Structure and optical properties under VUV excitation of Mn2+ doped alkaline earth aluminate phosphors

phys. stat. sol. (a) 203, No. 5, 930–940 (2006) / DOI 10.1002/pssa.200521372

Structure and optical properties under VUV excitation

2+

of Mn 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 16 October 2005, accepted 13 January 2006

Published online 20 March 2006

PACS 61.66.Fn, 78.55.Hx

Polycrystalline hexagonal alkaline earth aluminates M Al O with M :BaMg , Ba Mg , and Sr Mg

1.5

3

16 27

3

2

1.5

doped by divalent manganese and prepared by solid state reaction were investigated concerning their crys-

tal structure and optical properties at room temperature. Rietveld procedures were used for structure pa-

2+

rameters refinement. The investigated aluminate phosphors crystallize in the space group P6 /mmc. Mn

3

ions occupy both tetrahedral 4f lattice positions of the small cations in the unit cell. Transitions between

4

6

5

2+

the terms T ( G) and A ( S) within the 3d configuration of the Mn ion cause the green emission of the

1

phosphors. It can be excited very efficient by VUV radiation. Spectral position of the emission band

which seems to consist of two superimposed bands and its band width at half height as well as the shape

of the excitation spectrum are almost independent of the composition of M in M Al O . The luminous

3

16 27

flux of the investigated aluminate phosphors depends on their composition.

Polykristalline hexagonale Erdalkalialuminate M Al O mit M : BaMg , Ba Mg und Sr Mg , dotiert

1.5

3

16 27

3

2

1.5

mit zweiwertigem Mangan und präpariert durch Festkörperreaktion, wurden bezüglich ihrer Kristallstruk-

tur und optischen Eigenschaften bei Raumtemperatur untersucht. Die Rietveld-Methode diente zur Ver-

feinerung der Strukturparameter. Die untersuchten Aluminat-Leuchtstoffe kristallisieren in der Raum-

2+

gruppe P6 /mmc. Mn -Ionen besetzen in der Elementarzelle die beiden tetraedrischen 4f-Gitterplätze der

3

4

6

5

kleinen Kationen. Übergänge zwischen den Termen T ( G) und A ( S) innerhalb der 3d Konfiguration

1

2+

der Mn Ionen bewirken die grüne Emission der Leuchtstoffe. Sie kann sehr effektiv durch VUV-

Strahlung angeregt werden. Die spektrale Position der Emissionsbande, die aus zwei überlagerten Banden

zu bestehen scheint, und ihre Halbwertsbreite sowie die Gestalt des Anregungsspektrums werden von der

Zusammensetzung des M im M Al O praktisch nicht beeinflusst. Die Lichtstrom-Werte sind von der

3

16 27

Zusammensetzung der Aluminat-Leuchtstoffe abhängig.

1 Introduction

The green emission of synthetic hexagonal alkaline earth aluminates doped by divalent manganese is

characterized by high quantum efficiencies under VUV excitation [1–3]. The emission intensities are

comparable with that of the “classic” green emitting phosphor Zn SiO :Mn [1, 4].

2

4

Basing on our reported investigations of Eu²⁺doped aluminates M Al O [5] now Mn²⁺ions which

3

16 27

are able to absorb directly VUV radiation [2, 6, 7] were used for doping of selected host lattices

M Al O to get phosphors with green emission. The relations between structural and optical properties

in dependence on the composition of the polycrystalline Mn²⁺doped alkaline earth aluminates M Al O

3

16 27

3

16 27

were investigated.

*

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

Original

Paper

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

931

2 Experimental

Alkaline earth carbonates, γ-Al O , and Mn(NO ) were used as starting materials for preparation

2 3 3 2

of polycrystalline aluminates of the compositions BaMg Mn Al O , Ba Mg Mn Al O , and

2–x x 16 27 1.5 16 27

1.5–x

x

Sr Mg Mn Al O with 0 ≤ x ≤ 0.15 by solid state reaction. The starting mixture was fired at 1703 K

1.5 1.5–x x 16 27

in a water vapour saturated reducing atmosphere (N /H ). Either Na AlF which amounts to 5% of the

2 2 3 6

aluminium needed or BaF substituting 20 to 70% of the BaCO necessary for preparation were used as

2 3

flux.

Powder diffraction data were recorded and structure parameters were refined as described earlier [5].

Beside bond lengths also bond angles were estimated by the program PowderCell (version 2.3, Decem-

ber 1999) in proof of tetrahedral surroundings of a part of the small cations in the aluminates.

The optical investigations were performed as reported in [5]. However, the excitation source for meas-

urement of the emission spectra was a xenon excimer lamp (Radium Xeradex 20 Excimer radiator, Ra-

dium Lampenwerk Wipperfürth).

3 Results and discussion

3.1 Structure

The investigated alkaline earth aluminates M Al O crystallize in the hexagonal crystal system with

3 16 27

β-Al O -structure [5, 8, 9]. The space group is P6 /mmc [10]. The unit cell consists of blocks with spinel

2 3 3

structure containing Al³⁺, Mg²⁺and O^2–ions separated of each other by layers with one Ba²⁺or Sr²⁺ion

which there can occupy different lattice positions and one O^2–ion. This oxygen ion is placed between

two aluminium ions in a small distance [10]. Mn²⁺ions occupy lattice positions of the small cations.

The X-ray patterns of the polycrystalline alkaline earth aluminates M Al O were compared with

3

16 27

those of BaMgAl O [11], Ba Mg Al

10 17 0.956 0.912

O [12] and SrMgAl O [13]. They show the character-

10 17

10.088 17

istical crystal structure of the hexagonal β-Al O as described before [5].

2 3

Small amounts of crystalline MgAl O [14] and Al O [15] (in BaMg Al O ) and SrAl O [16] (in

2 4 2 16 27 2

3

2

4

Sr Mg Al O ) could be found as additional phases. Only a trace of BaAl O [17] was identified as

1.5 1.5 16 27 2 4

additional phase in Ba Mg Al O [5]. After doping by Mn²⁺ions the amount of MgAl O increases

1.5

16 27

2

4

somewhat in BaMg Al O , and in Sr Mg Mn Al O beside SrAl O a trace of MgAl O could be

2 0.1 2 2 4

16 27

1.5

1.4

16 27

4

identified. The amount of BaAl O as additional phase is a little bit higher in Ba Mg Mn Al O than

2 4 1.5 0.1 16 27

1.4

in the host lattice Ba Mg Al O . Use of BaF instead of Na AlF as flux causes a decrease of the

1.5 1.5 16 27 2 3

6

BaAl O part in Ba Mg Mn Al O . In case of BaMg Mn Al O the amount of MgAl O remains

2 4 1.5 2 4

1.4

0.1

16 27

1.9

0.1

16 27

almost constantly, but a small amount of BaAl O could be observed additionally in samples which were

2 4

prepared with BaF as flux.

2

The structure model of Ba Mg Al

0.956 0.912

O [18] was used to carry out the structure parameter re-

10.088 17

finements [5]. The final refinement parameters of the materials are in the range 0.11 to 0.26 for R and

wp

between 1.6 and 2.4 in case of Gof.

Substitution of 25% of the Mg²⁺ions by Ba²⁺ions in BaMg Al O causes a very small increase of the

2

16 27

lattice constant c , the ratio c /a and the volume of the unit cell. The lattice constant a decreases some-

0 0 0 0

what (Table 1). It is known [9] that 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⁺. It is assumed

2

16 27

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. Ba Mg , 3. Sr Mg [5].

3

2

1.5

sample

a [pm]

0

c [pm]

0

c /a

0 0

V [Å³]

1

2

3

562.29(3)

562.21(2)

562.54(2)

2263.43(12)

2265.46(7)

2236.74(11)

4.0254

4.0296

3.9761

619.75(6)

620.13(3)

613.00(4)

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932

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

that with growing content of Ba²⁺ions in the aluminates the Ba²⁺ions occupy increasingly a position of

the large cation in the layer between the spinel blocks which causes a decrease of the number of oxygen

ions on this lattice position. Besides, also additional Ba²⁺ions can be inserted into the layer between the

spinel blocks [9]. Changes in this layer influence hardly the lattice parameters because of the relative

wide range of the layer. The merely small changes of the lattice parameters after this substitution can be

explained in this way.

Substitution of the Ba²⁺ions in Ba Mg Al O by the smaller Sr²⁺ions (ionic radii for coordination

1.5

16 27

number eight are 156 and 140 pm respectively [19]) causes a decrease of the volume of the unit cell

and of the ratio c /a because of a distinct diminution of the lattice constant c . The lattice constant a in-

0

Table 2 Atomic coordinates of aluminate samples M Al O with M : 1. BaMg , 2. Ba Mg ,

3

16 27

3

2

1.5

3. Sr Mg [5].

1.5

component

multiplicity

Wyckoff

letters

sample atomic coordinates

x

y

z

Ba

Sr

6h

2d

1

2

3

0.6802(48)

0.6826(25)

0.6667

0.3604(96)

0.3651(49)

0.3333

0.2500

Al(1)

12k

1

2

3

0.8323(16)

0.8330(7)

0.8358(8)

0.6646(33)

0.6660(13)

0.6716(15)

0.1072(2)

0.1064(1)

0.1089(1)

Al(2) /Mg

0.47

4f

1

2

3

0.3333

0.6667

0.0271(4)

0.0252(2)

0.0285(3)

0.53

0.47

Al(2) /Mg

0.53

Al(2) /Mg

0.53

Al(3)

Al(4)

O(1)

O(2)

O(3)

O(4)

O(5)

4f

1

2

3

0.3333

0.6667

0.1733(4)

0.1745(2)

0.1742(3)

2a

1

2

3

0.0000

12k

4f

1

2

3

0.1498(22)

0.1534(10)

0.1508(10)

0.2996(45)

0.3069(20)

0.3016(21)

0.0530(3)

0.0521(1)

0.0523(2)

1

2

3

0.5069(22)

0.5022(10)

0.5018(12)

0.0139(44)

0.0044(19)

0.0038(24)

0.1447(4)

0.1480(2)

0.1495(3)

1

2

3

0.6667

0.3333

0.0598(7)

0.0603(3)

0.0606(4)

4e

1

2

3

0.0000

0.1422(9)

0.1449(4)

0.1555(6)

2c

1

2

3

0.3333

0.6667

0.2500

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Original

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

933

creases somewhat (Table 1). Obviously, the values for a depends preferentially on the size of the spinel

0

blocks in the unit cell and this size seems to be influenced hardly by the substitution of the Ba²⁺ions by

Sr²⁺ions.

Ba²⁺ions are in the lattice on a 6h cation position, but Sr²⁺ions occupy a 2d cation position because of

the different size of both cations. The differences in the atomic coordinates between Ba²⁺and Sr²⁺ions

are small (Table 2). The atomic coordinates of the little cations and the oxygen ions in the alkaline earth

aluminates M Al O differ of each other merely in a very small range when the composition of M is

3

16 27

3

varied. Only the y-coordinate of O(2) changes when 25% of the Mg²⁺ions were substituted by Ba²⁺or

Sr²⁺ions and the z-coordinate of O(4) shows a variation after substitution of the Ba²⁺ions by Sr²⁺ions

(Table 2).

Al³⁺ions occupy four different lattice positions in the alkaline earth aluminates M Al O (Table 2).

3

16 27

Al(1) and Al(4) ions are coordinated octahedrally. Al(2) and Al(3) ions occupy tetrahedrally coordinated

4f lattice positions [10]. Mg²⁺ions substitute tetrahedrally coordinated Al³⁺ions. Therefore, a part of the

Al(2) ions in the aluminates M Al O was replaced by Mg²⁺ions (Table 2). It is assumed that Mn²⁺ions

3

16 27

substitute partially Mg²⁺ions in the Mn²⁺doped alkaline earth aluminates M Al O because Mn²⁺ions

3

16 27

prefer tetrahedrally – but not octahedrally – coordinated lattice positions in compounds with spinel struc-

ture [8, 9, 20]. Moreover, Mn²⁺ions could occupy also the second 4f cation position in the unit cell and

thereby replace a part of the Al(3) ions which are on this position. Antoine et al. [21] have described that

this 4f position is occupied preferentially by Mn²⁺ions in case of Mn²⁺doped alkaline aluminates with

β-Al O -structure.

2

3

Al(2) ions are coordinated tetrahedrally by three O(1) ions and one O(3) ion. The first coordination

sphere of Al(3) ions is a tetrahedron which consists of three O(2) ions and one O(5) ion.

The single bond lengths Al(2)–O(1) of each of the investigated samples are equal and smaller than the

distance between Al(2) and O(3). Therefore, the environment of the Al(2) ions is a slightly stretched

tetrahedron. The bond lengths Al(2)–O(1) decrease somewhat after substitution of 25% of the Mg²⁺ions

by Ba²⁺or Sr²⁺ions. Also the distance between Al(2) and O(3) is smaller in Ba Mg Al O than in

1.5

16 27

BaMg Al O . The largest bond length Al(2)–O(3) could be observed in Sr Mg Al O (Table 3).

2 16 27 1.5 1.5 16 27

Thereby the bond length Al(2)–O(3) changes in dependence on the composition of M in M Al O in

3 3 16 27

the same way as the lattice constant a (Tables 1 and 3). The average distance Al(2)–O(1/3) decreases

0

somewhat when 25% of the Mg²⁺ions were replaced by Ba²⁺or Sr²⁺ions. This decrease is smaller in

Sr Mg Al O than in Ba Mg Al O (Table 3).

1.5 16 27 1.5 1.5 16 27

1.5

3+

Table 3 Bond lengths of the Al ions on the 4f cation positions to the oxygen ions of the first coordina-

tion sphere for aluminates M Al O with M : 1. BaMg , 2. Ba Mg , 3. Sr Mg .

3

16 27

3

2

1.5

sample

bond lengths [pm]

Al(2)–O(1)

Al(2)–O(3)

196.7

Al(3)–O(2)

Al(3)–O(5)

173.6

1

188.1

181.1

181.0

av. 190.3

av. 179.2

171.0

2

185.5

185.4

193.7

175.1

175.0

av. 187.5

av. 174.1

169.5

3

185.7

185.6

199.3

173.3

av. 189.1

av. 172.4

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934

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

Table 4 Bond angles O–Al–O for the aluminium ions Al(2) and Al(3) on the 4f cation positions for

aluminates M Al O with M : 1. BaMg , 2. Ba Mg , 3. Sr Mg .

3

16 27

3

2

1.5

sample

bond angles [°]

O(1)–Al(2)–O(1) O(1)–Al(2)–O(3) O(2)–Al(3)–O(2) O(2)–Al(3)–O(5)

1

110.8

110.7

108.2

107.9

111.0

110.9

2

3

109.8

109.7

109.2

108.9

110.1

112.1

106.7

110.3

108.6

The surroundings of the Al(3) ions is a somewhat compressed tetrahedron. In each of the investigated

samples the single distances between Al(3) and O(2) are equal and larger than the bond length Al(3)–

O(5). These values and moreover the average bond length Al(3)–O(2/5) decrease when 25% of the Mg²⁺

ions were substituted by Ba²⁺ions and all these bond lengths are smallest in Sr Mg Al O (Table 3).

1.5

16 27

The bond lengths of the tetrahedrally coordinated Al³⁺ions to their oxygen ligands of the first coordi-

nation sphere are longer for Al(2) than for Al(3) (Table 3). Therefore, Mg²⁺and Mn²⁺ions should replace

preferentially Al(2) ions on their 4f cation position because of the larger size of Mg²⁺and Mn²⁺ions in

comparison with the Al³⁺ions (ionic radii of the fourfold coordinated ions are 53 pm (Al³⁺), 71 pm

(Mg²⁺), and 80 pm (Mn²⁺); the ionic radius of the twofold coordinated oxygen ion equals 121 pm [19]).

Covalent bonds should participate in the bond between Mn²⁺and O^2–ions because the observed bond

lengths are smaller than the sum of the ionic radii (201 pm). Moreover, the lattice constants a and c do

0

not change after doping of the alkaline earth aluminates M Al O by Mn²⁺ions as it was obvious from

3

16 27

our investigations. Covalent parts in the bond between Mn²⁺and O^2–ions could be explained by the dis-

tinct smaller electropositive character of the Mn²⁺ion in comparison with that of the Mg²⁺ion.

The measured bond lengths of the Al(2) and Al(3) ions to their oxygen ligands of the first coordina-

tion sphere differ scarcely from the values given for BaMgAl O :Eu²⁺[10].

10 17

The single bond angles O(1)–Al(2)–O(1) or O(1)–Al(2)–O(3) and O(2)–Al(3)–O(2) or O(2)–

Al(3)–O(5) are near the tetrahedron angle of 109.5°. The smallest deviations were observed for the alu-

minate Ba Mg Al O (Table 4). The average bond angle O–Al(2)–O or O–Al(3)–O calculated from

1.5

16 27

the single values given in Table 4 equals 109.5° or 109.4°.

3.2 Optical properties

The undoped aluminate BaMgAl O exhibits a broad emission with maximum at 265 nm under VUV

10 17

excitation by λ < 190 nm. This emission results from exciton recombination at Ba–O groups in the layers

of the lattice with the large cation [22]. The investigated undoped alkaline earth aluminates M Al O

3

16 27

show a similar emission which was detected using synchrotron radiation for excitation.

6 4

The S level within the 3d⁵configuration is the ground state of the Mn²⁺ion. The G levels of this

5/2

J

configuration at about 390 nm [23] are the excited states of the free Mn²⁺ion at the lowest energy. Above

these levels there are other quartet terms and at still higher energies several doublet terms. In the free

Mn²⁺ion all optical absorption transitions within the 3d⁵configuration are parity and spin forbidden.

However, the parity and spin selection rules are relaxed when Mn²⁺ions are under the influence of a

crystal field generated by the surroundings of the Mn²⁺ions for example in the Mn²⁺doped alkaline earth

aluminates M Al O .

3

16 27

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Original

Paper

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

935

100

80

60

40

20

0

120

100

80

1

2

3

1

2

3

60

40

20

0

500

550

600

500

550

600

Wave length [ nm ]

Fig. 1 Emission spectra in dependence on the

Wave length [ nm ]

Fig. 2 Emission spectrum of the aluminate

composition for samples M Mn Al O with

0.1

Ba Mg Mn Al O with: (1) measured band,

1.5 1.4 0.1 16 27

2.9

16 27

2+

M : (1) BaMg , (2) Ba Mg , (3) Sr Mg .

2.9

(2) band of Mn on Al(2) position, (3) band of

2+

1.9

1.5

1.4

1.5

1.4

Mn on Al(3) position.

Mn²⁺ions occupy preferentially tetrahedrally coordinated lattice positions in these aluminates and the

parity selection rule is relaxed in tetrahedral symmetry because there is no centre of symmetry [23]. The

4

⁴G level is split into the states T , T , E, and A under the influence of a tetrahedral crystal field [23,

24]. The ⁴T state of the ⁴G level becomes the excited state at the lowest energy in the Mn²⁺doped alka-

1

2

1

4

line earth aluminates M Al O and the transition T (⁴G) → ⁶A (⁶S) within the Mn²⁺ions causes the

3

16 27

1

observed emission of these ions [1, 23, 24].

4

The spectral position of the Mn²⁺emission band depends on the crystal field splitting of the G level

and the energy gap between the ⁶S ground state and the centre of gravity of the ⁴G level.

5/2

The emission band of the starting phosphor BaMg Mn Al O ranges from 470 to 600 nm with a

1.9 0.1 16 27

maximum at 513 nm and a band width at half height of 28 nm (Fig. 1, Table 5). After substitution of

25% of the Mg²⁺ions in the host lattice by Ba²⁺ions the emission intensities increase. The emission band

of Ba Mg Mn Al O is peaking at 514 nm and its band width at half height stays at 28 nm (Fig. 1,

1.5

1.4

0.1

16 27

Table 5 Emission properties of samples M Mn Al O with M : 1. BaMg , 2. Ba Mg , 3. Sr Mg

1.4

2.9

0.1

16 27

2.9

1.9

1.5

1.4

1.5

(Φ spectral radiant flux of the emission peak (measured emission band Φ = 100), λ wave length of the

emission peak, H band width at half height).

sample measured emission emission band of Mn²⁺ions

emission band of Mn²⁺ions

on Al(3) lattice position

band

on Al(2) lattice position

λ [nm]

H [nm] Φ [%]

λ [nm]

H [nm] Φ [%]

λ [nm]

H [nm]

1

2

3

513

514

28

88.3

84.1

91.4

512

513

22

24

35.5

34.7

18.9

528

530

531

29

33

34

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936

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

Table 5). The same spectral position of the emission band and the equal band width at half height could

be observed when the Ba²⁺ions in Ba Mg Mn Al O were substituted by Sr²⁺ions. However, the

1.5

1.4

0.1

16 27

emission intensities are smaller than those of the starting phosphor (Fig. 1, Table 5).

Shape and spectral position of the described emission bands are very similar to the emission spectrum

of the hexagonal aluminate BaAl O :Mn [1, 2, 25].

12 19

The influence of the Mn²⁺content on the emission properties was investigated for the host lattice

Ba Mg Al O which shows the strongest emission of the alkaline earth aluminates M Al O after

1.5 1.5 16 27 3 16 27

doping by Mn²⁺ions. With rising content of Mn²⁺ions in the phosphor Ba Mg Mn Al O with

1.5

1.5–x

x

16 27

0 ≤ x ≤ 0.15 the emission increases up to x = 0.10 and then it decreases. The spectral position of the

emission band is almost independent of x.

The measured emission band of the Mn²⁺ions in the aluminates M Al O :Mn (Fig. 1) should be

3

16 27

composed of two bands if the Mn²⁺ions occupy both tetrahedrally coordinated 4f cation positions in the

unit cell of the Mn²⁺doped aluminates M Al O . Really, two superimposed bands could be found by

3

16 27

means of curve splitting procedures [26] in the emission spectra. Therefore, Mn²⁺ions occupy both tetra-

hedrally coordinated 4f cation positions in the unit cell of these aluminates, none octahedrally coordi-

nated cation positions 12k or 2a were occupied. The bond lengths to the ligands of the first coordination

sphere are longer for Mn²⁺ions on the 4f cation position of Al(2) than for Mn²⁺ions on the 4f cation

position of Al(3) (Table 3). As a consequence of this difference in the bond lengths the emission band of

Mn²⁺ions which occupy the lattice position of Al(2) is at higher energies than that of Mn²⁺ions on Al(3)

lattice positions because a crystal field of enhanced strength acts upon the latter Mn²⁺ions. The emission

band of Mn²⁺ions on the 4f lattice position of Al(2) shows distinctly higher intensities and therefore

Mn²⁺ions should be preferentially on this 4f cation position. The band width at half height of the domi-

nating emission band is smaller in comparison with that of the emission band which results from excited

Mn²⁺ions on Al(3) lattice positions (Fig. 2, Table 5). This indicates a stronger coupling of the emission

process of the latter Mn²⁺ions with vibrations of the host lattice due to the smaller distance of these Mn²⁺

ions to the oxygen ligands of the first coordination sphere (Table 3). The spectral position of the meas-

ured emission band is almost independent of the composition of M in the Mn²⁺doped aluminates

3

M Al O (Fig. 1, Table 5). The same observation is also valid for the emission band which results from

3 16 27

excited Mn²⁺ions occupying the 4f lattice position of Al(2). Obviously, the differences in the average

bond length Al(2)–O(1/3) between the samples presented in Table 3 are too small to influence

the strength of the crystal field acting upon the Mn²⁺ions on this lattice position and therefore to influ-

ence visibly the spectral position of the emission band. The differences in the average bond length

Al(3)–O(2/5) between the samples are more distinct. This bond length decreases in the sequence

BaMg Mn Al O –Ba Mg Mn Al O –Sr Mg Mn Al O (Table 3). In agreement with this

1.9

0.1

16 27

1.5

1.4

0.1

16 27

1.5

1.4

0.1

16 27

observation the emission band is shifted a little bit to smaller energies in the same sequence (Tables 3

and 5). It was to be assumed from the differences in the observed values for the spectral radiant flux Φ of

the emission peak that less Mn²⁺ions are on the 4f lattice position of Al(3) – and therefore more

on the 4f lattice position of Al(2) – in the sample which contains Sr²⁺ions as the large cation compared

with the two samples without Sr²⁺ions (Table 5). This could be due to the observation that in

Sr Mg Mn Al O the difference between the sum of the ionic radii of manganese and oxygen ions

1.5

1.4

0.1

16 27

(201 pm) and the observed average bond length Al(3)–O(2/5) of 172.4 pm is largest (Table 3).

The luminescence of the Mn²⁺doped aluminate BaMgAl O additional doped by Eu²⁺ions leads to

10 17

the conclusion that Mn²⁺ions occupy the tetrahedrally coordinated 4f lattice position of Al(3) also in this

phosphor. The emission under VUV excitation (147 nm) increases by the additional doping with Eu²⁺

ions due to an efficient energy transfer from the Eu²⁺ions which occupy the 2d lattice position of the

large cation in the layer between the spinel blocks in the unit cell [5] to Mn²⁺ions in the spinel blocks

[3]. The efficiency of a such energy transfer increases with decreasing distance between both ions. This

distance is distinctly smaller for Mn²⁺ions occupying the 4f lattice position of Al(3) than for Mn²⁺ions

on the 4f lattice position of Al(2) in the host lattices M Al O (Table 2) or BaMgAl O .

3

16 27

10 17

As described above the ground state of the free Mn²⁺ion with 3d⁵configuration is the sextet term ⁶S

5/2

which is not degenerated. Several quartet and doublet terms with a degeneracy of 2L+1 are excited states

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

937

of the free Mn²⁺ion. All optical absorption transitions within the 3d⁵configuration of the free Mn²⁺ion

are spin and parity forbidden. The degeneracy of the excited states may be lifted and the spin or parity

selection rule can be relaxed when Mn²⁺ions are brought under the influence of a crystal field. Thereby

the lift of the degeneracy increases with decreasing symmetry of the crystal field. Many absorption bands

of the Mn²⁺ions with different intensities could appear in the excitation spectra of Mn²⁺doped com-

pounds due to transitions from the S ground state of the Mn²⁺ion to the single crystal field compo-

6

5/2

nents of the different quartet and doublet terms. In the hexagonal aluminate of the composition

La O ·11Al O :Mn²⁺, for example, the absorption in the UV region is only weak [24]. In the VUV re-

2

3

2

3

6

gion there should be the absorption transitions from the S ground state of the Mn²⁺ion to the single

5/2

crystal field components of the doublet terms. Moreover, in this spectral region strong absorption could

be observed due to absorption by the host lattice and besides caused by the transition between the 3d⁵

and 3d⁴4s¹configuration of the Mn²⁺ions peaking around 195 nm in case of the hexagonal aluminates

BaAl O :Mn²⁺[2] and (Ba,Mg,Sr)O·nAl O :Mn²⁺with 4.1 ≤ n ≤ 4.9 [27]. High efficient energy trans-

12 19

2

3

fer from the host lattice to the Mn²⁺ions is described for these aluminates. A strong absorption band was

also observed in the absorption spectrum of the technical phosphor Zn SiO :Mn²⁺ascribed to a transition

2

4

3d⁵→ 3d⁴4s¹within the Mn²⁺ion or to Mn²⁺→ Mn³⁺ionization [24].

In the Mn²⁺doped aluminates M Al O the environment of the Mn²⁺ions on the 4f lattice positions

3

16 27

of Al(2) and Al(3) is a slightly distorted tetrahedron. Therefore, the degeneracy of the quartet and dou-

blet terms of the 3d⁵configuration of the excited Mn²⁺ion should be somewhat smaller than in case of its

tetrahedral coordination.

The excitation spectra of the Mn²⁺doped aluminates M Al O seem to be composed of several super-

3

16 27

imposed bands in the VUV region from 100 to 200 nm due to the absorption by the host lattice and by

the Mn²⁺activator ions itself. Above 200 nm the excitation decreases distinctly with increasing wave

length and it is almost equal to zero at 254 nm (Fig. 3). Therefore, the Mn²⁺doped aluminates M Al O

3

16 27

100

80

60

40

20

0

120

100

80

60

40

20

0

1

2

3

1

2

3

4

100

150

200

250

100

150

200

250

Wave length [ nm ]

Fig. 3 Excitation spectra in dependence on the

Wave length [ nm ]

Fig. 4 Excitation spectra for different phosphors:

composition for samples M Mn Al O with

2.9

(1) Ba Mg Mn Al O , (2) Ba Mn Al O ,

1.5 1.4 0.1 16 27 0.9 0.1 12 19

0.1

16 27

M : (1) BaMg , (2) Ba Mg , (3) Sr Mg .

2.9

(3) Zn SiO : Mn, (4) (Ce,Tb)MgAl O :Tb.

2 4 11 19

1.9

1.5

1.4

1.5

1.4

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938

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

Table 6 Optical properties of the phosphors 1. (Ce,Tb)MgAl O : Tb (CAT, for comparison),

11 19

2. BaMg Mn Al O , 3. Ba Mg Mn Al O , 4. Sr Mg Mn Al O , 5. Ba Mg Mn Al O , 6.

16 27

1.9

0.1

16 27

1.5

1.4

0.1

16 27

1.5

1.4

0.1

16 27

1.5

1.41

0.09

Ba Mg Mn Al O , 7. Ba Mn Al O , 8. Zn SiO : Mn.

4

1.5

1.37

0.13

16 27

0.9

0.1

12 19

2

sample VUV excitation

emission

maximum

[nm]

int. 147 nm int. 172 nm maximum

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

luminous

Xe-ND

[a.u.]

flux

Xe-Exc.

[a.u.]

1

175

79

99

546 (line)

100

2

3

4

5

6

101

76

79

77

72

63

66

64

68

64

513

514

515

147

191

129

188

174

103

120

95

103

104–117

104

115

110

101

7

8

102

101

79

84

70

75

515

525

176

152

115

114

cannot be excited by the “classical” 254 nm radiation of a low-pressure mercury arc lamp. A such excita-

tion becomes only possible after additional doping of these aluminates with Eu²⁺ions. This was found in

our recent investigations and is described also in Ref. [3]. The emission of the investigated Mn²⁺doped

aluminates M Al O can be excited more efficient by the 147 nm radiation of a low-pressure xenon arc

3

16 27

lamp than by the 172 nm radiation of a xenon excimer lamp (Fig. 3). Only in the spectral region around

150 nm the excitation spectra of the Mn²⁺doped aluminates M Al O are similar to those of the Eu²⁺

3

16 27

doped aluminates M Al O with the same host lattice. In the spectral range between 180 and 215 nm

3 16 27

the excitation intensities are higher after doping of the aluminates M Al O by Mn²⁺ions than by Eu²⁺

ions [5]. This observation should be due to the 3d⁵→ 3d⁴4s¹absorption of the Mn²⁺ions [2, 27].

The shape of the excitation spectrum is scarcely influenced by the composition of M in the Mn²⁺

3

16 27

3

doped aluminates M Al O . The main excitation band of the starting phosphor BaMg Mn Al O is

3 16 27 1.9 0.1 16 27

peaking at 101 nm (Fig. 3, Table 6). The strong excitation in this spectral region results from the absorp-

tion by the host lattice and additional by the Mn²⁺ions which is obvious from comparison of this excita-

tion spectrum with that of the phosphor Ba Eu Mg Al O investigated before [5]. In the spectral re-

0.9

0.1

2

16 27

gion around 145 nm there is a strong absorption by the host lattice. The resulting excitation band appears

as shoulder in the long wave length flank of the dominating excitation band (Fig. 3).

The shape of the excitation spectrum changes scarcely when 25% of the Mg²⁺ions in the host lattice

BaMg Al O were substituted by Ba²⁺ions. Only the excitation intensities increase distinctly. The main

2

16 27

excitation band of the aluminate phosphor Ba Mg Mn Al O is peaking at 103 nm. After substitu-

1.5 1.4 0.1 16 27

tion of the Ba²⁺ions in the latter aluminate by Sr²⁺ions the excitation intensities decrease distinctly and

also the shape of the excitation spectrum changes a little bit. The maximum of the dominating excitation

band(s) appears as plateau between 104 and 117 nm (Fig. 3, Table 6).

A comparison of the excitation spectrum of the aluminate Ba Mg Mn Al O with the measured

1.5 1.4 0.1 16 27

excitation spectra of the hexagonal aluminate Ba Mn Al O and the technical phosphors Zn SiO : Mn

0.9 0.1 12 19 2 4

and (Ce,Tb)MgAl O :Tb shows the high efficiency of the excitation of the first phosphor (Fig. 4). The

11 19

shape of the excitation spectrum of the two Mn²⁺doped aluminates is almost equal and it is very similar

to that of the phosphor Zn SiO :Mn in the VUV region. Nearly the same shape of the excitation spec-

2 4

trum of Zn SiO :Mn is given by Ronda [1] but in case of BaAl O :Mn there are distinct differences –

2 4 12 19

especially below 140 nm – between our excitation spectrum and that of Ronda [1].

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 6.

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

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

939

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 by a V corrected photomultiplier are given. The values

λ

of the hexagonal aluminate Ba Mn Al O and the technical phosphors Zn SiO :Mn and

0.9 0.1 12 19 2 4

(Ce,Tb)MgAl O :Tb are presented for comparison in Table 6.

11 19

The values of the luminous flux increase when 25% of the Mg²⁺ions were substituted by Ba²⁺ions in

the host lattice BaMg Al O . They decrease when the Ba²⁺ions after a such substitution were replaced

2

16 27

completely by Sr²⁺ions (Table 6). The intensity sequence of the luminous flux (Table 6) equals that for

excitation (Fig. 3) and emission (Fig. 1) of these aluminate phosphors. The Mn²⁺doped alkaline earth

aluminate Ba Mg Mn Al O shows the highest values of the luminous flux. These values decrease

1.5

1.4

0.1

16 27

when the Mn²⁺content in the host lattice Ba Mg Al O becomes smaller or larger than 0.1 (Table 6)

1.5

16 27

in agreement with the observations concerning the excitation and emission intensities of the samples.

4 Conclusions

The investigated Mn²⁺doped alkaline earth aluminate phosphors M Al O crystallize in the hexagonal

3

16 27

crystal system with β-Al O -structure in the space group P6 /mmc. The unit cell consists of blocks with

2 3 3

spinel structure containing Al³⁺, Mg²⁺and O^2–ions separated from another by layers which contain one

Ba²⁺or Sr²⁺ion and one O^2–ion. Al³⁺ions occupy four different lattice positions. Al(1) and Al(4) ions are

coordinated octahedrally on the lattice positions 12k and 2a. Al(2) and Al(3) ions occupy tetrahedrally

coordinated 4f lattice positions. Mg²⁺ions substitute tetrahedrally coordinated Al³⁺ions. Al(2) ions are in

the centre of three O(1) ions and one O(3) ion. The first coordination sphere of Al(3) ions is a tetrahe-

dron which consists of three O(2) ions and one O(5) ion. The bond lengths of the tetrahedrally coordi-

nated Al³⁺ions to their oxygen ligands of the first coordination sphere are longer for Al(2) than for Al(3).

Mn²⁺ions occupy both tetrahedrally coordinated 4f cation positions in the unit cell but not the octahe-

drally coordinated cation positions.

The lattice constant a is scarcely influenced by changes of the composition of M in M Al O . Obvi-

0 3 3 16 27

ously, its value depends preferentially on the size of the spinel blocks in the unit cell. Substitution of

25% of the Mg²⁺ions by Ba²⁺ions in BaMg Al O causes a very small increase of the lattice constant c

2

16 27

0

due to the facts that additional Ba²⁺ions can be incorporated into the layer between the spinel blocks and

that changes in this layer hardly influence the lattice parameters because of the relative wide range of this

layer. The lattice constant c decreases distinctly when Ba²⁺ions in Ba Mg Al O were substituted

0

1.5

16 27

completely by the smaller Sr²⁺ions.

The emission band of the Mn²⁺ions in the aluminates M Al O :Mn is peaking around 514 nm with a

3

16 27

band width at half height of 28 nm. It seems to consist of two superimposed bands. The emission band of

Mn²⁺ions which occupy the lattice position of Al(2) is at higher energies than that of Mn²⁺ions on Al(3)

lattice positions. The excitation spectra of the Mn²⁺doped aluminates M Al O are composed of several

3

16 27

superimposed bands in the VUV region from 100 to 200 nm due to the absorption by the host lattice and

by the Mn²⁺ions itself. Above 200 nm the excitation decreases distinctly with increasing wave length and

it is almost equal to zero at 254 nm. Therefore, the Mn²⁺doped aluminates M Al O cannot be excited by

3

16 27

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

is scarcely influenced by the composition of M in the Mn²⁺doped aluminates M Al O . The main exci-

3

16 27

tation band is peaking around 101 nm. The intensities of the emission or excitation band and the values of

the luminous flux depend on the composition of M in the Mn²⁺doped aluminates M Al O .

The green emission of these luminophors due to thetransition ⁴T (⁴G) → ⁶A (⁶S) within the Mn²⁺ions

3

16 27

1

can be excited more effective by the 147 nm radiation of a low-pressure xenon arc lamp than by the

172 nm radiation of a xenon excimer lamp. These excitation properties are very important concerning a

use of the Mn²⁺doped aluminates M Al O in mercury-free xenon discharge lamps for example in the

3

16 27

light publicity which was the practical aim of the investigations described here.

The phosphors developed as result of this research work good fulfil the demands of the light publicity

which is obvious from the promising values of the luminous flux of Mn²⁺doped aluminates M Al O -

3

16 27

especially in case of the sample Ba Mg Mn Al O .

1.5 1.4 0.1 16 27

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

Acknowledgements The authors wish to thank Dr. M. Kirm (experimental station SUPERLUMI of HASYLAB at

DESY Hamburg, contract II-01-033) and H. 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

13N8152). The authors only have the responsibility for the content of this paper.

References

[1] C. R. Ronda, J. Lumin. 72–74, 49 (1997).

[2] G. Hong, X. Zeng, H. You, Ch.-H. Kim, Ch.-H. Pyun, Ch.-H. Park, B.-Y. Yu, H.-S. Bal, and I.-F. Kwon, Fa-

guang Xuebao 20(4), 311 (1999).

[3] K. Y. Jung, H. W. Lee, Y. Ch. Kang, S. B. Park, and Y. S. Yang, Chem. Materials 17(10), 2729 (2005).

[4] B.-F. Lei, Y.-L. Liu, Z.-R. Je, and Ch.-S. Shi, Faguang Xuebao 26(1), 67 (2005).

[5] D. Nötzold, H. Wulff, S. Jilg, L. Kantz, and L. Schwarz, phys. stat. sol. (a) 203, 919 (2006) (this issue).

[6] S. Tanimizu, T. Suzuki, M. Shiiki, and Ch. Okazaki, in: Proceedings of the 8th International Symposium

on the Science & Technology of Light Sources, Greifswald, Germany, 30 August–3 September 1998, pp. 322,

323.

[7] V. P. Denks, M. P. Kerikmyae, A. L. Lust, and T. I. Savikhina, Phys. Solid State 42(2), 261 (2000).

[8] P. B. Braun, Philips Res. Rep. 12, 491 (1957).

[9] A. L. N. Stevels, J. Lumin. 17, 121 (1978).

[10] M. Stephan, P. C. Schmidt, K. C. Mishra, M. Raukas, A. Ellens, and P. Boolchand, Z. Phys. Chem. 215, 1397

(2001).

[11] International Centre of Diffraction Data/Powder Diffraction File 2, 26-0163.

[12] International Centre of Diffraction Data/Powder Diffraction File 2, 84-0818.

[13] International Centre of Diffraction Data/Powder Diffraction File 2, 26-0879.

[14] International Centre of Diffraction Data/Powder Diffraction File 2, 82-2424.

[15] International Centre of Diffraction Data/Powder Diffraction File 2, 76-0144.

[16] International Centre of Diffraction Data/Powder Diffraction File 2, 74-0794.

[17] International Centre of Diffraction Data/Powder Diffraction File 2, 82-2001.

[18] N. Iyi, Z. Inoue, and S. Kimura, J. Solid State Chem. 61, 236 (1986).

[19] R. D. Shannon, Acta Crystallogr. A 32, 751 (1976).

[20] A. L. N. Stevels and J. M. P. J. Verstegen, J. Lumin. 14, 207 (1976).

[21] J. Antoine, D. Vivien, J. Livage, J. Thery, and R. Collongues, Mater. Res. Bull. 10, 865 (1975).

[22] B. Howe and A. L. Diaz, J. Lumin. 109(1), 51 (2004).

[23] G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer-Verlag, Berlin, Heidelberg, 1994), chap. 2.3.

[24] S. Shionoya and W. M. Yen (eds.), Phosphor Handbook (CRC Press, Boston, New York, Washington D.C.,

1999), chap. 3.2.

[25] D. Y. Lee, Y. C. Kang, H. D. Park, and S. K. Ryu, J. Alloys Compd. 353(1/2), 252 (2003).

[26] D. Nötzold, phys. stat. sol. (b) 203, 247 (1997).

[27] L.-J. Shen, Z.-Y. Zhang, L. Han, Y.-B. Zhou, Ch.-L. Liu, Y. Tao, and X. Ju, Faguang Xuebao (Chin. J. Lumin.)

24(2), 189 (2003).

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Article Doi

Structure and optical properties under VUV excitation of Mn2+ doped alkaline earth aluminate phosphors

DOI: 10.1002/pssa.200521372

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