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High-pressure luminescence spectroscopy of EuAl2O4 phosphor
Radiation Measurements 42 (2007) 652 – 656 www.elsevier.com/locate/radmeas
High-pressure luminescence spectroscopy of EuAl2O4 phosphor Yu. Zorenko a , V. Gorbenko a , M. Grinberg b,∗ , R. Turos-Matysiak b , B. Kukli´nski b a Laboratory of Optoelectronic Materials (LOM), Department of Electronic, Ivan Franko National University of Lviv, 79017 Lviv, Ukraine b Institute of Experimental Physics (IEP), Gda´nsk University, 80 952 Gda´nsk, Poland
Received 18 December 2006; accepted 31 January 2007
Abstract EuAl2 O4 powder phosphor was prepared by solid-state reaction of EuO and Al2 O3 oxides in vacuum. The influence of conditions of preparation on spectral lineshape of Eu2+ emission was analyzed. It was found that the fluorescence spectra of vacuum-prepared EuAl2 O4 samples at 300 K present the superposition of three bands peaked at 430, 500 and 528 nm, corresponding to the 4f 6 5d1 → 4f 7 (8 S7/2 ) transition of Eu2+ ions in the different sites of EuAl2 O4 lattice. The luminescence of Eu2+ centers in EuAl2 O4 host was also studied using the high-pressure spectroscopy up to 67 kbar. It was found that the bright green-yellow fluorescence of EuAl2 O4 at 300 K in the band peaked at 520.530 nm range can be presented by superposition of two Gaussian sub-bands. The different pressure shifts −23 and −27 cm−1 /kbar for two sub-bands were found. Such a structure of the emission spectrum was attributed to the existence of two different Eu2+ centers in the Eu2+ II sites of EuAl2 O4 lattice with higher coordination number. © 2007 Elsevier Ltd. All rights reserved. Keywords: EuAl2 O4 , Eu2+ luminescence; Solid-state reaction; High-pressure spectroscopy
1. Introduction The Eu2+ concentrated EuAl2 O4 belongs to the class of MAl2 O4 (M = Sr, Ca, Ba) phosphors (Blasse and Bril, 1968; Abbruscato, 1971). It shows a bright yellow-green emission with maximum at 520 nm and very weak phosphorescence in the region of room temperatures (RT) (Schierning et al., 2005). This emission can be effectively excited with a wavelength of 320–460 nm due to the presence of several absorption bands corresponding to the electric dipole transitions from the ground state of 4f 7 electronic configuration, 8 S7/2 , to the eg and t2g states of the excited 4f 6 5d1 electronic configuration of the Eu2+ ion located in different sites of MeAl2 O4 host (Poort et al., 1995; Wang et al., 2002; Meister et al., 2007). For this reason the EuAl2 O4 is suitable for the purposes of light conversion in white light-emitting diode (LED) lamps where the system is
∗ Corresponding author. Tel.: +48 58 5529544; fax: +48 58 3413175.
E-mail address: fi[email protected] (M. Grinberg). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.01.060
excited in the blue and UV by LED and emits in the yellowgreen and red spectral ranges (Schierning et al., 2005, Meister et al., 2007). The EuAl2 O4 and Eu1−x Mx Al2 O4 powder samples which are synthesized under various conditions by solid state reaction from Eu2 O3 , Al2 O3 and MCO3 charge have been investigated previously by Schierning et al. (2005). It has been found that application of the reducing atmosphere during the synthesis causes an increase of intensity of Eu2+ fluorescence. This is due to fact that reducing atmosphere stabilizes europium ions in the charge state of Eu2+ . One expects that condition of reducing treatment can also strongly influence the local surrounding of Eu2+ ions that, in turn, affects the shape of emission spectra and quantum efficiency of EuAl2 O4 phosphors. In this work we investigate the luminescence of EuAl2 O4 powder samples, sintered by solid-state reaction in vacuum (1.3 Pa) at temperatures of 900–1100 ◦ C. For studying the local surrounding and distribution of Eu2+ centers in EuAl2 O4 host the high-pressure spectroscopy of Eu2+ luminescence in the pressure up to 67 kbar was applied.
Yu. Zorenko et al. / Radiation Measurements 42 (2007) 652 – 656
2. Samples and experimental technique 2.1. Sample preparation The EuAl2 O4 powder samples were prepared in LOM (Lviv University) with equal molar parts of raw EuO (5 N purity) and Al2 O3 oxides (4 N) by solid state reaction. The charge was mixed in agate mortise and pressed in tablets with a diameter of 10 mm and a thickness of 1–2 mm. The tablets labeled as 1, 2 and 3 were annealed in the quarts tubes for 4 h in a furnace with resistive heating in vacuum (1.3 Pa) at temperatures 900, 1000 and 1100 ◦ C, respectively. The walls of quarts tubes were covered by grapheme layers. Dependence of sample composition on annealing temperature and treatment duration was analyzed by X-ray diffraction (XRD) and photoluminescence (PL). The XRD measurement was carried out using a DRON X-ray diffractometer (W-anode operated at 35 keV). XRD pattern of a main part of EuAl2 O4 powder samples 2 and 3 shows that the value of 2 is very close to the wellknown SrAl2 O4 pattern (monoclinic structure C22 with the P21 space group), which is identical to EuAl2 O4 structure due to al˚ and Sr 2+ (1.32 A) ˚ most the same ionic radii for Eu2+ (1.31 A) ions (http://abulafia.mt.ic.ac.uk/shannon/radius.php). From the XRD measurement it has been also found that sample 1 contains a small amount of EuAlO3 and Al2 O3 oxides. It is worth noting that the Eu2+ ions in EuAl2 O4 structure can substitute EuI and EuII positions with the coordination number close to 6.07 and 6.23, respectively (Schulze and Müller-Buschbaum, 1981; Meister et al., 2007). The color of the samples depends on the temperature of treatment and changes from the light-green for samples annealed at 900 ◦ C to the green and deep-green for samples annealed at 1000 and 1100 ◦ C, respectively. PL spectra of vacuum-prepared samples were compared with the emission of the sample labeled as sample 4 (Schierning et al., 2005) prepared in WW6 (Erlangen University) by the solid-state reaction synthesis in an induction heated furnace in graphite crucible at 1215 ◦ C in strongly reducing conditions (95% N2 + 5% H2 atmosphere).
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sion frequency. The reason for the adjustment is that for the broad band luminescence consisting of a number of vibronic components the radiative rates of those emitting at higher frequency are enhanced by the energy-dependent photon density of states (Nakazawa, 1999). 3. Results and discussions 3.1. PL of powder samples The PL spectra of EuAl2 O4 powder samples under excitation with a wavelength of 330 nm (sample 1) and 460 nm (samples 2–4) in the region of the 4f 7 (8 S7/2 ) → 4f 6 5d1 transitions of Eu2+ ions are shown in Fig. 1. Shape of the emission spectra of samples 1–4 substantially depends on the excitation wavelength and annealing temperature. The PL spectrum of sample 1 under excitation at wavelength of 330 nm (Fig. 1, curve 1) presents a complex band in the 390–750 nm range which is a superposition of couples of Eu2+ -related bands peaked approximately at 430 and 450 nm (dark-blue emission), as well as the bands peaked at 505 and 555 nm (green-yellow emission). Under excitation at wavelength 460 nm the emission of sample 1 also shows the week luminescence in green-yellow (520–530 nm) range (not show in Fig. 1). With increasing the annealing temperature to 1000 ◦ C (sample 2) and to 1100 ◦ C (sample 3) the contribution of the emission in the blue region strongly decreases whereas the intensity of the bands in the green region notably increases and their main maximum under excitation at a wavelength of 460 nm shifts to 555 nm for sample 2 and to 530 nm for sample 3 (Fig. 1, curves 2 and 3, respectively). It should be noted that for vacuumprepared sample 3 only emission band peaked at 530 nm with small bump at 555 nm is observed. This emission practically coincides with the spectra of sample 4 peaked at 523 nm which was prepared by annealing in the 95% N2 + 5% H2 atmosphere at 1215 ◦ C (Schierning et al., 2005).
2.2. High pressure luminescence spectra High pressure PL spectra of the EuAl2 O4 powder samples were measured in IEP Gdansk University. All high-pressure measurements were made at RT. For applying the pressure we have used a diamond anvil cell (DAC) with dimethylosiloxane as a pressure transmitting medium. Luminescence of ruby crystal has been used for estimation of quantity of pressure. Experiments have been performed for the pressure range from ambient to 67 kbar. He–Cd laser emission with wavelength =325 nm has been used for excitation. Emission has been dispersed with a PGS2 monochromator and collected by a cooled Hamamatsu R943-02 photomultiplier working in the photon counting regime. All luminescence spectra have been corrected with respect to the apparatus response. The luminescence intensities were adjusted by the factor of −3 , where is the emis-
Fig. 1. PL spectra of EuAl2 O4 powder samples 1–4 under excitation at 330 nm (samples 1) and 460 nm (samples 2–4).
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Fig. 2. (a) Excitation spectra of the luminescence at a wavelength of 430 nm (1) and 500 nm (2, 3) and emission spectra (4, 5) under excitation at a wavelength of 330 nm of EuAl2 O4 powders samples 2 and 3; (b) excitation spectra (1, 2) of the luminescence at a wavelength of 530 nm and emission spectra (3, 4) under excitation at a wavelength of 330 nm of EuAl2 O4 powder sample 4 at 80 K (1, 3) and 300 K (2, 4), respectively.
The luminescence bands shown in Fig. 1 have been attributed to the 4f 6 5d1 → 4f 7 (8 S7/2 ) transitions of Eu2+ ions. The observed dependence of the peak position and intensity of Eu2+ -related bands peaked at 520–560 nm in samples 2–4 indicates the existence of the strong dependence of the Eu2+ ions emission lineshape on temperature of preparation. The existence of complicated Eu2+ -related emission bands peaked in the dark-blue (430–450 nm) and green-yellow (505–545 nm) regions (curve 1 for samples 1) testifies the existence of several Eu2+ related emission centers located in different sites of EuAl2 O4 lattice. This conclusion is strongly supported by the shape of the excitation spectra of Eu2+ luminescence in samples 1–4 (Fig. 2). The excitation spectra of dark-blue emission monitored at 430 nm in sample 2 (Fig. 2a, curve 1) consists
of the two bands peaked at 250 and 308 nm related to the 4f 7 (8 S7/2 ) → 4f 6 5d1 (eg and t2g ) transitions of Eu2+ ions, located in the first, Eu1 site of EuAl2 O4 lattice with lower coordinate number (Poort et al., 1995). It is worth to note that the low energy tail of the 308 nm band is significantly widened. The excitation spectra of the emission monitored at wavelength 500 nm in samples 2, 3 and 4 are presented in the Fig. 2a and b, respectively. The excitation spectra of samples 2 and 3 (Fig. 2a, curves 2 and 3) apart of the bands peaked at 250 and 308 nm contains the dominant band peaked at 323 nm and additional low-intensity band around 440 nm, related to the 4f 7 (8 S7/2 ) → 4f 6 5d1 (eg and t2g ) transitions of Eu2+ ions, located probably in the second, EuII site of EuAl2 O4 lattice. The low energy tail of the 324 nm band contains additionally the bump peaked at 374 nm. It is worth to note that the intensity of the 323 nm excitation bands in vacuum-prepared EuAl2 O4 samples 2 and 3 notably increases with rise of annealing temperature from 1000 to 1100 ◦ C (Fig. 2a, curves 2 and 3, respectively). This effect reflects the increase of the concentration of Eu2+ II centers. Further increase of the annealing temperature beyond 1200 ◦ C results in rearrangement of Eu2+ in different sites of EuAl2 O4 lattice. Specifically, for sample 4 annealed at 1215 ◦ C in the 95% N2 + 5% H2 atmosphere, the bands peaked 324 and 430 nm of EuII centers dominate in the excitation spectra of the green-yellow (530 nm) luminescence at RT (Fig. 2b, curve 1). At the same time, the another couple of excitation bands, peaked approximately at 380 and 467 nm, becomes apparent in the excitation spectra of luminescence at 530 nm in samples 4 at 80 K (Fig. 2b, curve 2). These excitation bands and resulted emission band peaked at 528 nm (curve 3) probably belong to another europium related centers in EuAl2 O4 lattice, the Eu2+ II . It is worth to note that excitation bands at 380 and 460 nm of the emission band at 520 nm in SrAl2 O4 :Eu phosphor was recently related to Eu2+ II centers with higher coordination number (Poort et al., 1995). Our results testify that at least two different luminescence centers exist in the EuII position of EuAl2 O4 host which emit in the bands peaked approximately in the 500–505 and 528–530 nm ranges (Fig. 2a and b, respectively). Thus, the annealing temperature influences strongly the distribution of Eu2+ ions over EuI and EuII or/and EuII positions of EuAl2 O4 lattice, which are responsible for dark-blue and greenyellow emission, respectively. The process of EuI → EuII and EuI → EuII redistribution notable increases for annealing temperatures above 1000 and 1200 ◦ C, respectively. It should be noted that the intensity of the excitation bands of Eu2+ I centers at 250 and 308 nm decreases with the increasing 2+ of Eu2+ II and EuII centers concentrations (Fig. 2a, curves 2 2+ and 3) due to effective Eu2+ → Eu2+ I II and EuII and energy transfer caused by the re-absorption of the dark-blue emission bands peaked at 430 nm by the absorption bands of Eu2+ II and 2+ EuII centers peaked at 430 and 467 nm, respectively. This results in complete quenching of the emission band at 430 nm and domination of the emission band peaked at 520–528 nm in the PL spectra of sample 4 (Fig. 2b). As a result, the EuAl2 O4 powder samples show very bright luminescence in green-yellow
Yu. Zorenko et al. / Radiation Measurements 42 (2007) 652 – 656
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Fig. 4. Positions of the emission peaks related to the luminescence of two different centers for Eu2+ ions in EuII sites of EuAl2 O4 host.
Fig. 3. Emission spectra of EuAl2 O4 powder sample 3 in a pressure range of 8–67 kbar under excitation at 325 nm.
region with quantum efficiency is approximately equal to 34% (Meister et al., 2007). The absence of concentration quenching (energy transfer between the same Eu2+ -related centers) in such phosphors is due to a small spectral overlapping of the emission and excitation spectra of sample 4 at 80 K (Fig. 2b, curves 1 and 3), although at RT this overlap is more notable (curves 2 and 4). 3.2. High pressure investigation The luminescence spectra of EuAl2 O4 (sample 3) obtained for different pressures are presented in Fig. 3. One should notice that when the pressure increases the spectrum shifts toward higher wavelength (lower energy). The luminescence spectrum has been considered as a superposition of two bands (Gaussian fitting is presented for the luminescence at 8 kbar). We have tentatively attributed these bands to two different centers of Eu2+ ions in EuAl2 O4 host labeled as EuII and EuII . Energies of the maxima of these peaks for different pressures are presented in Fig. 4. It is seen that both bands shift to the lower energy with pressure rising. The respective shifts rates for the pressure below 40 kbar are −23 and −27 cm−1 /kbar for peaks with higher and lower energy, respectively. It should be noted that when we consider the pressure range up to 67 kbar, the pressure shift for the peak with lower energy raises up to −52 cm−1 /kbar (dashed line). The situation is presented in Fig. 4. Above results demonstrate the significant dependence of lineshape of emission spectra of EuAl2 O4 powder samples on condition of their preparation (temperature of sintering and O2− pressure in the atmosphere of treatment). The main influence on the EuAl2 O4 emission spectra plays the temperature of annealing. Beyond the 1000 ◦ C the thermostimulated redistribution over Eu2+ and Eu2+ I II positions of
EuAl2 O4 host occurs that results in increase of the Eu2+ II centers concentration with the domination emission in the band peaked at 500 nm. In turn, further increase of the temperature of sample’s preparation beyond the 1200 ◦ C leads to the increase of the concentration of the Eu2+ II centers with the bright emission in the green-yellow (520–530 nm) range and small overlapping with the corresponding excitation spectra. Additional support of this conclusion was obtained under the high pressure investigation. The different pressure shifts of high and low energy sub-bands equal to −23 and −27 cm−1 /kbar, respectively (even −52 kbar in range up to 67 kbar for low energy sub-band) were found for the green-yellow (500–530 nm) luminescence of vacuum-prepared at 1100 ◦ C sample. These two bands and their different shift were tentatively attributed to the existence of two different centers in the Eu2+ II sites in EuAl2 O4 host. Since the samples were prepared in reducing conditions large influence of oxygen vacancies on the EuAl2 O4 spectra have been observed (Schierning et al., 2005). The oxygen vacancy in the neighborhood of Eu2+ ion can create the dipole Eu2+ –VO or more complicated aggregate center, and a large amount of Eu2+ centers in the different sites of EuAl2 O4 host are distorted. The Eu2+ –VO centers are characterized by bright fluorescence, more efficient than emission of non-perturbed Eu2+ ions. High quantum yield of the perturbed centers is caused by the oxygen vacancies that leads to subsequent decrease of overlapping between the Eu2+ –VO and Eu2+ wave functions and thus reduces concentration quenching of the luminescence. 4. Conclusion Our luminescence, luminescence excitation and high pressure photoluminescence studies of the EuAl2 O4 powders yield the following conclusions. The strong dependence of the spectral characteristics of EuAl2 O4 powder samples on conditions of their preparation (temperature of annealing in 1000–1215 ◦ C range, vacuum or 95% N2 + 5% H2 treatments, etc.) gives the evidence that the main influence on the intensity and the lineshape of the
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Yu. Zorenko et al. / Radiation Measurements 42 (2007) 652 – 656
emission spectra plays the temperature of sintering. The distribution of the Eu2+ ions between two different EuI and EuII positions of EuAl2 O4 host is responsible for luminescence in the dark-blue (430 nm) and green-yellow (500–530 nm) ranges, respectively. The existence of oxygen vacancies in EuAl2 O4 powders can originate the additional plurality of emission centers in the EuAl2 O4 lattice. For study of local environment of the Eu2+ centers in the EuAl2 O4 host the high-pressure spectroscopy was performed. The different pressure shifts of high- and low-energy sub-bands equal to −23 and −27 cm−1 /kbar, respectively (even −52 cm−1 /kbar in range 40–67 kbar for the lower energy sub-band) were found. These two bands were tentatively attributed to existence of the two different types of Eu2+ centers in the Eu2+ II sites of EuAl2 O4 lattice with higher coordination number. Acknowledgments The work was fulfilled in framework of MESU project SL77 F and cooperation project “Luminescence of Eu2+ -ions in phosphors based on oxides and bromides” between the LOM Lviv University and IEP Gda´nsk University. The authors
express their gratitude to Dr. I. Solsky and V. Baluk (SIA Carat) for the XRD measurements as well as to Dr. M. Batentschuk (Erlangen University) for EuAl2 O4 sample preparing in the 95% N2 + 5% H2 atmosphere. References Abbruscato, V., 1971. Optical and electrical properties of SrAl2 O4 :Eu2+ . J. Electrochem. Soc. 118 (6), 930. Blasse, G., Bril, A., 1968. Fluorescence of Eu2+ -activated alkaline-earth aluminates. Philips Res. Rep. 23, 201–206. Meister, F., Batentschuk, M., Dröscher, S., Osvet, A., Stiegelschmitt, A., Weidner, M., Winnacker, A., 2007. Eu2+ luminescence in the EuAl2 O4 concentrated phosphor. Radiat. Meas., in this volume. Nakazawa, E., 1999. Absorption and emission of light. In: Shionoya, W., Yen, W.M. (Eds.), Phosphor Handbook. CRC Press, Boca Raton, pp. 11–20. Poort, S.M.H., Blokpoel, W.P., Blasse, G., 1995. Luminescence of Eu2+ in barium and strontium aluminate and galate. Chem. Mater. 7, 1547–1551. Schierning, G., Batentschuk, M., Osvet, A., Stiegelschmitt, A., Winnacker, A., 2005. The influence of lattice defects on fluorescence and phosphorescence in the europium aluminate EuAl2 O4 . Phys. Status Solidi (c) 2, 109–112. Schulze, A.-R., Müller-Buschbaum, Hk., 1981. Zur Struktur von monoklinem SrAl2 O4 . Z. anorg. Allg. Chem. 475, 205–210. Wang, D., Yin, Q., Li, Y., Wang, M., 2002. Concentration quenching of Eu2+ in SrO · Al2 O3 phosphor. J. Lumin. 97, 1–6.
High-pressure luminescence spectroscopy of EuAl2O4 phosphor Yu. Zorenko a , V. Gorbenko a , M. Grinberg b,∗ , R. Turos-Matysiak b , B. Kukli´nski b a Laboratory of Optoelectronic Materials (LOM), Department of Electronic, Ivan Franko National University of Lviv, 79017 Lviv, Ukraine b Institute of Experimental Physics (IEP), Gda´nsk University, 80 952 Gda´nsk, Poland
Received 18 December 2006; accepted 31 January 2007
Abstract EuAl2 O4 powder phosphor was prepared by solid-state reaction of EuO and Al2 O3 oxides in vacuum. The influence of conditions of preparation on spectral lineshape of Eu2+ emission was analyzed. It was found that the fluorescence spectra of vacuum-prepared EuAl2 O4 samples at 300 K present the superposition of three bands peaked at 430, 500 and 528 nm, corresponding to the 4f 6 5d1 → 4f 7 (8 S7/2 ) transition of Eu2+ ions in the different sites of EuAl2 O4 lattice. The luminescence of Eu2+ centers in EuAl2 O4 host was also studied using the high-pressure spectroscopy up to 67 kbar. It was found that the bright green-yellow fluorescence of EuAl2 O4 at 300 K in the band peaked at 520.530 nm range can be presented by superposition of two Gaussian sub-bands. The different pressure shifts −23 and −27 cm−1 /kbar for two sub-bands were found. Such a structure of the emission spectrum was attributed to the existence of two different Eu2+ centers in the Eu2+ II sites of EuAl2 O4 lattice with higher coordination number. © 2007 Elsevier Ltd. All rights reserved. Keywords: EuAl2 O4 , Eu2+ luminescence; Solid-state reaction; High-pressure spectroscopy
1. Introduction The Eu2+ concentrated EuAl2 O4 belongs to the class of MAl2 O4 (M = Sr, Ca, Ba) phosphors (Blasse and Bril, 1968; Abbruscato, 1971). It shows a bright yellow-green emission with maximum at 520 nm and very weak phosphorescence in the region of room temperatures (RT) (Schierning et al., 2005). This emission can be effectively excited with a wavelength of 320–460 nm due to the presence of several absorption bands corresponding to the electric dipole transitions from the ground state of 4f 7 electronic configuration, 8 S7/2 , to the eg and t2g states of the excited 4f 6 5d1 electronic configuration of the Eu2+ ion located in different sites of MeAl2 O4 host (Poort et al., 1995; Wang et al., 2002; Meister et al., 2007). For this reason the EuAl2 O4 is suitable for the purposes of light conversion in white light-emitting diode (LED) lamps where the system is
∗ Corresponding author. Tel.: +48 58 5529544; fax: +48 58 3413175.
E-mail address: fi[email protected] (M. Grinberg). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.01.060
excited in the blue and UV by LED and emits in the yellowgreen and red spectral ranges (Schierning et al., 2005, Meister et al., 2007). The EuAl2 O4 and Eu1−x Mx Al2 O4 powder samples which are synthesized under various conditions by solid state reaction from Eu2 O3 , Al2 O3 and MCO3 charge have been investigated previously by Schierning et al. (2005). It has been found that application of the reducing atmosphere during the synthesis causes an increase of intensity of Eu2+ fluorescence. This is due to fact that reducing atmosphere stabilizes europium ions in the charge state of Eu2+ . One expects that condition of reducing treatment can also strongly influence the local surrounding of Eu2+ ions that, in turn, affects the shape of emission spectra and quantum efficiency of EuAl2 O4 phosphors. In this work we investigate the luminescence of EuAl2 O4 powder samples, sintered by solid-state reaction in vacuum (1.3 Pa) at temperatures of 900–1100 ◦ C. For studying the local surrounding and distribution of Eu2+ centers in EuAl2 O4 host the high-pressure spectroscopy of Eu2+ luminescence in the pressure up to 67 kbar was applied.
Yu. Zorenko et al. / Radiation Measurements 42 (2007) 652 – 656
2. Samples and experimental technique 2.1. Sample preparation The EuAl2 O4 powder samples were prepared in LOM (Lviv University) with equal molar parts of raw EuO (5 N purity) and Al2 O3 oxides (4 N) by solid state reaction. The charge was mixed in agate mortise and pressed in tablets with a diameter of 10 mm and a thickness of 1–2 mm. The tablets labeled as 1, 2 and 3 were annealed in the quarts tubes for 4 h in a furnace with resistive heating in vacuum (1.3 Pa) at temperatures 900, 1000 and 1100 ◦ C, respectively. The walls of quarts tubes were covered by grapheme layers. Dependence of sample composition on annealing temperature and treatment duration was analyzed by X-ray diffraction (XRD) and photoluminescence (PL). The XRD measurement was carried out using a DRON X-ray diffractometer (W-anode operated at 35 keV). XRD pattern of a main part of EuAl2 O4 powder samples 2 and 3 shows that the value of 2 is very close to the wellknown SrAl2 O4 pattern (monoclinic structure C22 with the P21 space group), which is identical to EuAl2 O4 structure due to al˚ and Sr 2+ (1.32 A) ˚ most the same ionic radii for Eu2+ (1.31 A) ions (http://abulafia.mt.ic.ac.uk/shannon/radius.php). From the XRD measurement it has been also found that sample 1 contains a small amount of EuAlO3 and Al2 O3 oxides. It is worth noting that the Eu2+ ions in EuAl2 O4 structure can substitute EuI and EuII positions with the coordination number close to 6.07 and 6.23, respectively (Schulze and Müller-Buschbaum, 1981; Meister et al., 2007). The color of the samples depends on the temperature of treatment and changes from the light-green for samples annealed at 900 ◦ C to the green and deep-green for samples annealed at 1000 and 1100 ◦ C, respectively. PL spectra of vacuum-prepared samples were compared with the emission of the sample labeled as sample 4 (Schierning et al., 2005) prepared in WW6 (Erlangen University) by the solid-state reaction synthesis in an induction heated furnace in graphite crucible at 1215 ◦ C in strongly reducing conditions (95% N2 + 5% H2 atmosphere).
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sion frequency. The reason for the adjustment is that for the broad band luminescence consisting of a number of vibronic components the radiative rates of those emitting at higher frequency are enhanced by the energy-dependent photon density of states (Nakazawa, 1999). 3. Results and discussions 3.1. PL of powder samples The PL spectra of EuAl2 O4 powder samples under excitation with a wavelength of 330 nm (sample 1) and 460 nm (samples 2–4) in the region of the 4f 7 (8 S7/2 ) → 4f 6 5d1 transitions of Eu2+ ions are shown in Fig. 1. Shape of the emission spectra of samples 1–4 substantially depends on the excitation wavelength and annealing temperature. The PL spectrum of sample 1 under excitation at wavelength of 330 nm (Fig. 1, curve 1) presents a complex band in the 390–750 nm range which is a superposition of couples of Eu2+ -related bands peaked approximately at 430 and 450 nm (dark-blue emission), as well as the bands peaked at 505 and 555 nm (green-yellow emission). Under excitation at wavelength 460 nm the emission of sample 1 also shows the week luminescence in green-yellow (520–530 nm) range (not show in Fig. 1). With increasing the annealing temperature to 1000 ◦ C (sample 2) and to 1100 ◦ C (sample 3) the contribution of the emission in the blue region strongly decreases whereas the intensity of the bands in the green region notably increases and their main maximum under excitation at a wavelength of 460 nm shifts to 555 nm for sample 2 and to 530 nm for sample 3 (Fig. 1, curves 2 and 3, respectively). It should be noted that for vacuumprepared sample 3 only emission band peaked at 530 nm with small bump at 555 nm is observed. This emission practically coincides with the spectra of sample 4 peaked at 523 nm which was prepared by annealing in the 95% N2 + 5% H2 atmosphere at 1215 ◦ C (Schierning et al., 2005).
2.2. High pressure luminescence spectra High pressure PL spectra of the EuAl2 O4 powder samples were measured in IEP Gdansk University. All high-pressure measurements were made at RT. For applying the pressure we have used a diamond anvil cell (DAC) with dimethylosiloxane as a pressure transmitting medium. Luminescence of ruby crystal has been used for estimation of quantity of pressure. Experiments have been performed for the pressure range from ambient to 67 kbar. He–Cd laser emission with wavelength =325 nm has been used for excitation. Emission has been dispersed with a PGS2 monochromator and collected by a cooled Hamamatsu R943-02 photomultiplier working in the photon counting regime. All luminescence spectra have been corrected with respect to the apparatus response. The luminescence intensities were adjusted by the factor of −3 , where is the emis-
Fig. 1. PL spectra of EuAl2 O4 powder samples 1–4 under excitation at 330 nm (samples 1) and 460 nm (samples 2–4).
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Yu. Zorenko et al. / Radiation Measurements 42 (2007) 652 – 656
Fig. 2. (a) Excitation spectra of the luminescence at a wavelength of 430 nm (1) and 500 nm (2, 3) and emission spectra (4, 5) under excitation at a wavelength of 330 nm of EuAl2 O4 powders samples 2 and 3; (b) excitation spectra (1, 2) of the luminescence at a wavelength of 530 nm and emission spectra (3, 4) under excitation at a wavelength of 330 nm of EuAl2 O4 powder sample 4 at 80 K (1, 3) and 300 K (2, 4), respectively.
The luminescence bands shown in Fig. 1 have been attributed to the 4f 6 5d1 → 4f 7 (8 S7/2 ) transitions of Eu2+ ions. The observed dependence of the peak position and intensity of Eu2+ -related bands peaked at 520–560 nm in samples 2–4 indicates the existence of the strong dependence of the Eu2+ ions emission lineshape on temperature of preparation. The existence of complicated Eu2+ -related emission bands peaked in the dark-blue (430–450 nm) and green-yellow (505–545 nm) regions (curve 1 for samples 1) testifies the existence of several Eu2+ related emission centers located in different sites of EuAl2 O4 lattice. This conclusion is strongly supported by the shape of the excitation spectra of Eu2+ luminescence in samples 1–4 (Fig. 2). The excitation spectra of dark-blue emission monitored at 430 nm in sample 2 (Fig. 2a, curve 1) consists
of the two bands peaked at 250 and 308 nm related to the 4f 7 (8 S7/2 ) → 4f 6 5d1 (eg and t2g ) transitions of Eu2+ ions, located in the first, Eu1 site of EuAl2 O4 lattice with lower coordinate number (Poort et al., 1995). It is worth to note that the low energy tail of the 308 nm band is significantly widened. The excitation spectra of the emission monitored at wavelength 500 nm in samples 2, 3 and 4 are presented in the Fig. 2a and b, respectively. The excitation spectra of samples 2 and 3 (Fig. 2a, curves 2 and 3) apart of the bands peaked at 250 and 308 nm contains the dominant band peaked at 323 nm and additional low-intensity band around 440 nm, related to the 4f 7 (8 S7/2 ) → 4f 6 5d1 (eg and t2g ) transitions of Eu2+ ions, located probably in the second, EuII site of EuAl2 O4 lattice. The low energy tail of the 324 nm band contains additionally the bump peaked at 374 nm. It is worth to note that the intensity of the 323 nm excitation bands in vacuum-prepared EuAl2 O4 samples 2 and 3 notably increases with rise of annealing temperature from 1000 to 1100 ◦ C (Fig. 2a, curves 2 and 3, respectively). This effect reflects the increase of the concentration of Eu2+ II centers. Further increase of the annealing temperature beyond 1200 ◦ C results in rearrangement of Eu2+ in different sites of EuAl2 O4 lattice. Specifically, for sample 4 annealed at 1215 ◦ C in the 95% N2 + 5% H2 atmosphere, the bands peaked 324 and 430 nm of EuII centers dominate in the excitation spectra of the green-yellow (530 nm) luminescence at RT (Fig. 2b, curve 1). At the same time, the another couple of excitation bands, peaked approximately at 380 and 467 nm, becomes apparent in the excitation spectra of luminescence at 530 nm in samples 4 at 80 K (Fig. 2b, curve 2). These excitation bands and resulted emission band peaked at 528 nm (curve 3) probably belong to another europium related centers in EuAl2 O4 lattice, the Eu2+ II . It is worth to note that excitation bands at 380 and 460 nm of the emission band at 520 nm in SrAl2 O4 :Eu phosphor was recently related to Eu2+ II centers with higher coordination number (Poort et al., 1995). Our results testify that at least two different luminescence centers exist in the EuII position of EuAl2 O4 host which emit in the bands peaked approximately in the 500–505 and 528–530 nm ranges (Fig. 2a and b, respectively). Thus, the annealing temperature influences strongly the distribution of Eu2+ ions over EuI and EuII or/and EuII positions of EuAl2 O4 lattice, which are responsible for dark-blue and greenyellow emission, respectively. The process of EuI → EuII and EuI → EuII redistribution notable increases for annealing temperatures above 1000 and 1200 ◦ C, respectively. It should be noted that the intensity of the excitation bands of Eu2+ I centers at 250 and 308 nm decreases with the increasing 2+ of Eu2+ II and EuII centers concentrations (Fig. 2a, curves 2 2+ and 3) due to effective Eu2+ → Eu2+ I II and EuII and energy transfer caused by the re-absorption of the dark-blue emission bands peaked at 430 nm by the absorption bands of Eu2+ II and 2+ EuII centers peaked at 430 and 467 nm, respectively. This results in complete quenching of the emission band at 430 nm and domination of the emission band peaked at 520–528 nm in the PL spectra of sample 4 (Fig. 2b). As a result, the EuAl2 O4 powder samples show very bright luminescence in green-yellow
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Fig. 4. Positions of the emission peaks related to the luminescence of two different centers for Eu2+ ions in EuII sites of EuAl2 O4 host.
Fig. 3. Emission spectra of EuAl2 O4 powder sample 3 in a pressure range of 8–67 kbar under excitation at 325 nm.
region with quantum efficiency is approximately equal to 34% (Meister et al., 2007). The absence of concentration quenching (energy transfer between the same Eu2+ -related centers) in such phosphors is due to a small spectral overlapping of the emission and excitation spectra of sample 4 at 80 K (Fig. 2b, curves 1 and 3), although at RT this overlap is more notable (curves 2 and 4). 3.2. High pressure investigation The luminescence spectra of EuAl2 O4 (sample 3) obtained for different pressures are presented in Fig. 3. One should notice that when the pressure increases the spectrum shifts toward higher wavelength (lower energy). The luminescence spectrum has been considered as a superposition of two bands (Gaussian fitting is presented for the luminescence at 8 kbar). We have tentatively attributed these bands to two different centers of Eu2+ ions in EuAl2 O4 host labeled as EuII and EuII . Energies of the maxima of these peaks for different pressures are presented in Fig. 4. It is seen that both bands shift to the lower energy with pressure rising. The respective shifts rates for the pressure below 40 kbar are −23 and −27 cm−1 /kbar for peaks with higher and lower energy, respectively. It should be noted that when we consider the pressure range up to 67 kbar, the pressure shift for the peak with lower energy raises up to −52 cm−1 /kbar (dashed line). The situation is presented in Fig. 4. Above results demonstrate the significant dependence of lineshape of emission spectra of EuAl2 O4 powder samples on condition of their preparation (temperature of sintering and O2− pressure in the atmosphere of treatment). The main influence on the EuAl2 O4 emission spectra plays the temperature of annealing. Beyond the 1000 ◦ C the thermostimulated redistribution over Eu2+ and Eu2+ I II positions of
EuAl2 O4 host occurs that results in increase of the Eu2+ II centers concentration with the domination emission in the band peaked at 500 nm. In turn, further increase of the temperature of sample’s preparation beyond the 1200 ◦ C leads to the increase of the concentration of the Eu2+ II centers with the bright emission in the green-yellow (520–530 nm) range and small overlapping with the corresponding excitation spectra. Additional support of this conclusion was obtained under the high pressure investigation. The different pressure shifts of high and low energy sub-bands equal to −23 and −27 cm−1 /kbar, respectively (even −52 kbar in range up to 67 kbar for low energy sub-band) were found for the green-yellow (500–530 nm) luminescence of vacuum-prepared at 1100 ◦ C sample. These two bands and their different shift were tentatively attributed to the existence of two different centers in the Eu2+ II sites in EuAl2 O4 host. Since the samples were prepared in reducing conditions large influence of oxygen vacancies on the EuAl2 O4 spectra have been observed (Schierning et al., 2005). The oxygen vacancy in the neighborhood of Eu2+ ion can create the dipole Eu2+ –VO or more complicated aggregate center, and a large amount of Eu2+ centers in the different sites of EuAl2 O4 host are distorted. The Eu2+ –VO centers are characterized by bright fluorescence, more efficient than emission of non-perturbed Eu2+ ions. High quantum yield of the perturbed centers is caused by the oxygen vacancies that leads to subsequent decrease of overlapping between the Eu2+ –VO and Eu2+ wave functions and thus reduces concentration quenching of the luminescence. 4. Conclusion Our luminescence, luminescence excitation and high pressure photoluminescence studies of the EuAl2 O4 powders yield the following conclusions. The strong dependence of the spectral characteristics of EuAl2 O4 powder samples on conditions of their preparation (temperature of annealing in 1000–1215 ◦ C range, vacuum or 95% N2 + 5% H2 treatments, etc.) gives the evidence that the main influence on the intensity and the lineshape of the
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emission spectra plays the temperature of sintering. The distribution of the Eu2+ ions between two different EuI and EuII positions of EuAl2 O4 host is responsible for luminescence in the dark-blue (430 nm) and green-yellow (500–530 nm) ranges, respectively. The existence of oxygen vacancies in EuAl2 O4 powders can originate the additional plurality of emission centers in the EuAl2 O4 lattice. For study of local environment of the Eu2+ centers in the EuAl2 O4 host the high-pressure spectroscopy was performed. The different pressure shifts of high- and low-energy sub-bands equal to −23 and −27 cm−1 /kbar, respectively (even −52 cm−1 /kbar in range 40–67 kbar for the lower energy sub-band) were found. These two bands were tentatively attributed to existence of the two different types of Eu2+ centers in the Eu2+ II sites of EuAl2 O4 lattice with higher coordination number. Acknowledgments The work was fulfilled in framework of MESU project SL77 F and cooperation project “Luminescence of Eu2+ -ions in phosphors based on oxides and bromides” between the LOM Lviv University and IEP Gda´nsk University. The authors
express their gratitude to Dr. I. Solsky and V. Baluk (SIA Carat) for the XRD measurements as well as to Dr. M. Batentschuk (Erlangen University) for EuAl2 O4 sample preparing in the 95% N2 + 5% H2 atmosphere. References Abbruscato, V., 1971. Optical and electrical properties of SrAl2 O4 :Eu2+ . J. Electrochem. Soc. 118 (6), 930. Blasse, G., Bril, A., 1968. Fluorescence of Eu2+ -activated alkaline-earth aluminates. Philips Res. Rep. 23, 201–206. Meister, F., Batentschuk, M., Dröscher, S., Osvet, A., Stiegelschmitt, A., Weidner, M., Winnacker, A., 2007. Eu2+ luminescence in the EuAl2 O4 concentrated phosphor. Radiat. Meas., in this volume. Nakazawa, E., 1999. Absorption and emission of light. In: Shionoya, W., Yen, W.M. (Eds.), Phosphor Handbook. CRC Press, Boca Raton, pp. 11–20. Poort, S.M.H., Blokpoel, W.P., Blasse, G., 1995. Luminescence of Eu2+ in barium and strontium aluminate and galate. Chem. Mater. 7, 1547–1551. Schierning, G., Batentschuk, M., Osvet, A., Stiegelschmitt, A., Winnacker, A., 2005. The influence of lattice defects on fluorescence and phosphorescence in the europium aluminate EuAl2 O4 . Phys. Status Solidi (c) 2, 109–112. Schulze, A.-R., Müller-Buschbaum, Hk., 1981. Zur Struktur von monoklinem SrAl2 O4 . Z. anorg. Allg. Chem. 475, 205–210. Wang, D., Yin, Q., Li, Y., Wang, M., 2002. Concentration quenching of Eu2+ in SrO · Al2 O3 phosphor. J. Lumin. 97, 1–6.