 1). The bond lengths Ba–O(2) and Ba–O(5) diminishes a little

bit when 25% of the Mg²⁺ions were substituted by Ba²⁺ions (Table 3) although the lattice constant c

0

and the volume of the unit cell increase somewhat after this substitution (Table 1). However, the ob-

served diminution of the bond lengths Ba–O(2) and Ba–O(5) is in agreement with the differences be-

tween both compounds in the atomic coordinates of the Ba²⁺ion and the O(2) ions (Table 2). The dis-

tances Sr–O(2) become distinctly longer when 25% of the Mg²⁺ions were substituted by Sr²⁺ions (Table

3) due to the differences between SrMg Al O and Sr Mg Al O concerning the lattice constant c

2

16 27

1.5

16 27

0

(Table 1) and the atomic coordinates of the O(2) ion (Table 2). Moreover, the bond lengths Eu–O(2)

were needed for comparison of the structural and optical properties of the Eu²⁺doped aluminates

M Al O . These bond lengths could be estimated by means of PowderCell substituting 10% of the large

3

16 27

cation by the Eu²⁺ion. The single bond lengths Ba–O(2) and Eu–O(2) differ of each other due to the

different lattice position of both cations although the average bond lengths X–O(2) are almost equal as it

is obvious from (Table 3).

3.2 Optical properties

Undoped BaMgAl O shows a broad emission peaking at 265 nm under VUV excitation with

10 17

λ < 190 nm. This emission is due to exciton recombination at Ba–O groups in the cation layer of the

lattice [23]. Undoped BaMg Al O exhibits the same property. This is obvious from our investigations

2 16 27

using synchrotron radiation.

The ⁸S state within the 4f⁷configuration is the ground state of the Eu²⁺ion and the ⁶P states of this

7/2

J

configuration at about 364 nm [24] are the excited states of the free ion with the lowest energy. A state

within the 4f⁶5d configuration becomes the excited state at the lowest energy in Eu²⁺doped alkaline earth

aluminates caused by the crystal field splitting of the orbital fivefold degenerated 5d states of this con-

figuration. Therefore only 4f⁷(⁸S ) – 4f⁶5d transitions within the Eu²⁺ions are observed in absorption

7/2

and emission [24]. The exciton emission of the host lattice overlaps excellently with the excitation band

of Eu²⁺ions in (Ba,Eu)Mg Al O . An effective energy transfer of the host lattice to the activator ions

2

16 27

also could be observed in all Eu²⁺doped alkaline earth aluminates investigated here. The spectral posi-

tion of the Eu²⁺emission band depends on the crystal field splitting of the 5d level and the energy gap

between the ⁸S ground state and the centre of gravity of the 5d level.

7/2

The emission band of the starting phosphor Ba Eu Mg Al O ranges from 380 to 580 nm peaking

0.9 0.1 2 16 27

at 453 nm with a band width at half height of 56 nm (Fig. 1, Table 4). This emission band is very similar

to those described for BaMgAl O :Eu²⁺in Ref. [25–27].

With growing substitution of the Ba²⁺ions by Sr²⁺ions according to Ba Sr Eu Mg Al O with

10 17

0.9–y

y

0.1

2

16 27

0 ≤ y ≤ 0.9 the emission band is shifted to smaller energies, its intensity diminishes and its band width at

half height increases. The emission band of Sr Eu Mg Al O is peaking at 461 nm with a band width

0.9

0.1

2

16 27

at half height of 64 nm (Fig. 1, Table 4).

The emission band is shifted to higher energies and the intensity decreases when the Mg²⁺ions were

substituted increasingly by Zn²⁺ions according to Ba Eu Mg Zn Al O with 0 ≤ u ≤ 2. The emission

0.9

0.1

2–u

u

16 27

band of Ba Eu Zn Al O is peaking at 445 nm with a band width at half height of 64 nm if only

0.9 0.1 2 16 27

Na AlF was taken as flux during the preparation of the samples. The use of NH Cl as the second flux

3 6 4

causes a shift of the emission band of Ba Eu Zn Al O to 448 nm. The maximum intensity is higher

0.9 0.1 2 16 27

after the additional use of NH Cl, but it is smaller than that of the starting phosphor. The band width at

4

half height stays at 64 nm (Fig. 1, Table 4).

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phys. stat. sol. (a) 203, No. 5 (2006)

925

Fig. 1 Emission spectra in dependence on the composition for

samples M Eu Al O with M : 1. Ba Mg ; 2. Sr Mg ;

100

1

2

3

4

2.9

0.1

16 27

2.9

0.9

2

0.9

2

3. Ba Zn ; 4. Sr Mg .

0.9

2

1.4

1.5

80

60

40

20

0

400

450

500

550

600

Wave length [ nm ]

The emission band is shifted scarcely, its band width at half height stays constantly, but its intensity

increases, when 25% of the Mg²⁺ions were substituted by Sr²⁺ions in Sr Eu Mg Al O . The emission

band of the Eu²⁺ions in Sr Eu Mg Al O is peaking at 462 nm with a band width at half height of

0.9

0.1

2

16 27

1.4

0.1

1.5

16 27

64 nm (Fig. 1, Table 4).

With rising Eu²⁺content in the mixed crystals the emission intensity increases up to z = 0.15 in case of

samples with the composition Ba Eu Mg Al O with 0 ≤ z ≤ 0.2. The optimal Eu²⁺concentration

1–z

z

2

16 27

equals z = 0.1 in the materials Sr Eu Mg Al O , Ba Eu Zn Al O , and Sr Eu Mg Al O in each

1–z z z 1.5 16 27

2

16 27

1–z

z

2

16 27

1.5–z

case with 0 ≤ z ≤ 0.2. The spectral position of the emission band does not change with rising Eu²⁺content

in these mixed crystals.

Table 4 Optical properties of samples M Al O with M : 1. (Ba,Eu)Mg (BAM, for comparison);

3

16 27

3

2

2. Ba Eu Mg ; 3. Ba Sr Eu Mg ; 4. Sr Eu Mg ; 5. Sr Eu Mg ; 6. Ba Eu Mg Zn ;

1

0.9

0.1

2

0.45

0.1

2

0.9

0.1

2

0.85

0.15

2

0.9

0.1

1

7. Ba Eu Zn ; 8. Sr Eu Mg .

0.1

0.9

0.1

2

1.4

1.5

sample VUV excitation

emission

maximum

[nm]

int. 147 nm int. 172 nm maximum

[% of max.] [% of max.] [nm]

luminous flux

Xe-ND

[a.u.]

Xe-Exc.

[a.u.]

1

171

90

100

450

100

2

3

4

5

6

7

8

147

169

163

161

145

175

146

100

76

83

93

82

73

74

98

96

453

457

461

451

448

462

149

167

208

149

107

110

223

115

144

168

129

81

98

93

99

78

134

204

100

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D. Nötzold et al.: Structure and optical properties under VUV/UV excitation

The differences in the spectral position of the emission band between the mixed crystals of the com-

position Ba Eu Mg Al O and Sr Eu Mg Al O (Table 4) are in agreement with the differences in

0.9

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the bond lengths Eu–O(2). These are smaller in Sr Eu Mg Al O (Table 3) and a crystal field of

0.9 0.1 2 16 27

larger strength there acts upon the Eu²⁺ions causing the observed shift of the emission band to smaller

energies (Fig. 1, Table 4).

The emission band is shifted somewhat to higher energies after substitution of the Mg²⁺by Zn²⁺ions

although the distances Eu–O(2) ions are longer in Ba Eu Mg Al O than in Ba Eu Zn Al O

16 27

0.9

0.1

2

16 27

0.9

0.1

2

(Tables 3 and 4). However, the strength of the crystal field acting upon the Eu²⁺ions should depend not

only on the bond lengths Eu–O(2) but also on the bond angles O(2)–Eu–O(2). The measured angles

indicate a smaller distance between the three O(2) ligands in the spinel blocks above and below the layer

with the Eu²⁺ions in Ba Eu Mg Al O than in Ba Eu Zn Al O . To explain the observed differ-

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ences in the spectral position of the emission bands it is assumed that the 4f⁶5d–4f⁷(⁸S ) transition

7/2

within the Eu²⁺ions is influenced more by the different arrangement of the O(2) ions than by the small

difference in the bond lengths which amounts only 2%. Therefore a stronger crystal field should be

around the Eu²⁺ions in Ba Eu Mg Al O .

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Substitution of 25% of the Mg²⁺ions by Sr²⁺ions in Sr Eu Mg Al O causes an increase of the

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bond lengths Eu–O(2) (Table 3). However, the expected shift of the emission band to higher energies

cannot be observed (Fig. 1, Table 4). The bond angles O(2)–Eu–O(2) stay unchanged after this substitu-

tion.

The emission band of the Eu²⁺ions in the investigated aluminates is symmetrically and consists of

only one band which is obvious from the results of curve splitting procedures [28]. As assumed, the

Eu²⁺ions occupy only one lattice position of the large cation in the investigated aluminates – the 2d posi-

tion.

The orbital fivefold degenerate state of the 5d electron within the 4f⁶5d configuration of the excited

free Eu²⁺ion can be split into three components under the influence of a hexagonal crystal field ignoring

any interaction of the 5d electron with the six 4f electrons. In the Eu²⁺doped aluminates M Al O the

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Eu²⁺ions occupy the 2d lattice position of the large cation with the low C point symmetry. Therefore,

the orbital fivefold degeneracy of the states of the 5d electron in the excited Eu²⁺ion is cancelled com-

pletely and up to five absorption bands of the Eu²⁺ions could appear in the excitation spectra of the

s

100

80

60

40

1

2

20

3

4

Fig. 2 Excitation spectra in dependence on the composition for

samples M Eu Al O with M : 1. Ba Mg ; 2. Sr Mg ;

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2.9

0.9

2

0.9

2

3. Ba Zn ; 4. Sr Mg .

1.5

0

0.9

2

1.4

100

150

200

250

Excitation wave length [ nm ]

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investigated aluminate phosphors due to transitions from the ⁸S ground state of the Eu²⁺ions to the five

7/2

5d crystal field components of the 4f⁶5d configuration. Moreover, in the VUV region of the excitation

spectra there are the absorption bands of the host lattice causing the observed effective energy transfer

from the host lattice to the Eu²⁺activator ions.

The excitation spectra of the Eu²⁺doped aluminates M Al O seem to be composed of three bands in

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the VUV region from 100 to 200 nm due to the absorption by the host lattice. They show several super-

imposed bands in the UV region above 200 nm resulting from the absorption of the Eu²⁺activator ions.

A distinct minimum of the excitation is in the spectral range around 200 nm. The emission of the investi-

gated Eu²⁺doped aluminates M Al O can be excited more effective by the 147 nm radiation of a

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low-pressure xenon arc lamp and/or by the 172 nm radiation of a xenon excimer lamp than by the “clas-

sical” 254 nm radiation of a low-pressure mercury arc lamp. The shape of the excitation spectrum –

especially in the VUV region – depends on the composition of M in the Eu²⁺doped aluminates

3

M Al O (Fig. 2).

3 16 27

The excitation spectrum of the starting phosphor Ba Eu Mg Al O is very similar to that of

0.9 0.1 2 16 27

Eu²⁺doped BaMgAl O given by Zhang et al. [29]. The main excitation band is peaking at 147 nm.

10 17

The maximum of a second excitation band with only a little bit smaller intensities appears at 163 nm

(Fig. 2).

This band becomes the main excitation band after substitution of the Ba²⁺by Sr²⁺ions. However, the

excitation intensities at 147 nm are almost equal to that at 163 nm. The excitation intensities diminish

after this substitution (Fig. 2).

In case of the Zn substituted sample Ba Eu Zn Al O the excitation intensities below 172 nm are

0.9 0.1 2 16 27

smaller and above 172 nm higher than those of the starting phosphor Ba Eu Mg Al O . The main

0.9 0.1 2 16 27

excitation band is peaking at 175 nm and the excitation band at about 147 nm appears only as a shoulder

in the high energy flank of the main excitation band (Fig. 2).

The excitation intensities increase distinctly when 25% of the Mg²⁺ions were substituted by Sr²⁺ions

in Sr Eu Mg Al O . Compared with the excitation spectrum of the starting phosphor the intensities

0.9 0.1 2 16 27

are somewhat smaller in the spectral region between 128 and 167 nm and higher above 167 nm. The

main excitation band is peaking at 146 nm and the second band with almost equal intensities appears at

166 nm (Fig. 2).

The spectral position of the excitation maximum and the excitation intensities at 147 and 172 nm in

comparison with that of the excitation maximum of some investigated samples are shown in Table 4.

Moreover, the spectral position of the emission band and the values of the luminous flux that means the

integral emission intensities during excitation by a low-pressure xenon arc lamp (“Xe-ND”) or by a

xenon excimer lamp (“Xe-Exc.”) measured with a V corrected photomultiplier are given. The values of

λ

an industrial BAM phosphor (Ba, Eu)Mg Al O are presented for comparison in Table 4.

2 16 27

The values of the luminous flux become larger when Ba²⁺ions are substituted increasingly by Sr²⁺

ions in Ba Eu Mg Al O (Table 4). Obviously, the shift of the emission band to larger wave lengths

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and the increase of the band width at half height of the emission band causing an enlargement of the

luminous flux dominate in comparison with the diminution of the excitation intensities during this substi-

tution. The luminous flux decreases with rising Eu²⁺concentration in Sr Eu Mg Al O (Table 4). This

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is in agreement with the observations concerning the emission intensities of these samples described

above.

The values of the luminous flux decrease with rising substitution of the Mg²⁺ions by Zn²⁺ions in

Ba Eu Mg Al O due to the diminution of the excitation and the shift of the emission band to smaller

0.9 0.1 2 16 27

wave lengths. However, the excitation, the emission intensities, and the values of the luminous flux of

Ba Eu Zn Al O can be enlarged distinctly when NH Cl is used as the second flux during the prepa-

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4

ration of this sample. The relative large values of the luminous flux of such a sample – especially during

172 nm excitation – are visible in Table 4.

The values of the luminous flux increase when 25% of the Mg²⁺ions were substituted by Sr²⁺ions in

Sr Eu Mg Al O (Table 4) which is in agreement with the observations concerning the emission and

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2

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the excitation of these samples (Figs. 1 and 2).

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D. Nötzold et al.: Structure and optical properties under VUV/UV excitation

4 Conclusions

The investigated Eu²⁺doped alkaline earth aluminate phosphors M Al O crystallize in the hexagonal

crystal system with β-Al O -structure in the space group P6 /mmc. Ba²⁺ions occupy a 6h lattice position

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2

3

of the large cation and the comparatively smaller Sr²⁺and Eu²⁺ions are on the 2d lattice position of the

large cation in the unit cell which consists of blocks with spinel structure containing Al³⁺, Mg²⁺and/or

Zn²⁺and O^2–ions separated of each other by layers containing one Ba²⁺or Sr²⁺or Eu²⁺ion and one

O^2–ion. The first coordination sphere of the Ba²⁺ions consists of six O(2) ions situated mirror-

symmetrically in the two spinel blocks including the layer with the large cation and two O(5) ions which

are in this layer. Sr²⁺and Eu²⁺ions have only the coordination number six due to their smaller size in

comparison with the Ba²⁺ions. Only the six O(2) ions are the ligands of the first coordination sphere.

The point symmetry of the large cation is C . The six O(2) ions are situated more symmetrically around

s

the 2d cation position than around the 6h position.

The different compositions of M in M Al O scarcely influence the lattice constant a . Obviously,

3 3 16 27 0

its value depends preferentially on the size of the spinel blocks in the unit cell. Only the lattice constant

c decreases when Ba²⁺ions were substituted completely by the smaller Sr²⁺ions. Substitution of 25% of

the Mg²⁺ions by Ba²⁺ions in BaMg Al O or by Sr²⁺ions in SrMg Al O causes only a very small

0

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2

16 27

increase of the lattice constant c because changes in the layer between the spinel blocks influence hardly

0

the lattice parameters due to the relative wide range of this layer.

The spectral position, the intensity, and the band width at half height of the emission band of the Eu²⁺

ions and moreover the shape of the excitation spectrum and the values of the luminous flux depend on

the compositions of M in the Eu²⁺doped aluminates M Al O because the strength of the crystal field

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acting upon the Eu²⁺ions changes with variation of the composition of M .

3

The blue emission in these phosphors due to transitions between 4f⁶5d and 4f⁷configurations of the

Eu²⁺ion can be excited more effective by the 147 nm radiation of a low-pressure xenon arc lamp and/or

by the 172 nm radiation of a xenon excimer lamp than by the “classical” 254 nm radiation of a low-

pressure mercury arc lamp. This is very important concerning a use of the Eu²⁺doped aluminate phos-

phors M Al O in mercury-free xenon discharge lamps for example in the light publicity which was the

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practical background of the investigations described here.

The promising values of the luminous flux of the Eu²⁺doped aluminate phosphors M Al O – espe-

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cially in case of the sample Sr Eu Mg Al O – show that these phosphors fulfil very well the de-

1.4 0.1 1.5 16 27

mands of light publicity.

Acknowledgements The authors wish to thank Dr. Marco Kirm (experimental station SUPERLUMI of HASY-

LAB at DESY Hamburg, contract II-01-033) and Mrs. Heike Witt for technical assistance. The scientific work

which is described in this paper was sponsored by the Bundesministerium für Bildung und Forschung of Germany

(BMBF contract I3N8152). The authors only have the responsibility for the content of this paper.

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Article Doi

Structure and optical properties under VUV/UV excitation of Eu2+ doped alkaline earth aluminate phosphors

DOI: 10.1002/pssa.200521208

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Article abstract of DOI:10.1002/pssa.200521208

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