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The effect of Ba/Mg impurities on the phase formation and photoluminescence properties of (Sr32xMx)Al2O6:Eu3+,Ho3+ (M 5 Ba, Mg) phosphors

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The effect of Ba/Mg impurities on the phase formation

and photoluminescence properties of (Sr

32x

M

x

)Al

2

O

6

:Eu

3+

,Ho

3+

(M 5 Ba, Mg) phosphors

Esra O¨ ztu¨rk1

Received: 27 November 2015 / Accepted: 13 May 2016 / Published online: 24 May 2016  Akade´miai Kiado´, Budapest, Hungary 2016

Abstract The Eu3?-activated strontium aluminate-based Sr3Al2O6:Eu3?,Ho3?, Sr2.99Ba0.01Al2O6:Eu3?,Ho3? and Sr2.99Mg0.01Al2O6:Eu

3?

,Ho3? phosphor systems were prepared by the solid-state reaction method under open atmosphere. DTA/TG analysis was conducted to obtain the thermal behaviours of the phosphors. Depending on the thermal analysis results, the heat treatments were carried out and the obtained single phase formations were char-acterised by X-ray diffraction. The effects of Ba2? and Mg2?, which were individually added as trace amounts, and also of the same activator (Eu3?) and co-dopant (Ho3?) that were used for all host lattices, on the photolumines-cence and phase formation properties of the hosts were investigated.

Keywords Strontium aluminate Solid-state reaction method Photoluminescence  Eu3? Ba2?  Mg2?  Phase formation

Introduction

Rare-earth ion-activated inorganic phosphors have fre-quently been the subject of research by scientists thanks to their remarkable photoluminescent properties and wide applications, such as for fluorescent lamps, light-emitting diodes (LEDs), X-ray imaging, display devices, radiation dosimetry and optical storage. It is well known and proved

that aluminate or silicate type phosphors are more chemi-cally and thermally stable, are lower in cost, and have even longer persistence, high quantum efficiency, etc., proper-ties compared with sulphide-based phosphors. Among the rare earths, trivalent europium (Eu3?) is used as the major dopant for many types of phosphor systems providing red region emissions, high efficiency and colour purity. Eu3?is an efficient red emission activator which is widely used because of its electronic transitions from the lowest 5D0 excited state to the 7FJ (J = 0, 1, 2, 3, 4) ground state, depending on the type of the host lattice. Eu2?-doping can provide intense and efficient broadband emissions, arising from the electronic transitions between the ground state of 4f7and the excited state of 4f65d1of Eu2? ions. Further-more, it has been well demonstrated that the initial inten-sity of the photoluminescence and afterglow (long lasting) of europium-activated phosphors is enhanced by adding a certain trace amount of Ln3? ions, such as the most com-monly used Dy, resulting in significantly improved emis-sion intensity.

In stoichiometric combinations of SrO–Al2O3, such as SrAl2O4, SrAl4O7, Sr4Al4O25, Sr3Al2O6and SrAl12O19, the compounds were generally prepared by the solid-state reaction method which includes the mechanical mixing of high purity and small particle sized raw materials (SrCO3 and Al2O3), which is necessary to prevent compositional inhomogeneities [1–4].

In this research, the purpose is to produce strontium aluminate-based Sr3-x-yAl2O6:xEu3?,yHo3?, Sr2.99-x-y Ba0.01Al2O6:xEu3?,yHo3? and Sr2.99-x-yMg0.01Al2O6: xEu3?,yHo3?phosphor systems using the solid-state reac-tion method in open atmosphere. The effect of Ba2? and Mg2? ions on the phase formation and also the photolu-minescent properties bearing the excitation and emission bands of the base sample Sr3Al2O6were studied.

& Esra O¨ztu¨rk

esracircir@gmail.com

1 Department of Metallurgical and Materials Engineering,

Engineering Faculty, Karamanog˘lu Mehmetbey University, Karaman, Turkey

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Materials and methods

The solid-state reaction method was chosen to prepare strontium aluminate-based Sr3-x-yAl2O6:xEu3?,yHo3?, Sr2.99-x-yBa0.01Al2O6:xEu3?,yHo3? and Sr2.99-x-y Mg0.01Al2O6:xEu3?,yHo3? phosphor systems. The com-positions of each sample were stoichiometrically calcu-lated, and appropriate amounts of high-purity raw materials including SrCO3 (99.9 %), Al2O3 (99.9 %), BaCO3 (99.9 %), 4MgCO3Mg(OH)25H2O (A.R.), Eu2O3 (99.99 %) and Dy2O3 (99.99 %) were precisely weighed and ground in an agate mortar to get a fine and homoge-nous mixing of particles. After mixing the powders, the thermal analysis of each system was determined to apply the heat treatments effectively. According to the thermal analysis results, the heat treatments were carried out in pure alumina crucibles in a muffle furnace (Protherm PTF 16/50/450) under open atmosphere, then cooled down to room temperature slowly. The sintered samples were ground to powder form for the characterisations.

The decomposition and oxidation processes of the samples were determined through differential thermal analysis (DTA) and thermogravimetric (TG) analysis (Seiko Instruments Inc./Exstar DTA/TG 6200) at a heating rate of 10C min-1 from 50 to 1300C. After applying the heat treatments to each of the systems according to the DTA/TG results, a BRUKER AXS D8 ADVANCE model X-ray diffractometer, which was run at 40 kV and 30 mA (Cu-Ka radiation) in a step-scan mode (0.02/2h), was used to investigate the phase formations. Finally, the photolu-minescent properties of the phosphor systems were anal-ysed by a spectrophotometer [Photon Technology International (PTI), QuantaMasterTM30].

Results and discussion

Thermal analysis

The thermal behaviours of the phosphor systems were determined in the range from 50 to 1300C (Figs.1–3). First of all, the DTA/TG results of Sr3-x-yAl2O6:xEu3?, yHo3? are given in Fig.1.

Figure1 shows that there is only one major mass loss between 800 and 1150C according to the TG curve, which is related to the decomposition of SrCO3 and the removal of CO2 in the system. The decomposition of SrCO3under heating can be given as follows:

SrCO3! D

SrOþ CO2 ð1Þ

The DTA curve gives endothermic peaks at 943 and 1045C which are attributed to an

orthorhombic-to-rhombohedral transition and the decomposition of SrCO3 to SrO, respectively [5,6]. The TG curve exhibits a mass loss of about 23.3 %, which is in agreement with the cal-culated mass loss (*23.8 %).

Figures2 and 3 exhibit the thermal behaviours of Sr2.99-x-yBa0.01Al2O6:xEu3?,yHo3? and Sr2.99-x-yMg0.01 Al2O6:xEu3?,yHo3?, respectively.

The DTA/TG curves of Sr2.99-x-yBa0.01Al2O6:xEu3?, yHo3? and Sr2.99-x-yMg0.01Al2O6:xEu3?,yHo3? are very close to the results of Sr3-x-yAl2O6:xEu3?,yHo3?(Fig.2) despite the presence of a trace amount of BaCO3 and 4MgCO3Mg(OH)25H2O.

It was proved that the results are very similar showing the decomposition of the major material, which was SrCO3, as expected. The additives, BaCO3and 4MgCO3Mg(OH)25H2O,

50 77 100 200 DTG/μg min–1 Endo 943 °C 1045 °C DTA/μV TG/% 400 600 Temperature/°C 800 1000 1200 1300 0 –25 –50 –75 25 50 75 100

Fig. 1 DTA/TG curves of Sr3-x-yAl2O6:xEu3?,yHo3?

50 200 400 600 Temperature/°C 800 1000 1200 1300 77 100 DTA/μV TG/% DTG/μg min–1 Endo 940 °C 1027 °C 0 10 –10 20 30 40 50 60 70

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did not affect the total mass losses and temperatures, as shown in the figures.

As shown in Figs.1–3, the dehydration and the decomposition of starting materials are complete after 1045, 1027 and 1023C, respectively. So, we carried out the sintering of the phosphors in two steps: Firstly, the samples were pre-fired at 800C for 2 h to achieve the dehydration. Secondly, the Sr2.99-x-yBa0.01Al2O6:xEu3?, yHo3?, Sr2.99-x-yMg0.01Al2O6:xEu3?,yHo3? and Sr3-x-y Al2O6:xEu

3?

,yHo3?powders were sintered at 1200C for 3 h in air to help the doped Eu3?, Ho3?, Ba2? and Mg2? ions to substitute for achieving phase-forming process. The XRD result showed that the main sintering temperature (at 1200C) is sufficient for the phase forming of the Sr2.99-x-yBa0.01Al2O6:xEu3?,yHo3? and Sr2.99-x-yMg0.01 Al2O6:xEu3?,yHo3? but it is not enough for the Sr3-x-y Al2O6:xEu3?,yHo3?. So, we decided to increase the sin-tering temperature at 1250C for 6 h, unfortunately the sintering temperature at 1250C was not enough for the phase forming of the Sr3-x-yAl2O6:xEu3?,yHo3?. The sintering process was finished because of melting may occur.

X-ray diffraction (XRD) analysis

According to thermal analysis results, we carried out the sintering of the phosphors in two steps: Firstly, the samples were pre-fired at 800C for 2 h to achieve the dehydration and to help the doped Eu3?, Ho3?, Ba2?and Mg2?ions to substitute; next, the Sr3-x-yAl2O6:xEu3?,yHo3? powder was sintered at 1200C for 3 h and 1250 C for 6 h in air. The Sr2.99-x-yBa0.01Al2O6:xEu3?,yHo3? and Sr2.99-x-y Mg0.01Al2O6:xEu3?,yHo3? powders were sintered at 1200C for 3 h.

After the heat treatments including the pre-sintering stage at 800 C for 2 h and major sintering processes, XRD analysis was applied.

Figure4 shows the comparative XRD patterns of Sr3-x-yAl2O6:xEu3?,yHo3? that was sintered at two dif-ferent temperatures.

In spite of increasing the sintering temperature to obtain the major single phase, Sr3Al2O6was not observed in the XRD pattern. The XRD results proved that the expected crystal system could not be indexed by increasing sintering temperature except, significantly, for the corundum phase, Al2O3.

Figures5 and 6 show the XRD patterns of the trace amounts of the Ba2?- and Mg2?-added Sr3-x-yAl2O6: xEu3?,yHo3?samples, respectively.

Figures5 and 6 show that the XRD patterns of the samples were identical to one another and were indexed with the same PDF card according to The International Centre for Diffraction Data (ICDD) [3,7]. The major phase determined in these two samples was mainly composed of Sr3Al2O6 as the single phase which was sintered at 1200C for 3 h. Furthermore, the result clearly proves that trace amounts of activator Eu3?and co-activator Ho3?ions were incorporated into the lattice and caused no change in the lattice structure, as can be seen in the unchanged diffraction patterns. Moreover, the individually added Mg2?and Ba2? ions were substituted in the major system before the secondary phases began to appear. It is well known that the major structure only tolerates the size mismatch between Sr2?and Ba2?or Mg2?up to a certain point. It has previously been postulated by reports [8, 9] that the addition of Ba2? (*135 pm) and Mg2? (65 pm) cations has a stabilising effect depending on the structure type. In this context, this means that the stabilising effect of

50 200 400 600 Temperature/°C 800 1000 1200 1300 77 100 DTA/μV TG/% 0 –20 20 40 60 DTG/μg min–1 Endo 940 °C1023 °C

Fig. 3 DTA/TG curves of Sr2.99-x-yMg0.01Al2O6:xEu3?,yHo3?

0 10 20 30 40 50 2θ/° Intensity/a.u. 60 70 1200 °C-3 h 1250 °C-6 h : Al2O3

*

*

*

*

*

*

*

*

* ** *

*

80 90 2200 4400 6600 8800 11000 13200 15400 17600

Fig. 4 XRD patterns of Sr3-x-yAl2O6:xEu3?,yHo3? obtained at

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these cations in certain amounts helped in the formation of the major phase. Also, it was not necessary to increase the sintering temperature in this research. The XRD patterns of Sr2.99-x-yBa0.01Al2O6:xEu3?,yHo3? as well as those of Sr2.99-x-yMg0.01Al2O6:xEu3?,yHo3? were in very good agreement with the PDF 00-024-1187 Sr3Al2O6card [3,7]. The two samples crystallised in the cubic structure with the lattice parameters a = 15.844 A˚ , b = 15.844 A˚ , c = 15.844 A˚ , a = 90, b = 90 and c = 90.

Photoluminescence properties

The PL studies of all the phosphors gave effective results with excitation and emission spectra. The PL spectra of the phosphors are given in Figs.7–9.

It can be clearly seen that the PL results of the three systems indicate prominent emission bands attributed to 5D

0?7F0 (572 nm), and electric dipole allowed 5D

0?7F2(609 and 610 nm) transitions of the Eu3? ion, respectively. The other emissions at about 541 nm, 647 and

10 2 1 1 3 2 1 4 0 0 4 2 2 4 3 0 4 5 0 6 3 2 5 5 1 7 2 2 8 0 0 8 4 4 8 8 0 1 2 4 0 8 8 8 0 3000 6000 9000 12000 15000 18000 21000 4 4 0 20 30 40 50 Intensity/cps 60 70 80 90 2θ/°

Fig. 5 XRD pattern of Sr2.99-x-yBa0.01Al2O6:xEu3?,yHo3?

phosphor 10 2 1 1 2 2 0 2 3 0 4 0 0 4 4 0 6 2 0 4 5 0 6 3 2 7 2 2 8 0 0 8 4 1 8 4 4 8 8 0 1 2 4 0 8 8 8 20 30 40 50 60 70 80 90 0 4000 8000 12000 16000 20000 21000 Intensity/cps 2θ/°

Fig. 6 XRD pattern of Sr2.99-x-yMg0.01Al2O6:xEu3?,yHo3?

phosphor 200 0 10 20 30 40 50 60 300 283 nm 5D0→7F0 5D0→7F4 5D0→7F2 572 nm 707 nm 615 nm Emission 357 nm 392 nm Excitation Intra 4f-transitions of Eu3+-ion 400 500 Wavelength/nm Intensity/a.u. The Eu 3+ –O 2– charge tr ansf er (CT) tr ansition 600 700 800 850

Fig. 7 PL spectra of Sr3-x-yAl2O6:xEu3?,yHo3?phosphor

200 250 300 350 400 450 500 550 600 650 700 750 Wavelength/nm 0 100 200 300 400 500 600 700 800 900 Intensity/a.u. The Eu 3+ –O 2– charge tr ansf er (CT) tr ansition Excitation Emission Intra 4f-transitions of Eu3+-ion 272 nm 394 nm468 nm 609 nm 587 nm 541 nm 646 nm 704 nm 5D0→7F2 5D0→7F1 5D0→7F3 5D0→7F4 5D1→7F0

Fig. 9 PL spectra of Sr2.99-x-yMg0.01Al2O6:xEu3?,yHo3?phosphor

200 250 300 350 400 450 500 550 600 650 700 750 Wavelength/nm 0 100 200 300 400 500 600 700 800 850 Intensity/a.u. The Eu 3+–O 2– charge tr ansf er (CT) tr ansition Excitation Intra 4f-transitions of Eu3+-ion 271 nm 362 nm 395 nm 465 nm 647 nm Emission 610 nm 588 nm 541 nm 703 nm 5D1→7F0 5D0→7F1 5D0→7F2 5D0→7F3 5D0→7F4

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707 nm were related to 5D1?7F0, 5D0?7F3 and 5D

0?7F4 transitions of Eu3? [1, 10–13]. The major excitation peaks of the samples, which were concentrated in the range of 270–285 nm, were associated with the charge-transfer state (CTS) band which again proved that the typical Eu3?-activated phosphors mostly show strong CTS transition band excitation around 200–300 nm. Fur-thermore, the CTS is related to an electron transferred from the oxygen 2p orbital to the empty 4f orbital of europium, which may be ascribed as ligand-to-Eu3? charge-transfer transitions (LMCT). Furthermore, the excitation peaks generally seen between 300 and 500 nm could be assigned to typical intra-4f transitions of the Eu3?ion [14]. Also, no other emission can be observed related to the co-dopant Ho3? in the samples, indicating that the doped Ho3? ion does not show any significant emission under those excitations.

Two important conclusions were reached in this study. First of all, the Sr3-x-yAl2O6:xEu3?,yHo3? phosphor which could not be obtained as single phase via heat treatments had good PL results that were due to the tran-sitions of Eu3?, as described above. The second one is that the PL results, including the excitation and emission spectra of all samples, had almost the same peak positions. The emission intensity at about 610 nm as a strong red emission apparently increased with the individual addition of Ba2?and Mg2?ions to the basis phosphor. Additionally, the excitation and emission intensities of the samples were dramatically changed by adding Ba2? and Mg2? ions namely, and the PL intensities were markedly increased. The most important factor explaining this result is, of course, the complete phase formation of Sr3Al2O6thanks to impurity ions. The results clearly indicate the great dependence of photoluminescent intensity on the crys-tallinity of the prepared powders.

Conclusions

The Sr3Al2O6-based phosphors, Sr3Al2O6:Eu3?,Ho3?, Sr2.99Mg0.01Al2O6:Eu3?,Ho3? and Sr2.99Mg0.01Al2O6: Eu3?,Ho3?, were prepared by the solid-state reaction method under open atmosphere. For the first sample, the Sr3Al2O6 single phase could not be indexed although higher sintering temperatures were applied. The expected phase was determined by XRD in the second and third samples which were mainly composed of Sr3Al2O6as the single phase. Because the addition of Ba2? and Mg2? cations has a stabilising effect depending on the structure type, the stabilising effect of these cations in certain amounts helped in the formation of the major phase in this research. The PL analysis exhibited that all of the activated systems, which were based on the same host and activator/

co-activator couples, exhibit excitations and red emissions due to Eu3? because of its f–f transitions. The most important result is that the Ba2?and Mg2? cations-added samples have remarkable PL results compared to the first sample because of single phase formation by means of those ions. Finally, it was proved that the PL intensities in the phosphor systems strongly depend on the phase-form-ing process.

Acknowledgements The authors would like to thank Karamanoglu Mehmetbey University, Scientific Research Projects Commission (BAP, Project Number: 05-YL-14) in the Republic of Turkey for its financial support.

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Şekil

Figure 1 shows that there is only one major mass loss between 800 and 1150 C according to the TG curve, which is related to the decomposition of SrCO 3 and the removal of CO 2 in the system
Figure 4 shows the comparative XRD patterns of Sr 3-x-y Al 2 O 6 :xEu 3? ,yHo 3? that was sintered at two  dif-ferent temperatures.
Fig. 9 PL spectra of Sr 2.99-x-y Mg 0.01 Al 2 O 6 :xEu 3? ,yHo 3? phosphor200 250 300 350 400 450 500 550 600 650 700 750Wavelength/nm0100200300400500600700800850

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