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Acta Chim. Slov. 2012, 59, 912-919
Scientific paper
Structural and Luminescent Properties of Eu2+-doped Aluminates Prepared by the Sol-gel Method
Natasa Celan Korosin, Anton Meden and Natasa Bukovec
Department of Inorganic Chemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, A{ker~eva cesta 5, SI-1000 Ljubljana, Slovenia * Corresponding author: E-mail: natasa.celan@fkkt.uni-lj.si Received: 08-05-2012
Abstract
Alkaline earth aluminates with the overall nominal compositions Mg0 5Sr0 5Al2O4 (MSA), Ca0 5Sr0 5Al2O4 (CSA) and Ca0 5Mg0 5Al2O4 (CMA) doped with 0.5 or 1 mol% of Eu2+ ions were obtained by a modified aqueous sol-gel method and annealed in a reductive atmosphere at 900, 1000, 1100 and 1300 °C. The sample compositions and their structures were studied by XRD employing the Rietveld method. Solid solubility was confirmed in CSA only, due to the similar ionic radii of Ca2+ and Sr2+. UV excited luminescence was observed in the blue region (X = 440 nm) in samples of CSA and CMA containing the monoclinic phase of CaAl2O4 and in the green region (X = 512 nm) in samples of MSA containing hexagonal or monoclinic phases of SrAl2O4.
Keywords: Sol-gel, aluminates, europium, persistent luminescence, Rietveld refinement, solid solubility
1. Introduction
Materials that possess persistent luminescence can emit in visible several hours after ceasing the irradiation,1'2 therefore are successfully used in various applications, e.g. luminous paints for buildings, airports and highways, textile products, display technology, lighting, etc. The desired properties for these materials are good chemical stability, high quantum efficiency and long-lived afterglow.1-3
Among others, the alkaline earth aluminates MAl2O4 (M = Ca, Mg, Sr) doped with the europium(2+) ion,4-8 or co-doped with other rare earth ions (Nd3+, Dy3+, Er3+)9-14 are potential persistent luminescent materials, displaying luminescence in the blue/green region. In order to achieve the longest and intense afterglow typical concentrations between 0.5 and 1 mol% of Eu2+ ions are suggested.1,4,9,10,12,13 The luminescence properties of these materials depend crucially upon their chemical homogeneity and microstructure. Most frequently they are prepared by solid state reactions, with long reaction times and at high calcining temperatures up to 1600 °C.4'10'12'13'15 Alternatively, the sol-gel technique is a more convenient method to achieve chemical homogeneity in such materials with smaller grain size particles, obtained at lower annealing temperatures (850-1250 °C).5,7,8,14
As previously reported, the structure of these compounds depends on their preparation route.5,8,16 The
hexagonal form of CaAl2O4 was prepared through the solgel method by calcining the xerogel at 850 °C, whereas the monoclinic phase was formed at 1200 °C5 or by a conventional solid state reaction at 1250 °C.16 The UV excited luminescence of monoclinic CaAl2O4 was ob ser -ved in the blue region, peaking at approximately 440 nm, while the hexagonal phase had a peak maximum at a slightly higher wavelength.16 SrAl2O4 exists in two crystallographic forms; the monoclinic (space group P21) and the hexagonal (space group P6322).17 Most frequently monoclinic SrAl2O4 was prepared by conventional solid state reaction at high temperatures 1200-1650 °C.11,18 The coexistence of the monoclinic and the hexagonal poly-morphs of SrAl2O4 was demonstrated, when it was prepared by the sol-gel method with a boron content of 1 mol% and annealed at temperatures of about 1000-1100 °C.19 When these aluminates were doped with Eu2+ ions, a strong broad band centred at 520 nm was observed under UV-excitation.6,10,18 MgAl2O4 doped with Eu3+/Eu2+ and Dy3+ ions, prepared by the citrate sol-gel method and annealed in air had a cubic spinel structure (space group Fd-3m, PDF No. 21-1152).14 Its emission spectra showed broad bands at 480 and 579 nm. Aluminates with
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formulae MxSr1-xAl2O4 (M: Ca, Ba; x = 0 to 1) and MgxSr1-xAl204 ( x = 0.05 to 0.25) doped with Eu2+ and Nd3+ ions were prepared by conventional solid state reactions with the aim of investigating their structural properties in relation to their luminescent abilities.15,20,21 In some cases Ca/Sr or Mg/Sr replacement enhanced the persistence of their luminescence. Solid solubility of Ca1_ xSrxAl204 was expected since the Sr2+ ion is only 11% larger than the Ca2+ ion and both parent compounds have similar tridymite-type structures.20,22
In this work, we studied alkaline earth aluminates with the overall nominal compositions Mg0 5Sr0 5Al204 (MSA), Ca05Sr05Al204 (CSA) and Ca^Mg^Al^ (CMA) doped with Eu2+ ions, obtained by a modified aqueous sol-gel method. The influence of annealing temperature on structure and consequently on the luminescent properties was investigated.
2. Experimental
2. 1. Sample Preparation
The sol-gel technique is a convenient method of achieving chemically homogeneous materials. The alkoxy sol-gel route5,8,16,23 using hydrolysis and condensation of metal alkoxides, or Pechini's gel methods19 using chelate polyesterification of hydrocarbonic acids containing at least one hydroxyl group with a polyfunctional alcohol and metal ions, were most often applied. Among the aqueous sol-gel methods, only the citrate sol-gel process was used to prepare ultrafine powders of pure MgAl204.24 However, this method is not convenient for the preparation of materials in a reductive atmosphere, which is needed to obtain Eu2+ ions as luminescent centres. Furthermore, in a reductive atmosphere a residue of carbon arises from the citric acid. We modified the preparation of boehmite sols employing the aqueous sol-gel route by using nitric acid as peptizing agent, as has been reported in detail elsewere.25
All starting chemicals (Al(N03)39H20, Ca(N03)2 ■4H20, Mg(N03)2 6H20, Sr(N03)2 and Eu203) were analytical grade reagents purchased from Aldrich.
The polycrystalline aluminates MSA, CSA and CMA doped with 0.5 or 1 mol% of Eu2+ (in mol% of the total alkaline earth metals amount), were prepared by the sol-gel method. Gaseous ammonia was introduced into a 0.05 M aqueous solution of Al(N03)3 to precipitate Al(0H)3 at pH 9; this was filtered and washed with deionised water. Then a transparent sol was prepared by peptizing Al(0H)3 with 1 M HN03, admixing appropriate amounts of solutions of Ca2+, Mg2+, Sr2+ and Eu3+ ions and heating at 80 °C for 4 hours. The xerogel was obtained by heating the sol in a Petri dish at 80 °C. Portions were then annealed in a tubular oven in a reductive atmosphere (Ar/H2-5%) at various temperatures (900, 1000, 1100, 1300 °C) for 3 hours.26
2.	2. Instrumental Methods
The phase composition of the calcined materials was determined by the Rietveld method from their X-ray powder diffraction patterns, collected using a PANalytical X'Pert PRO diffractometer with CuKa radiation (X = 1.5406 A) in the range of 26 = 5°- 80° in steps of 0.034° with a total integration time of 100 s per step (the full range of the 128 channel linear RTMS detector was used, so that each channel integrated the intensity for about 0.78 s at each step). Total collection time was 28.8 min.
The same patterns were first used for phase identification by Crystallographica Search Match27 using the PDF-4 database.28
Rietveld refinement was performed using the TOPAS 4.1 software.29 The structural data were taken from the ICSD database,30 refining only the unit cell parameters, scale factors and profile and background parameters. Any amorphous phase possibly present was not considered, so that the results represent only the relative amounts of identified crystalline phases. In order to estimate errors, some measurements were repeated.
The luminescence spectra were measured at room temperature using a Perkin Elmer LS-5 spectrometer in the range of 400-650 nm using a powder sample holder. 25 mg of the sample was distributed on the surface of the holder with a surface area of 1 cm2. The excitation and emission slits widths were set to 5 nm and 8 nm, respectively. The excitation wavelength was 350 nm.
Persistent luminescence spectra were measured with a Mettler Toledo HP DSC827e analyser equipped with a PCO SensiCam at room temperature after exposure to an Hg lamp for 5 min. The delay between the initial irradiation and the afterglow measurements was 3 min. 8 mg of the sample was distributed in a 40 pL aluminium holder with a surface area of approximately 28 mm2. During measurement the camera shutter was set to f/0.95 and the exposure time was 3 seconds. Sampling utilized a 12 bit DAC (digital to analogue converter), therefore sample values range between 0 and 4095 in arbitrary units.
3. Results and Discussion
The structure and consequent luminescence properties of these materials depend on their thermal treatment. Various phases of the material with different luminescent properties were obtained by dynamic TG and DSC measurements of the xerogels up to 1100 °C, and by their isothermal heat treatment in a reductive atmosphere for 3 hours at 900, 1000 and 1100 °C as reported.26 In order to explain such behaviour, investigation of their structures at the quantitative level was necessary.
3.	1. Phase Compositions
All samples of MSA, CSA and CMA doped with 0.5 or 1 mol% of Eu2+ annealed at the temperatures mentio-
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ned above, contained up to four of the following phases: monoclinic CaAl2O4 (PDF No. 23-1036, ICSD No. 241240), cubic Ca3Al2O6 (PDF No. 38-1429, ICSD No. 151369), hexagonal SrAl2O4 (PDF No. 31-1336, ICSD No. 160300), monoclinic SrAl2O4 (PDF No. 34-379, ICSD No. 160297), cubic MgAl2O4 (PDF No. 75-1796, ICSD No. 91379), hexagonal SrMgAl10O17 (PDF No. 26-0879) or SrMg0.94Al1006O17 (ICSD No. 155527) and orthorhombic phase isostructural with orthorhombic Ca20Al26Mg3Si3O68 (PDF No. 35-133, ICSD No. 26353). The formulae of the phases, listed in the databases are given for clarity, although it is known that in some systems (especially Ca-Sr aluminates) solid solutions are formed.22 The Rietveld refinement was performed for all samples and acceptable fits for quantitative analysis were achieved. Rp and Rwp values are listed in Table 1. Somewhat higher values of the reliability indices in the CSA samples are probably due to preferred orientation of both major phases, however, the quantitative mass ratios are
Table 1. Selected results of Rietveld refinement.
not significantly affected as the intensity mismatch is compensated using the whole profile. The phase compositions obtained are presented in Table 1. The results are given in terms of wt.%; possibly present amorphous phase was not considered and the formulae are given for pure compounds for clarity. The relative errors, estimated from some repeated measurements, are about 5% when the phase concentration is high (above 20%) and increase at lower phase concentrations.
As shown in Table 1, beside the cubic phase of MgAl2O4, the hexagonal phase of SrAl2O4 is present in the samples of MSA annealed at 900, 1000 and 1100 °C, while its monoclinic phase appears and its amount increases when they are annealed at 1000 and 1100 °C. Finally, only the monoclinic phase was identified in the samples annealed at 1300 °C. The fact that the phase mass ratio of MgAl2O4 versus SrAl2O4 almost perfectly reaches its theoretical value (41:59) at high temperatures confirms that the solid solubility between these two phases is very
Temperature Sample	900 °C	1000 °C	1100 °C	1300 °C
MSA:0.5Eu2+	SrAl2O4 hexagonal 44% MgAl2O4 cubic 56% Rp = 62.04 Rwp = 8.44	SrAl2O4 hexagonal 24% SrAl2O4 monoclinic 28% MgAl2O4 cubic 48% Rp = 99.85 Rwp = 12.42	SrAl2O4 hexagonal 24% SrAl2O4 monoclinic 32% MgAl2O4 cubic 44% Rp = 10.45 Rwp = 13.29	SrAl2O4 monoclinic 60% MgAl2O4 cubic 40% SrMg0.94Al10.06O17 hexagonal < 1% Rp = 12.36 Rwp = 16.30
MSA:1Eu2+	SrAl2O4 hexagonal 41% MgAl2O4 cubic 59% Rp = 82.79 Rwp = 11.24	SrAl2O4 hexagonal 25% SrAl2O4 monoclinic 26% MgAl2O4 cubic 50% Rp = 8.11 Rwp = 10.12	SrAl2O4 hexagonal 22% SrAl2O4 monoclinic 35% MgAl2O4 cubic 43% Rp = 11.18 Rwp = 14.25	SrAl2O4 monoclinic 63% MgAl2O4 cubic 36% SrMg0.94Al10.06O17 hexagonal < 2% Rp = 10.84 Rwp = 13.70
CSA:0.5Eu2+	CaAl2O4 monoclinic 42% SrAl2O4 hexagonal 45% SrAl2O4 monoclinic 5% Ca,Al2O6 cubic 8% Rp = 11.56 Rwp = 15.20	CaAl2O4 monoclinic 48% SrAl2O4 hexagonal 42% SrAl2O4 monoclinic 3% Ca,Al2O6 cubic 7% Rp = 14.10 Rwp = 18.28	CaAl2O4 monoclinic 53% SrAl22O44hexagonal 40% SrAl2O4 monoclinic 3% Ca3Al2O6 cubic 4% Rp3= 125.369 Rwp = 20.91	CaAl2O4 monoclinic 66% SrAl2O4 hexagonal 18% SrAl2O4 monoclinic 13% Ca3A2l2O4 6 cubic 4% Rp = 12.81 Rwp = 16.76
CSA:1Eu2+	CaAl2O4 monoclinic 48% SrAl2O4 hexagonal 41% SrAl2O4 monoclinic 7% Ca,Al2O6 cubic 4% Rp = 12.35 Rwp = 16.14	CaAl2O4 monoclinic 51% SrAl2O4 hexagonal 39% SrAl2O4 monoclinic 5% Ca,Al2O6 cubic 5% Rp = 14.69 Rwp = 18.69	CaAl2O4 monoclinic 65% SrAl2O4 hexagonal 33% Ca3Al2O6 cubic < 2% Rp = 17.16 Rwp = 23.72	CaAl2O4 monoclinic 78% SrAl22O44hexagonal 20% Ca3Al2O6 cubic < 2% Rp = 17.25 Rwp = 23.81
CMA:0.5Eu2+	Poorly crystalline phase of MgAl2O4 could be seen only	Poorly crystalline phase of MgAl2O4 could be seen only	MgAl2O4 cubic 63% CaAl2O4 monoclinic 37% Rp = 11.84 Rwp = 16.01	MgAl2O4 cubic 50% CaAl2O4 monoclinic 50% Rp = 12.85 Rwp = 17.33
CMA:1Eu2+	Poorly crystalline phase of MgAl2O4 could be seen only	Poorly crystalline phase of MgAl2O4 could be seen only	MgAl2O4 cubic 48% CaAl2O4 monoclinic 40% Ca20Al26Mg3Si3O68 orthorhombic 12% Rp = 10.58 Rwp = 13.58	MgAl2O4 cubic 41% CaAl2O4 monoclinic 48% Ca20Al26Mg3Si3O68 orthorhombic 11% Rp = 11.86 Rwp = 16.02
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MSA:0.5Eit!+
" S"A < )j nwi •MgALO,
10 20 30 40 50 60 70 80
Zen
Figure 1. Crystallization and phase development with increasing temperature in a typical sample of MSA:0.5Eu2+ as shown by XRD. Peaks of individual phases are marked by symbols according to the legend.
small. This is also confirmed by comparison of the refined unit cells of the present phases. The unit cell parameters of the present phases are nearly equal in all samples and very close to the reference values from PDF and ICSD. The maximum difference in the unit cell volumes of the phases is less than 0.5%. Fig. 1 shows the increase of crystallinity and phase development with increasing temperature of a typical sample of MSA.
In the samples of CSA the hexagonal and monocli-nic phases of SrAl2O4 were identified in all samples except CSA:1Eu2+ annealed at 1100 and 1300 °C (Table 1, Fig. 2). The hexagonal phase of SrAl2O4, was also found when preparing these aluminates by conventional solid state reactions at 1200 and 1250 °C.20,21 It was accompanied by two calcium aluminates present in all samples
of this composition: monoclinic CaAl2O4 and cubic Ca3Al2O6. The amount of monoclinic CaAl2O4 was high in all samples of this composition, especially in the samples annealed at 1300 °C (the theoretical mass ratio between CaAl2O4 and SrAl2O4 if they were pure would be 43:57) (Fig. 2). This indicates that a substantial amount of Sr is in fact incorporated into the crystal lattice of a monoclinic CaAl2O4 forming a solid solution; this has also been reported elsewhere.22 Solid solubility was also confirmed by comparing the refined unit cells of the present phases. Table 2 shows the unit cells with maximal and minimal refined volume, compared to the PDF and ICSD references. It is seen that the differences among the samples are small (differences in the unit cell volume are within 1%), which means that the maximum solid solubility is similar at all studied temperatures. Significant increase of the unit cells of the calcium phases with respect to the PDF and ICSD (unit cell volume differs for up to 3.7%) confirms inclusion of Sr into the Ca sites in these phases. Unit cells of the strontium phases are smaller than that of the reference PDF and ICSD ones, which mean that Ca is included into the Sr sites in these phases (maximum difference is 7%). However, it has to be noted, that the reference data themselves also differ by 2% between the PDF and ICSD data, so that it is not possible to draw very decisive conclusions from these comparisons.
The samples of CMA remain amorphous when annealed at 900 °C. The cubic phase of MgAl2O4 first appears at 1000 °C as shown in Fig. 3 in contrast to samples of MSA which display this phase after annealing at 900 °C. The monoclinic phase of CaAl2O4 crystallizes not earlier than at 1100 °C in the samples of this composition. However, its crystallization is probably complete at 1300 °C, while the proportions of the two phases achieve their theoretical mass ratio (53:47) in the samples with 0.5 mol% of Eu2+ (Table 1), confirming that solid solubility in
Table 2. Unit cells with maximal and minimal refined volume, compared to the PDF and ICSD references for CSA:Eu2+ samples.
Phase	Sample/Reference	a [A]	b [A]	c [A]	P[°]	Volume [A3]
SrAl2O4_hex	CSA:1Eu2+-900 CSA:1Eu2+-1300 PDF No. 31-1336 ICSD No. 160300	5.0839(4) 5.0822(5) 5.1400 5.1666(1)		8.2345(8) 8.213(2) 8.462 8.5485(3)		184.31(4) 183.70(4) 193.61 197.62
Ca3Al2O6_cub	CSA:0.5Eu2+-1300 CSA:0.5Eu2+-900 PDF No. 38-1429 ICSD No. 151369	7.668(2) 7.681(3) 15.2631 7.624				450.8(3) 453.2(4) 444.5 443.1
CaAl2O4_mon	CSA:1Eu2+-900 CSA:0.5Eu2+-1300 PDF No. 23-1036 ICSD No. 241240	8.778(2) 8.809(3) 8.698 8.6942(2)	8.212(0) 8.245(3) 8.092 8.09299(19)	15.313(3) 15.280(5) 15.208 15.20965(32)	89.87(2) 89.94(9) 90.14 90.1665(19)	1103.9(5) 1109.7(6) 1070.4 1070.2
SrAl2O4_mon	CSA:0.5Eu2+-1100 CSA:1Eu2+-1000 PDF No. 34-379 ICSD No. 160297	8.42(1) 8.406(6) 8.4424 8.5066(5)	8.80(1) 8.790(5) 8.822 8.8898(5)	5.223(5) 5.295(6) 5.1607 5.1732(3)	92.1(1) 91.6(2) 93.415 92.875(4)	387.1(7) 391.1(6) 383.7 393.4
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Figure 2. Rietveld plot of CSA:0.5 mol% Eu2+-1300. The black pattern is measured, the grey calculated; the grey line below is the difference and the vertical bars at the bottom are the reflection positions of the phases observed (see legend). Strongest peaks of individual phases are marked by symbols in the legend.
this system is very small. Like in the case of MSA, this was confirmed by the unit cell volumes. The unit cells of the present phases are nearly equal in all samples and very close to the reference values from PDF and ICSD. The maximum difference in the unit cell parameters of the phases is less than 0.4%.
When 1 mol% of Eu2+ is added to samples of CMA an unexpected phase was identified at high temperatures (1100 and 1300 °C). In the PDF database it is described as orthorhombic Ca20Al26Mg3Si3O68 (PDF No. 35-133). The synthesis and analyses of these samples were double-checked and are highly reproducible, and show that there is no Si present in the samples. Rietveld refinement, on the other hand, confirmed the structural model, so that the most probable explanation is that Eu stabilizes the phase isostructural with Ca20Al26Mg3Si3O68 where Si is replaced by Al and charge balance is likely achieved by oxidisation
of Eu or by oxygen vacancies (Ca17Eu3Al29Mg3O68, Ca20Al29Mg3O66 5), or a combination of the two. The first possibility was checked by UV excited fluorescence, but it was found that oxidation to Eu3+ is very limited and insufficient to cover the charge balance by itself, so that the second effect probably takes place. For the purpose of phase quantity estimation, the formula and structure of this phase were left as containing Si, as the scattering factors of Si and Al are so similar that this does not cause any significant error in the Rietveld refinement.
3. 2. UV Excited and Persistent Luminescence
The luminescence properties of these materials depend on their crystal structures and frequently on the quantity of Eu2+ (Table 1). The luminescence of the Eu2+ ion arises from transitions between the 4J65d1 and 4J7 configurations. A shift in the luminescence band position for the
Figure 3. Crystallization and phase development with increasing temperature in a typical sample of CMA:Eu2+ as shown by XRD. Peaks of individual phases are marked by symbols according to the legend.
Figure 4. UV excited (Aexc = 350 nm) luminescence spectra of MSA:Eu2+ (0.5 mol% or 1 mol% of Eu2+) annealed at various temperatures, measured at room temperature. The curves legend is ordered by appearance.
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different host lattices can be explained by a change in the crystal field effect on the Eu2+ ion.16'31 Therefore the structure of the host lattice affects the luminescent properties of the material.
The host lattice for Eu2+ ions (0.5 and 1 mol%) in MSA is one of the phases of SrAl2O4 (hexagonal or monoc-linic), since only the characteristic band of pure SrAl2O4 observed in the green region (Amax ~ 512 nm) appears in the UV excited luminescent spectra,15'18 as shown in Fig. 4.
The lifetime of persistent luminescence depends on quantity of Eu2+ ions and the annealing temperature. Luminescence was measured at room temperature as long as it could be perceived by the naked eye.
Appropriate initial persistent luminescence intensities and afterglow lifetimes were obtained when the samples of MSA doped with 0.5 mol% Eu2+ were heated at 900 or 1300 °C (Fig. 5). Both samples contained pure phases of SrAl2O4, the hexagonal phase when the sample was annealed at 900 °C, and the monoclinic phase when it was annealed at 1300 °C (Table 1).
1 10 100 Time [min]
Figure 5. Persistent luminescence lifetimes of samples with nominal formula MSA:Eu2+, measured at room temperature. The inset graph represents magnification of curves tails. The curves legend is ordered by appearance.
Annealing the samples at 1000 °C caused weakening of the initial persistent luminescence intensity, together with shortening of the afterglow lifetime (sample with 1 mol% of Eu2+) as well as quick quenching of the persistent luminescence with initial high intensity (sample with 0.5 mol% of Eu2+). The luminescent properties practically disappeared in the samples heated at 1100 °C (Fig. 4). The reason for this behaviour could be explained by the presence of the two phases of SrAl2O4 (hexagonal and monoclinic) in the samples annealed at 1000 or 1100 °C and at the ratio in the amounts of these two phases, which decrease or even quench the luminescent behaviour of these materials (Table 1).
UV excited luminescence of CSA and CMA doped with 0.5 or 1 mol% Eu2+ was observed in the blue
= 350 nm	-CSA:0.5EU!"-1000 ■■■■*'■■ CSA:1EU!'-10M CSA:0.5Eu;*-900 -CSA:0.5Eu!"-130D —o- CSA1Eu!*-130C
/ \	CSA:1EU!'-900
	CMA:1Eu'*-11W
	---CMA:1Eu!*-13M -----CMA:C.5Eu;,-1300
/ /15 51 \ \	CMA:0.5Eu!'-l1[)0
	CSA:1Eu'*-11MJ
/ ii' s' \	V -----CSA:0.5Eu!'-1100
If// '	
H4' 'i '¿X. Me /// X J/s	
400 450 500 550 600 650 Wavelength [nm]
Figure 6. UV excited (Aexc = 350 nm) luminescence spectra of CSA:Eu2+ and CMA:Eu2+' (0.5 mol% or 1.0 mol% of Eu2+) annealed at various temperatures, measured at room temperature. The curves legend is ordered by appearance.
region (Amax ~ 440 nm), as shown in Fig. 6. These aluminates contain the monoclinic phase of CaAl2O4, indicating that this phase is mostly responsible for the luminescent properties of the material.810 In MSA (0.5 and 1 mol% Eu2+) luminescence is observed in the green region (Amax ~ 512 nm) corresponding to that of pure SrAl2O4. But the Eu2+ ions seem to prefer the CaAl2O4 host lattice in those aluminates which contain it, since only the single band characteristic of pure CaAl2O4 is present in the UV excited luminescence spectra of CMA. This band is also dominant in the UV excited luminescent spectra of CSA, with a prolonged shoulder indicating that the SrAl2O4 phase causes some luminescent activity in the green region, which is manifested as a bright cyan colour, especially in the samples annealed at 900 and 1000 °C. This also confirms the structural results (Table 1), indicating the existence of some pure SrAl2O4 phases beside a solid solution of strontium in the CaAl2O4 phase. The luminescence properties disappear when the samples of CSA are annealed at 1100 °C (Fig. 6). The reason for this behaviour could be explained by change in the amounts of phases at various annealing temperatures (Table 1).
The lifetime of CSA samples persistent luminescence depends on quantity of Eu2+ ions and the annealing temperature (Fig. 7). In CSA samples annealed at 1000 °C, 1 mol% of Eu2+ ions provided nearly nine hours of luminescence activity. The initial afterglow from the sample doped with 0.5 mol% of Eu2+ was stronger than that from doped with 1 mol% of Eu2+, but its afterglow lifetime was shorter by almost four times. Several shorter lifetimes were achieved in the samples doped with 0.5 mol% of Eu2+ annealed at 1300 °C (7.5 hours). The sample doped with 1 mol% of Eu2+ annealed at this temperature had a low initial persistent luminescence with a short lifetime (2.7 hours).
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Figure 7. Persistent luminescence lifetimes of CSA:Eu2+ materials, measured at room temperature. The inset graph represents magnification of curves tails. The curves legend is ordered by appearance.
Samples of CMA acquire luminescent properties on annealing them at 1100 or 1300 °C, when the monoclinic phase of CaAl2O4 is formed (Table 1). As shown in Fig. 8, the samples doped with 1 mol% of Eu2+ display longer afterglow lifetimes irrespective of annealing temperature, with a higher initial intensity in the sample annealed at 1100 °C. It seems that the orthorhombic phase (Table 1), which appears in the samples doped with 1 mol% of Eu2+, enhances their luminescent properties. The samples doped with 0.5 mol% of Eu2+ and annealed at 1300 °C have very short afterglow lifetimes (17 min), although the initial afterglow is stronger than those annealed at 1100 °C.
200-
150-
Ž*
100
50-
04
\3.5 h		-CMA:1EU!*-1100
		CMA:0.5EuJ'-1300
		---CMA:1Eu!*-1300
		CMA:0.5EU2*-1 100
V \	5	
V \		\
	€.	\
\ \	<	
\ \	&	\\
\ \	1>	
\ \		
\\	—	
s\		
Ov		ZP « » 5P ZOO
		Time [min]
	T--	_ 1
1 10 100 Time [min]
Figure 8. Persistent luminescence lifetimes of CMA:Eu2+ materials, measured at room temperature. The inset graph represents magnification of curves tails. The curves legend is ordered by appearance.
4. Conclusions
A modified preparation of boehmite sols by the aqueous sol-gel route using nitric acid as peptizing agent was employed to obtain the xerogels as precursors of
alkaline earth aluminates doped with Eu2+ ions, thus enabling preparation of the materials in a reductive atmosphere without a carbon residue.
Structural studies showed the presence of various phases obtained at different annealing temperatures. They confirmed the solid solubility of strontium in the monoclinic phase of CaAl2O4 in CSA at all annealing temperatures, beside some pure phases (hexagonal and monoclinic) of SrAl2O4 Solid solubility was not observed in CMA and MSA. In CMA the presence of magnesium somewhat hindered the crystallization process of the material up to 1100 °C and an additional orthorhombic phase (beside monoclinic CaAl2O4 and cubic MgAl2O4) appeared in the samples doped with 1 mol% of Eu2+. Nevertheless, pure phases of SrAl2O4 (hexagonal at 900 °C and monoclinic at 1300 °C), or mixtures of both phases (at 1000 or 1100 °C), were identified in MSA.
Appropriate luminescent properties could be achieved at annealing temperatures lower than those required in conventional solid state reactions.
UV excited luminescence was observed in the blue region (A = 440 nm) in CSA and CMA containing the monoclinic phase of CaAl2O4, indicating that this phase primarily defined the luminescent properties of the materials. However, in CSA the luminescence turned out to be a bright cyan colour, corresponding to the presence of some pure phases of SrAl2O4, which caused additional UV excited luminescence in the green region (A = 520 nm). In MSA only UV excited luminescence in the green region with a maximum at ~512 nm was observed, corresponding to that of pure SrAl2O4.
The best materials CSA:1Eu2+ annealed at 1000 °C provided nearly nine hours of cyan luminescence activity, CSA:0.5Eu2+ annealed at 1300 °C 7.5 hours and MSA:0.5Eu2+ prepared at 1300 °C exhibited bright and green luminescence in darkness for more than six hours, which makes them suitable for application in various optoelectronic devices.
5. Acknowledgements
Financial support from the Slovenian Research Agency (ARRS), Ljubljana, (P-0134) is gratefully acknowledged.
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Povzetek
Z modificirano sol-gel metodo smo pripravili zemeljskoalkalijske aluminate z nominalno sestavo Mg05Sr05Al2O4 (MSA), Ca0 5Sr0 5Al2O4 (CSA), Ca05Mg05Al2O4 (CMA), dopirane z 0,5 ali 1 mol% Eu2+ ionov, ki smo jih žgali pri 900, 1000, 1100 in 1300 °C. Sestavo vzorcev in njihovo strukturo smo proučevali z XRD analizo s pomočjo Rietveldove metode. Trdno topnost, ki je posledica podobnih ionskih radijev, smo potrdili v vzorcih CSA. V CSA in CMA vzorcih, ki vsebujejo monoklinsko fazo CaAl2O4, smo UV vzbujeno luminiscenco opazili v modrem območju (X = 440 nm), v vzorcih MSA, ki vsebujejo heksagonalno ali monoklinsko fazo SrAl2O4 pa v zelenem območju (X = 512 nm).
Korošin et al.: Structural and Luminescent Properties of Eu2+-doped