KYW2O8:Eu3+ – A closer look on its photoluminescence and structure

Journal of Luminescence 159 (2015) 251–257

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

KYW2O8:Eu3 þ – A closer look on its photoluminescence and structure Sebastian Schwung a, David Enseling a, Volker Wesemann b, Daniel Rytz b, Birgit Heying c, Ute Ch. Rodewald c, Birgit Gerke c, Oliver Niehaus c, Rainer Pöttgen c, Thomas Jüstel a,n a

Fachbereich Chemieingenieurwesen, Fachhochschule Münster, Stegerwaldstrasse 39, 48565 Steinfurt, Germany Forschungsinstitut für mineralische und metallische Werkstoffe-Edelsteine/Edelmetalle-GmbH (FEE), Struthstrasse 2, 55743 Idar-Oberstein, Germany c Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany b

art ic l e i nf o

a b s t r a c t

Article history: Received 8 September 2014 Received in revised form 22 October 2014 Accepted 29 October 2014 Available online 5 November 2014

High-quality single crystals of partially europium-substituted KYW2O8 (KYW) were grown by the topseeded solution growth technique. The structures of four crystals with different europium content were refined on the basis of single crystal X-ray diffractometer data. The trivalent character of europium in these crystals was manifested through 151Eu Mössbauer spectra and magnetic susceptibility measurements. Moreover, reflection and photoluminescence spectra were recorded and from these spectra the quantum efficiency, lumen equivalent, and CIE1931 color point were calculated. It turned out that Eu3 þ doped KYW is an efficient photoluminescent material at room temperature, while the thermal quenching temperature T1/2 is at about 633 K (360 1C), thus making the material a potential radiation converter for light emitting diodes. & 2014 Published by Elsevier B.V.

Keywords: Tungstates Single crystals Van Vleck paramagnetism Eu3 þ photoluminescence Thermal quenching Reflection spectroscopy

1. Introduction The mineral Scheelite CaWO4 was one of the first solid state gain materials in the early 60s [1,2]. Starting from this structure an evolution to so the called double tungstates and molybdates has taken place [3–6]. In these compounds, two tungstate units per formula are formally split between two bivalent calcium ions into a monovalent alkali metal, especially potassium, and in a trivalent lanthanide, which allows the implementation of a high concentration of other rare-earth ions as luminescent activators. On the basis of this idea many different laser gain materials were detected and investigated for solid state lasing [5]. One of the commonly used host material for typical NIR emitting solid state lasers is KYW2O8, mostly activated by Neodymium, Erbium or Ytterbium [7,8]. In addition to the solid state laser community, research groups active in the search and characterization of host materials for trivalent europium ions for red emitting phosphors for LED application are interested in molybdates and tungstates. This is driven by the situation that phosphor converted (pc) LEDs for general lighting have still some shortcomings: For instance, the dissipation of heat and re-absorption phenomena limits the efficiency and the color point stability over lifetime of pcLEDs. In order to solve these

n

Corressponding author at: Fachbereich Chemieingenieurwesen, Fachhochschule Münster, Stegerwaldstrasse 39, 48565 Steinfurt, Germany. Tel.: þ49 2551 9 62205; fax: þ 49 2551 9 62202. E-mail addresses: [email protected] (D. Rytz), [email protected] (R. Pöttgen), [email protected] (T. Jüstel). http://dx.doi.org/10.1016/j.jlumin.2014.10.067 0022-2313/& 2014 Published by Elsevier B.V.

problems a change of architecture is necessary. In remote phosphor LEDs the conversion screen is not a component of the LED package anymore; it is a component of the LED comprising luminaries. This enables the entry of ceramics and even single crystalline converter materials into the area of solid state lighting. Due to the high quantum efficiency, stability and well-established production of single crystals, this research work focuses on trivalent europium doped molybdates and tungstates. In a recent study double tungstates doped with Eu3þ , as for instance KGdW2O8 [9,10] or tungstates with a rare earth site comprising 100% europium, i.e. KEuW2O8 [11] were investigated and showed very efficient luminescence. Furthermore, the energy transfer between Yb3þ and Eu3 þ in KYbW2O8 [12] was investigated. However, the Eu3þ luminescence in KYW2O8 is only shown in a single study so far, in which solely 2% Eu3 þ was doped into KYW2O8 [13]. In this study, the optical properties and structure of potassium yttrium double tungstate doped with trivalent Europium in dependency of the doping concentration in a temperature range from 100 to 800 K in single crystalline form are presented.

2. Experimental 2.1. Synthesis and crystal growth Eu doped KYW2O8 (KYW) single crystals have been grown by the top-seeded solution growth (TSSG) technique with Eu doping levels (with respect to Y) of 10%, 20% and 30 mol%. Using K2W2O7 as

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a flux and a composition zKYWþ(1 z)K2W2O7 with z¼0.20, crystal growth of KYW:Eu at a liquidus temperature near 900 1C leads to growth results similar to what has already been described in the literature for other dopants such as, for example, Yb or Tm [14]. KYW crystals of up to 15 g were obtained in air from 100 g melts contained in 90 ml platinum crucibles, based on cooling rates of 0.1–0.3 1C/h over an interval of about 15 1C. Following the extraction of the crystal from the liquid solution at the end of the growth cycle, cooling to room temperature took place at a rate of 15 1C/h. The resulting crystals yielded high quality, inclusion and crack free volumes of up to 15 mm3. A typical boule is shown in Fig. 1. It should be noted that scaling this growth process up to boules with weights around or in excess of 100 g should be readily feasible based on our experience to grow KYW crystals with other dopants than Eu. The grown crystals show divergences between the dopant concentration of the melt mixture and the calculated concentrations by the X-ray diffraction results. The KYW:Eu(10%) melt leads in a KY0.913Eu0.087W2O8 crystal, KYW:Eu(20%) in a KY0.786Eu0.214W2O8 crystal, and the melt of KYW:Eu(30%) in a crystal according to KY0.735Eu0.265W2O8. Both, the stoichiometric mixture of the melt and the calculated crystal mixture are used synonymous in the following. All measurements presented here were obtained with polished single crystalline samples (Fig. 2) or on milled single crystal powders. 2.2. EDX data The single crystals studied on the diffractometer were analyzed using a Zeiss EVO MA10 scanning electron microscope in variable pressure mode. MAD-10 feldspar, EuF3 and elemental tungsten were used as standards for the semiquantitative EDX analysis. No

impurity elements heavier than sodium (detection limit of the instrument) were observed. The experimentally determined compositions were close to those refined from the single crystal X-ray data. 2.3. X-ray diffraction Well shaped crystal fragments of differently substituted KYW2O8: Eu3 þ specimens were obtained by careful mechanical fragmentation. The crystals were glued to quartz fibers using bees wax and their quality for intensity data collection was checked by Laue photographs on a Buerger camera (white molybdenum radiation, image plate technique, Fujifilm, BAS-1800). Four suitable crystals were measured at room temperature on an IPDS II (graphite monochromatized Mo Kα radiation; oscillation mode) or StadiVari (m-source, Mo Kα radiation, λ¼71.073 pm, oscillation mode, Dectris Pilatus 100 K detector) diffractometer. Numerical absorption corrections were applied to the data sets. All relevant crystallographic data and details of the data collections and evaluations are listed in Table 1. 2.4. Structure refinements The four data sets showed C-centered monoclinic lattices and the systematic extinctions were compatible with space group C2/c. The atomic positions of the neutron powder diffraction study by Galluci et al. [15] were taken as starting values and the structures were refined with anisotropic displacement parameters for all atoms with SHELXL-97 (full-matrix least-squares on F2o) [16,17]. The very good data quality even allowed for an anisotropic refinement of the oxygen sites. The Y3 þ /Eu3þ mixed occupancies were refined as a least-squares variable leading to the compositions listed in Table 1. The final difference electron-density syntheses were flat. The positional parameters and interatomic distances (exemplarily for KYW2O8 and KY0.735Eu0.265W2O8) are listed in Tables 2 and 3. Further details on the structure refinement may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry no's. CSD–428407 (KYW2O8), CSD–428408 (KY0.913Eu0.087W2O8), CSD–428409 (KY0.786Eu0.214W2O8) and CSD– 428410 (KY0.735Eu0.265W2O8). 2.5. 151Eu Mössbauer spectroscopy and magnetic susceptibility measurements

Fig. 1. Boule of KYW2O8 doped by 10% Eu grown with a b-axis oriented seed (initially located on top, in the middle) by TSSG. The boule weight was 10.7 g, the length scale is 10 mm per division.

The 21.53 keV transition of 151Eu with an activity of 130 MBq (2% of the total activity of a 151Sm:EuF3 source) was used for the Mössbauer spectroscopic experiment, which was conducted in the usual transmission geometry. The measurement was performed with a commercial helium-bath cryostat at 78 K. The sample was enclosed in a small PMMA container at a thickness corresponding to about 10 mg Eu/cm2. Fitting of the spectrum was performed with the Normos-90 program system [18]. The magnetic measurements were carried out on a quantum design physical property measurement system (PPMS) using the

Fig. 2. Photographs of a KYW2O8:Eu polished crystal at ambient light (left) and upon illumination by a 410 nm emitting (In,Ga)N diode (right).

S. Schwung et al. / Journal of Luminescence 159 (2015) 251–257

253

Table 1 Crystal data and structure refinement results for KYW2O8, KY0.913Eu0.087W2O8, KY0.786Eu0.214W2O8, and KY0.735Eu0.265W2O8, space group C2/c, Z¼ 4. Compound

KYW2O8

KY0.913Eu0.087W2O8

KY0.786Eu0.214W2O8

KY0.735Eu0.265W2O8

a (pm) b (pm) c (pm) β (deg) V (nm3) Molar mass (g mol–1) Calculated density (g cm–3) Absorption coefficient (mm–1) F(000) (e) Crystal size (mm3) Transm. ratio (min/max) θ range (deg) Range in hkl Total no. reflections Independent reflections (Rint) Reflections with IZ 2σ(I) (Rσ) Data/parameters Goodness-of-fit on F2 R1/wR2 for IZ 2σ(I) R1/wR2 for all data Extinction coefficient Largest diff. peak/hole (e Å–3)

1063.25(19) 1033.21(18) 755.39(14) 130.87(1) 0.6275 623.71 6.60 46.4 1080 20  40  70 0.025/0.432 4–33 7 16, 7 15, 7 11 4186 1145/0.0485 1008/0.0306 1145/57 1.006 0.0261/0.0609 0.0287/0.0619 0.0196(5) 2.65/  2.69

1063.44(6) 1035.28(4) 755.71(4) 130.75(1) 0.6303 629.23 6.63 46.3 1088 20  30  70 0.144/0.741 4–33 7 16, 7 16, 7 11 4596 1227/0.0308 1029/0.0230 1227/58 0.890 0.0137/0.0282 0.0189/0.0289 0.00350(8) 2.20/  2.00

1064.61(5) 1037.17(4) 756.40(3) 130.74(1) 0.6328 637.20 6.69 46.2 1101 40  40  70 0.044/0.200 4–33 7 16, 7 16, 711 4490 1228/0.0475 1063/0.0354 1228/58 0.896 0.0151/0.0281 0.0199/0.0288 0.0263(3) 1.78/  1.61

1064.99(5) 1037.87(3) 756.77(3) 130.76(1) 0.6336 640.42 6.71 46.2 1105 40  60  80 0.054/0.253 4–33 7 16, 7 16, 7 11 4503 1232/0.0521 1144/0.0333 1232/58 1.071 0.0277/0.0677 0.0293/0.0670 0.0063(3) 5.56/  5.02

Table 2 Refined atomic positions and displacement parameters of KYW2O8, KY0.913Eu0.087W2O8, KY0.786Eu0.214W2O8 and KY0.735Eu0.265W2O8. Ueq is defined as one-third of the trace of the orthogonalized Uij-tensor. The exponent of the anisotropic displacement parameters is defined through exp{–2π2  (U11h2a*2 þ …þ U12hka*b*)}. Atom

Wyckoff position

x

y

z

U11

U22

U33

U12

U13

U23

Ueq

KYW2O8 K Y W O1 O2 O3 O4

4e 4e 8f 8f 8f 8f 8f

0 0 0.19615(2) 0.1268(4) 0.0236(4) 0.2256(4) 0.1900(4)

0.7002(2) 0.27158(6) 0.00012(1) 0.5802(3) 0.1085(3) 0.3419(3) 0.9239(3)

3/4 3/4 0.23582(3) 0.1897(6) 0.9704(6) 0.1256(5) 0.9409(6)

236(7) 143(3) 115(1) 170(15) 146(14) 194(16) 185(15)

198(6) 118(2) 124(1) 189(13) 139(11) 149(12) 166(13)

231(6) 125(2) 116(1) 191(15) 133(12) 148(14) 164(13)

0 0 –1(1) 18(11) 8(9) –20(10) 4(10)

152(6) 87(2) 72(1) 123(13) 86(12) 104(13) 124(13)

0 0 6(1) –21(11) 17(9) –16(10) 3(10)

222(3) 129(1) 121(1) 180(6) 144(5) 170(6) 164(6)

KY0.913Eu0.087W2O8 K 4e 91.3(3)% Yþ 4e

0 0

0.69980(11) 0.27163(4)

3/4 3/4

129(5) 58(2)

126(4) 23(2)

134(5) 38(2)

0 0

74(4) 33(2)

0 0

138(2) 39(1)

8.7(3)% Eu W O1 O2 O3 O4

0.19602(1) 0.1258(3) 0.0238(3) 0.2251(3) 0.1906(3)

0.00013(1) 0.5797(2) 0.1083(2) 0.3422(2) 0.9244(2)

0.23581(2) 0.1881(5) 0.9704(4) 0.1256(5) 0.9422(5)

–1(1) 25(8) –10(7) –21(7) –1(7)

19(1) 39(9) 35(9) 42(9) 69(9)

6(1) –17(8) 13(7) –24(8) 10(8)

KY0.786Eu0.214W2O8 K 4e 78.6(3) % Y þ 4e

0 0

0.70037(11) 0.27170(3)

3/4 3/4

151(5) 75(2)

153(4) 47(1)

151(5) 51(2)

0 0

89(5) 40(1)

0 0

159(2) 58(1)

21.4(3)% Eu W O1 O2 O3 O4

0.19583(1) 0.1264(3) 0.0241(3) 0.2256(3) 0.1896(3)

0.00011(1) 0.5796(2) 0.1082(2) 0.3420(2) 0.9242(2)

0.23582(3) 0.1889(4) 0.9706(4) 0.1269(4) 0.9411(4)

55(1) 93(10) 64(10) 121(11) 117(10)

52(1) 139(10) 70(9) 85(9) 98(9)

47(1) 100(11) 72(10) 75(11) 102(10)

–1(1) 27(8) –8(7) –31(8) –9(8)

30(1) 67(10) 35(9) 49(10) 81(9)

6(1) –17(8) 0(7) –28(7) 1(7)

53(1) 108(4) 76(4) 105(4) 98(4)

KY0.735Eu0.265W2O8 K 4e 73.5(5)% Yþ 4e

0 0

0.70053(16) 0.27172(4)

3/4 3/4

131(6) 64(2)

153(6) 50(2)

148(6) 40(2)

0 0

77(5) 30(2)

0 0

155(3) 55(1)

26.5(5)% Eu W O1 O2 O3 O4

0.19571(2) 0.1269(4) 0.0236(4) 0.2263(4) 0.1899(4)

0.00012(1) 0.5793(3) 0.1083(3) 0.3424(3) 0.9244(3)

0.23580(3) 0.1888(5) 0.9707(5) 0.1269(5) 0.9414(5)

53(2) 106(13) 70(11) 105(13) 108(13)

59(1) 133(13) 99(12) 92(11) 103(12)

–1(1) 16(10) –6(9) –31(9) –24(10)

25(1) 60(11) 37(10) 38(11) 66(10)

6(1) –15(9) 11(8) –31(9) –16(9)

55(1) 109(5) 82(5) 103(5) 96(5)

8f 8f 8f 8f 8f

8f 8f 8f 8f 8f

8f 8f 8f 8f 8f

36(1) 71(9) 68(9) 96(10) 92(10)

30(1) 94(9) 52(8) 47(8) 83(9)

32(1) 70(11) 50(11) 71(11) 75(11)

39(1) 84(12) 62(10) 68(11) 83(11)

34(1) 84(4) 59(4) 81(4) 72(4)

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Table 3 Interatomic distances (pm), calculated with the powder lattice parameters of KYW2O8 and KY0.735Eu0.265W2O8. All distances within the first coordination spheres are listed. Standard deviations are all equal or less than 0.3 pm. YKW2O8

Y0.735Eu0.265KW2O8

K

2 2 2 2 2 2

O4 O4 O1 O2 O3 O1

277.3 280.9 292.0 302.0 312.7 335.0

K

2 2 2 2 2 2

O4 O4 O1 O2 O3 O1

278.6 282.3 292.9 302.7 313.2 336.0

Y

2 2 2 2

O2 O1 O3 O3

226.5 227.5 232.9 268.0

Y/Eu

2 2 2 2

O2 O1 O3 O3

227.6 229.1 234.6 268.7

W

1 1 1 1 1 1

O4 O1 O3 O2 O2 O4

177.5 177.7 182.2 195.6 209.3 232.3

W

1 1 1 1 1 1

O4 O1 O3 O2 O2 O4

177.9 177.9 182.3 196.0 209.7 232.5

O1

1 1 1 1

W Y K K

177.7 227.5 292.0 335.0

O1

1 1 1 1

W Y/Eu K K

177.9 229.1 292.9 336.0

O2

1 1 1 1

W W Y K

195.6 209.3 226.5 302.0

O2

1 1 1 1

W W Y/Eu K

196.0 209.7 227.6 302.7

O3

1 1 1 1

W Y Y K

182.2 232.9 268.0 312.7

O3

1 1 1 1

W Y/Eu Y K

182.3 234.6 268.7 313.2

O4

1 1 1 1

W W K K

177.5 232.3 277.3 280.9

O4

1 1 1 1

W W K K

177.9 232.5 278.6 282.3

powder samples. Thermal quenching measurements in the temperature range from 100 to 500 K were conducted by using a “MicrostatN” cryostat from Oxford Instruments in a copper sample holder. The temperature range from 300 to 800 K was investigated in a self-build heating chamber also in copper sample holders. For detection, a R2658P single-photon counting photomultiplier tube (Hamamatsu) was used. All luminescence spectra were recorded with a spectral resolution of 0.2 nm, a dwell time of 0.4 s in 0.2 nm steps, and three repeats.

3. Results and discussion 3.1. Crystal chemistry The KYW2O8:Eu3 þ crystals grown by the top-seeded solution growth technique. had high-quality and allowed for the collection of complete data sets by single crystal X-ray diffraction. The structure refinements converged smoothly to the residuals listed in Table 1.

Fig. 3. View of the KYW2O8 structure approximately along the c axis. Potassium, yttrium, tungsten, and oxygen atoms are drawn as light blue, yellow, green, and red circles, respectively. The oxygen polyhedra around the metal atoms are emphasized. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

vibrating sample magnetometer (VSM) option. Data were collected from 2.5 to 305 K with a magnetic flux densities of 10 kOe. 2.6. Optical property measurements Reflection spectra of polished single crystalline samples were recorded on an Edinburgh Instruments FS900 spectrometer equipped with a 450 W Xe arc lamp, a cooled single-photon counting photomultiplier (Hamamatsu R928) and an integration sphere coated with barium sulfate. A BaSO4 sample (99% SigmaAldrich) was used as reflectance standard. The quantum efficacy was also determined by using the Edinburgh Instruments FS900 spectrometer in an integrated sphere using the absolute luminescence quantum yields method according to Suzuki et al. [19]. A BaSO4 coated integrated sphere and a BaSO4 white standard was used to determine the excitation beam intensity and compare this with the reflectance of the sample at the excitation wavelength to determine the absorption strength of the phosphor. By comparison of the integral of the absorbed radiation with the integral of the emitted light the quantum efficacy can be calculated. Excitation and emission spectra of crushed single crystals were recorded with a fluorescence spectrometer FLS920 (Edinburgh Instruments) equipped with a 450 W ozone-free xenon arc lamp (OSRAM) and a sample chamber installed with a mirror optic for

Fig. 4. View of the KYW2O8 structure approximately along the b axis. Potassium, yttrium, tungsten, and oxygen atoms are drawn as light blue, yellow, green, and red circles, respectively. Rows of corner and edge-sharing WO6 octahedra are emphasized. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

100

KYW:Eu 10% KYW:Eu 20%

90

70 60

F0 - D3

50

300

7

5

F0,1 - D1

5 7

5

F0 - L6

7

5

F0 - Gj, L7

5

5

0

F0 - D4

10

CT

7

20

7

30

F0 - D2

5

40

7

Reflectance [%]

80

400

500

600

700

800

Wavelength [nm] Fig. 5. Reflection spectra of KYW2O8 doped by 10% or 20% Eu vs. BaSO4 as a white standard.

5

5

7

4.0x105

5

7

F0,1 - D3

7

7

6.0x105

F0,1 - D2

5 3

F0,1 - HJ

8.0x105

7

5

F0,1 - D4

1.0x106

2.0x10

CT

5

0.0 250

300

350

400

450

500

Wavelenth [nm] Fig. 6. Excitation spectra of KYW2O8 doped by 10% or 20% Eu monitored at 614 nm.

5 D - 7F 0 2

1.0x105

KYW:Eu 10% KYW:Eu 20%

0.0

600

5D - 7F 0 4 650

5D - 7 F 0 5

2.0x104

5D - 7F 0 3

4.0x104

5D - 7F 0 1

6.0x104

5D - 7F 0 0

Intensity [counts]

Reflection spectra of both KYW:Eu crystals show a reflectance of about 75% in the visible range, which is in line with the calculated result from the Fresnel equation taking a refractive index of n¼2.08 for KYW at 633 nm into account. Moreover, it demonstrates that the strength of the [Xe]4f6–[Xe]4f6 absorption lines in the range from 350 to 800 nm is dependent on the Eu3 þ concentration as expected. In contrast, the Eu3 þ concentration does not change the shape of the absorption edge at 320 nm (Fig. 5). According to our interpretation of the excitation spectra this absorption edge is caused by a ligand-to-metal charge transfer (LMCT) within the tungstate units and causes a broad excitation band ranging from 270 to 340. It is obvious that the LMCT absorption band as part of the reflection spectra is not or only little changed by the Eu3 þ concentration, which is in line with its constant shape and position. To eliminate effects of the optical orientation of the crystals by the polarization of the incoming light, crushed single crystals was used to observe statistic spectra of the emission and excitation. Furthermore, the excitation spectrum shows the typical [Xe]

F0,1 - GJ, L7

1.2x106

8.0x104

3.2. Optical properties

5

KYW: 20% Eu KYW: 10% Eu

1.4x106

7

1.6x106

Intensity [counts]

Our structural data are more precise than those of previous reports [20,21]. Although the focus of our work lies on the characterization of the optical properties of the KYW2O8:Eu3 þ samples, we briefly discuss the crystal chemical details of the host structure. The structure of KYW2O8 is presented in Figs. 3 and 4. The three differently charged cations exhibit different coordination number. The small W6 þ cations have strongly distorted octahedral oxygen coordination with W–O distances ranging from 178 to 232 pm. Tungstates with tetrahedral coordination have smaller ranges of W–O distances, e.g. 176–177 pm in Sc2(WO4)3 [22]. Two WO6 octahedra in KYW2O8 share a common edge and these double units condense via common corners in c direction (Fig. 4). Adjacent chains show a staggered arrangement (a consequence of the C centering). The Y3 þ and K þ ions fill the space between the octahedral chains (Fig. 3): Y@O8 with 226–268 pm Y–O and K@O12 with 277–335 pm K–O. Both polyhedra are substantially distorted. Substitution of Y3 þ by the larger Eu3 þ ions leads to a slight overall increase of the interatomic distances (Table 3) in the order of ca. 1 pm, but no pronounced changes in the local coordination geometry. The Y/Eu substituition is also reflected by the course of the lattice parameters. We observe an almost linear increase from pure KYW2O8 to KY0.735Eu0.265W2O8 with the highest substitution rate (Table 1). Our findings are similar to related substitution experiments in KGdW2O8 [23], KYbW2O8 [24], and KLuW2O8 [25].

255

F0,1 - L6

S. Schwung et al. / Journal of Luminescence 159 (2015) 251–257

700

750

Wavelength [nm] Fig. 7. Emission spectra KYW2O8 doped by 10% or 20% Eu upon excitation at 394 nm.

4f6–[Xe]4f6 intraconfiguration transitions from the 7F0 ground state of Eu3 þ to the excited states 5D2, 5D3, 5L6 and so on, while the transitions to the 3HJ levels are overlaid by the broad LMCT band (Fig. 6). Both crystals show the most intense photoluminescence upon excitation into the 5L6 state at 394 nm, whereby the quantum efficiency is higher for the 20% sample. This remarkable result can be explained by the fact that 394 nm photons are absorbed by crystal defect and by the 7F0–5L6 transitions. The presence of some crystal defects is proven by the weak UrbachTailing visible in the reflection spectrum between 330 and 420 nm. Under the assumption that the absorption by the defect states and subsequent energy transfer to Eu3 þ is not as efficient as the direct absorption by trivalent Europium, one would expect higher quantum efficiency with an increasing Eu3 þ concentration. The emission spectrum (Fig. 7) of both single crystals exhibits the typical emission line multiplets originating from the 5D0–7FJ emission transitions. The intensity and width of the forbidden 5D0–7F0 transition of trivalent europium is an indicator for the number and symmetry of the crystal sites hosting the activator [26,27]. Furthermore, the ratio of the 5D0–7F1 to 5D0–7F2 emission intensity is an indicator for the symmetry of the crystal site for Eu3 þ . For a inversion symmetric crystallographic site of Eu3 þ the magneticdipole 5D0–7F1 transition (J¼1) is favoured due to the selection rules. In case of non-inversion symmetry the electric-dipole transition with

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Integral [counts]

Emission Spectra 8x10

7

7x10

7

6x10

7

5x10

7

4x10

7

3x10

7

2x10

7

3+

KY(WO 4)2 : Eu (20%) Struck-Fonger model (-E/(k*T)) I = I0/(1 + B*e ) B = 690 (+/- 281) E = 0,357 eV (+/- 0,023 eV) T1/2= 633 K

1x107 0

Fig. 8. Simplified energy level diagram of Eu3 þ in KYW and LiEuMo2O8 (LEM) with related emission intensity of the emission from 5D0 to the 7FJ levels. Dotted energy levels were not measured and are taken from literature [24].

J¼0, 72 is no more strictly forbidden and behaves hypersensitive [28]. That leads to a dominating 5D0–7F2 transition in emission spectra of most Eu3 þ doped materials as we observe it. This confirms the X-ray diffraction results that Eu3 þ is located onto non-inversion symmetric crystal sites in the KYW2O8 matrix. As mentioned above, the distorted polyhedra leads to a lower symmetry of the crystal sites where Eu3 þ is located and results in a remarkable intensity of the 5D0–7F0 transition in contrast to some related materials like LiEuMo2O8, which was studied before also on crushed single crystals [29]. Taking the spectral data of this study and of the earlier studied LiEuMo2O8 into account, a simplified energy level diagram for Eu3 þ in KYW2O8 and in LiEuMo2O8 based on the Dieke diagram [30] is presented (Fig. 8). This reveals the slight energy differences of the energy levels related to the [Xe]4f6 configuration of trivalent Europium. Furthermore, the diagram comprises the relative intensity fraction of the 5 D0–7F5–0 transitions relative to the total emission intensity. Furthermore, our results are in good agreement with other studies on single crystalline KYW:Eu. The energy of the 5D0–7F5–0 transitions are close to the results of KYW:Eu(2%), which shows also hypersensitive 5D0–7F2 emission [13]. For a study of the thermal quenching of the photoluminescence, emission spectra of the 20% Eu3 þ comprising sample in the range from 100 to 800 K were measured (Fig. 9). The plot of the temperature dependant emission integrals was fitted by using the Struck–Fonger model for thermal quenching [31] I¼

I0 1 þ Bne  E=knT

ð1Þ

whereby I is the intensity, I0 is the intensity at low temperature (0 K), B is the phonon frequency of the quenching mechanism, E is the threshold energy of the quenching mechanism, k is the Boltzmann constant, and T is the temperature. From the fit function the thermal quenching temperature T1/2 is derived to by 633 K (360 1C) and the threshold energy to about 0.357 eV (2880 cm  1). This is five to six times the energy distance from the emitting 5D0 level to the ground state 7F2 level. This is the typical energy distance at which multiphonon relaxation sets in [32]. The quantum efficacy is almost 40% for the single crystal doped by 10% Eu3 þ and 47% for the one doped by 20% Eu3 þ . The increase shows that concentration quenching is not an issue here and furthermore that a higher Eu3 þ concentration leads to higher luminescence intensities. CIE 1931 color point and luminous efficacy are typical for Eu3 þ activated luminescent materials, wherein the activator is located in non-centrosymmetric sites and are summarized in Table 4.

100 150 200 250 300 350 400 450 500 550 600 650 700 750 800

Temperature [K] Fig. 9. Experimental thermal quenching curve of 20% Europium doped KYW (squares) and the result of the fitting by using the Struck–Fonger model (dasheddotted line).

Table 4 Photoluminescence data of KYW crystals doped by 10% or 20% Eu3 þ . Single crystal

Luminous efficacy [lm/W]

Quantum efficiency [%]

CIE1931 color point x; y

KYW2O8:Eu (10%) KYW2O8:Eu (20%)

269

39

0.673; 0.327

257

47

0.673; 0.326

all data are given for 394 nm excitation.

3.3.

151

Eu Mössbauer spectroscopy and magnetic susceptibility

A 151Eu Mössbauer spectrum (78 K data) of the KYW2O8 crystal doped with 30% Eu is presented in Fig. 10 together with a transmission integral fit. The spectrum was well reproduced by a single signal at an isomer shift of δ¼0.68(3) mm s–1, subjected to quadrupole splitting of ΔEQ ¼3.4(3) mm s–1, a consequence of the non-cubic site symmetry of the europium atoms. The isomer shift indicates purely trivalent europium. The experimental line width of 2.1(1) mm s–1 is in the usual range. The temperature dependence of the magnetic susceptibility of KYW2O8 and KY0.7Eu0.3W2O8 is presented in Fig. 11 KYW2O8 shows a diamagnetic signal over the whole temperature range. The weak upturn towards low temperature can be attributed to traces of paramagnetic impurities. The absolute value of –3.0(1)  10–4 emu/mol is comparable to an estimate of –1.49  10–4 emu/mol from the diamagnetic increments [33]. The europium substituted sample shows the typical van Vleck paramagnetism of trivalent europium, underlining the Mössbauer spectroscopic data.

4. Conclusions Europium doped single crystals of KYW2O8 comprise the dopant solely in the trivalent state according to the photoluminescence and Mössbauer spectra. This observation is confirmed by the magnetic susceptibility as function of temperature. It shows van Vleck paramagnetism as typical for Eu3 þ , while the undoped sample (KYW) is diamagnetic. This excludes the formation of tungsten in lower valence states caused by the potential loss of Oxygen, even though

S. Schwung et al. / Journal of Luminescence 159 (2015) 251–257

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References

Fig. 10. Experimental (data points) and simulated (red and blue line) 151Eu Mössbauer spectrum of KY0.735Eu0.265W2O8.at 78 K. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Temperature dependence of the magnetic susceptibility of KYW2O8 and KY0.735Eu0.265W2O8.

the crystals were grown at a high temperature of 900 1C. Therefore, the host material KYW is obviously stable against reduction up to 900 1C. The photoluminescence of both Eu3 þ doped crystals is typical for trivalent Europium, while the reflection spectrum confirms the high refractive index of KYW and shows the absorption edge caused by the LMCT at about 320 nm. Both crystals show a high quantum efficiency, while the 20% samples is thermally quenched by 50% at about 633 K (360 1C). It thus turns out that Eu3 þ doped single crystals for KYW are suitable as converter materials for (In,Ga)N LEDs. Acknowledgments This work was financially supported by the Deutsche Forschungsgemeinschaft. B.G. and O.N. acknowledge a research fellowship from the Fond der Chemischen Industrie and the NRW Graduate School of Chemistry.

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