Tamkang Journal of Science and Engineering, Vol. 5, No. 2, pp. 81-84 (2002)
81
Effect of Host Composition on the Luminescent Properties of Eu2+-Activated SrAl2B2O7 Ceramic Phosphors Chia-Pin Lin, Sheng-Tsong Chen and Teng-Ming Chen Department of Applied Chemistry National Chiao Tung University Hsinchu, Taiwan 300, R.O.C. E-mail:
[email protected]
Abstract Alkaline earth (M) dialuminodiborates have been reported to be hosts with better crystallinity, lower synthetic temperature and higher radiant efficiency, compared to corresponding borates or aluminates. This research was attempted to investigate the effect of host compositions on the photoluminescent properties of SrAl2B2O7:Eu2+ phases by systematically substituting Sr2+ with M2+ (M = Ca, Ba) ions. The emission of Eu2+ attributed to the 4f65d1 → 4f7 transition was found to be very sensitive to the changes of crystal field of the host due to Ca2+ and Ba2+ substitution, as indicated by the photoluminescence (PL) spectra. The synthesis, dopant-content dependent PL spectra and the decay lifetimes for M-substituted SrAl2B2O7:Eu2+ are presented in this paper. Key Words: (Sr,Ca)Al2B2O7:Eu2+, (Sr,Ba)Al2B2O7:Eu2+, Photoluminescence Spectra, Host Composition Effect, Fluorescence Decay Lifetime
1. Introduction Alkaline earth metal (M) dialuminodiborates (MAl2B2O7) have been known to be hosts for phosphor materials with better crystallinity, lower synthetic temperature and higher radiant efficiency, as compared to the corresponding borates or aluminates. The crystal structure for CaAl2B2O7 was reported by Chang et al. [1] to be a trigonal structure (space group R3c) consisting of double layers of (Al2O7)8- uints with tetrahedral oxygen-coordinated Al3+ ions and trigonal-planar (BO3)3- groups, between which octahedral oxygen-coordinated Ca2+ ions are located. Recently, Lucas et al. [2] investigated the temperature-dependent luminescence of Eu2+ for SrAl2B2O7:Eu2+. Furthermore, the Eu2+ ion is unique as an activator since its broad band 4f65d1 → 4f7 transition is parity-allowed and its radiative lifetime is relatively short (ca. 1 msec), and, most interestingly, its emission is strongly dependent on
the type of host, with possible emission wavelengths (λem) ranging from ultraviolet to red spectral region, as indicated in the investigations of SrB4O7:Eu2+ by Machida et al. [3]. Motivated by the investigations described above, we were prompted to study the effect of host compositions on the luminescent properties for SrAl2B2O7:Eu2+ by systematically substituting Sr2+ in the host lattice with Ba2+ or Ca2+ ion. Our goals are to study the possibility of tuning the emissive hues for SrAl2B2O7:Eu2+ by tuning the host composition through isovalent cation substitution and to investigate the effect of crystal field strength dependence of the 4f65d1 level for Eu2+ on the photoluminescence (PL) spectra and luminescence decay lifetime (τ) for SrAl2B2O7:Eu2+.
2. Experimental Polycrystalline samples of (Sr0.99-xBaxEu0.01) Al2B2O7 (abbrev. as (Sr1-xBax)Al2B2O7:Eu2+; x = 0,
Chia-Pin Lin et al.
0.2, 0.4, 0.6, 0.8, 0.99) and (Sr0.99-yCayEu0.01) Al2B2O7 (abbrev. as (Sr1-yCay)Al2B2O7:Eu2+; y = 0, 0.2, 0.4, 0.6, 0.8, 0.99) were synthesized by reacting stoichiometric amounts of CaCO3 (or BaCO3, 99.99%), Al2O3 (99.99%), SrCO3 (99.9%), H3BO3 (99.5%) and Eu2O3 (99.9%) in an alumina crucible at 650 oC for 1 h and then reduced in an atmosphere of 7% H2/ 93%Ar at 900 oC, 850 oC, o and 750 C for (Sr1-yCay)Al2B2O7:Eu2+, 2+ and (Sr0.99Eu0.01)Al2B2O7 (Sr1-xBax)Al2B2O7:Eu (abbrev. as SrAl2B2O7:Eu2+) phases, respectively.
3. Results and Discussion An analysis of the XRD profiles for both series of M-substituted SrAl2B2O7:Eu2+ (M = Ca and Ba) phases indicated the formation of complete solid solutions. Furthermore, to determine the optimal excitation wavelength (λexc) and compare the luminescent properties of pristine and M-substituted SrAl2B2O7:Eu2+ phases, we have measured the photoluminescence excitation (PLE) and photoluminescence (PL) spectra for pristine SrAl2B2O7:Eu2+ by using a Spex Fluorolog-3 spectrofluorometer equipped with a 450 W xenon lamp as the excitation source and the results are shown in Figure 1. Regardless of λexc (227 nm or 323 nm) used in the measurements, the PL spectra (solid curves shown in Figure 1) for SrAl2B2O7:Eu2+ were found to exhibit an broad emission peaking at 417 nm, typically observed for Eu2+ as an activator in a matrix of aluminoborates [2]. λ
em
= 420 nm
λ
ex
transition can be rationalized by considering the splitting of Eu2+ d-level into eg and t2g. Consequently, we ascribed the absorption bands between 280 and 400 nm to the 4f65d1 (t2g) level, with that of the 4f65d1 (eg) level being located below 259 nm. In particular, the formation of shoulder, next to the major absorption band occurring at 323 nm, is related to the crystal-field dependent splitting for 4f65d level. On the other hand, the emission peak observed at 364 nm is a typical transition (i.e., 6P7/2 → 8S7/2) from 4f7 level, and the broadband emission at 417 nm is thus attributed to the typical Eu2+ 4f-5d transitions. In order to evaluate and understand the effect of host compositions on the luminescence of SrAl2B2O7:Eu2+, we have investigated the PL spectra for both series of (Sr1-xBax)Al2B2O7:Eu2+ and (Sr1-yCay)Al2B2O7:Eu2+ phases as a function of x or y and the results are summarized in Figures 2 and 3, respectively. 6
7
2+
4f 5d → 4f ( Eu )
λ exc = 325 nm
417 x=0
413
Intensity (a.u.)
82
409
x = 0.2 x = 0.4
406 x = 0.6 401 300
x = 0.8 400
500
600
700
Wavelength (nm)
Figure 2. PL spectra as a function of x for 2+ (Sr1-xBax)Al2B2O7:Eu with λexc=325 nm
= 323 nm
7
4f →
6
4f 5d → 4f
6
4f 5d(t2g)
7 6
2+
420
7
4f → 6
λ
4f 5d (eg)
6
ex
Intensity (a.u.)
Intensity (a.u.)
7
4f 5d → 4f (Eu )
417
= 227 nm
8
P7/2 → S7/2
300
400
500
600
Figure 1. PLE (-----) and 2+ SrAl2B2O7:Eu
x=0
423
x = 0.2
426
x = 0.4
430
x = 0.6
440
x = 0.8 x = 0.99
700
Wavelength (nm)
PL (——) spectra for
Our observation was found to be consistent with that (i.e., 410 nm) observed by Lucas et al. [2] for SrAl2B2O7:Eu2+ excited with an ultraviolet of 350 nm at 9 K. The PLE spectra (dashed curve shown in Figure 1) attributed to Eu2+ 4f7 → 4f65d
λexc = 334 nm
300
400
500
600
700
800
Wavelength (nm)
Figure 3. PL spectra as a function of y for (Sr1-yCay)Al2B2O7:Eu2+ with λexc =334 nm
With increasing x an apparent blue shift in λem maxima from 417 nm (x = 0) to 401 nm (x = 0.8) was observed in the PL spectra for (Sr1-xBax)Al2B2O7:Eu2+ phases represented in Figure 2. Briefly, the transitions
2+
Effect of Host Composition on the Luminescent Properties of Eu -Activated SrAl2B2O7 Ceramic Phosphors
from the lower 4f65d1 component to the 8S level for Eu2+ are generally found to result in band emissions and strongly dependent on the crystal field strength (i.e., △) [4]. Therefore, the observation of blue shift in λem maxima for (Sr1-xBax)Al2B2O7:Eu2+ phases can be rationalized by the apparent weakening of crystal field strength experienced by Eu2+ luminescent center and larger lattice dimension when Sr2+ is gradually substituted by Ba2+ ions. With decreasing △ the energy corresponding to the transition from the lower 4f65d1 component to the 8S level is expected to become larger and a blue shift inλem maxima results. On the other hand, an apparent red shift in λem maxima from 417 nm (y = 0) to 440nm (y = 0.99) was observed with increasing y in the PL spectra for (Sr1-yCay)Al2B2O7:Eu2+ phases represented in Figure 3. The observed trend of red shift in λem due to Ca2+ substitution can also be explained by considering the same crystal-field energy level diagram for Eu2+ proposed by Blasse [4]. The observation can be attributed to the apparent strengthening of crystal field and smaller lattice dimensions due to Ca2+ substitution for Sr2+ ion. Thus, with increasing △ the energy corresponding to the transition from lower 4f65d1 component to the 8S state is thus expected to become smaller and a red shift in λem maxima results. As represented in Figure 4, the λem maxima were plotted as a function of M (M = Ba and Ca) content for M-substituted SrAl2B2O7:Eu2+ phases to understand the shifting of λem maxima due to M substitution. An apparent blue shift in λem maxima of 16 nm was found in the (Sr1-xBax)Al2B2O7:Eu2+ phases with x increasing from 0 to 0.8, whereas a red shift in λem maxima of 23 nm was also clearly observed for (Sr1-yCay)Al2B2O7:Eu2+ with y increasing from 0 to 0.99.
M = Ca
λem (nm)
420 M = Ba
410
The decay lifetimes (τ) for MAl2B2O7:Eu2+ phases shown in Table 1 were measured with a Spectra Physics GCR-5 Nd-YAG laser giving an ultraviolet wavelength (λexc) of 340 nm and found to be in the submicro- or micro-second range. Our observations of lifetime can be rationalized by the fact that the ground state of 4f7 is 8S and the spin multiplicity of the excited state is either 6 or 8; the sextet portion of the excited state probably also contributes to the spin-forbidden character of the 4f-5d transition and is expected to be longer, as proposed by Blasse [4]. Table 1. The decay lifetimes (τ) for some MAl2B2O7:Eu2+. Host Composition
τ (µsec)
BaAl2B2O7
2.16
(Ba0.6Sr0.4)Al2B2O7
1.81
SrAl2B2O7
2.20
(Sr0.6Ca0.4)Al2B2O7
6.13 x 10-1
CaAl2B2O7
5.86 x 10-1
Moreover, the τ value for pristine SrAl2B2O7:Eu2+ was found to be significantly longer, as compared to that for Ba2+ or Ca2+-substituted SrAl2B2O7:Eu2+ phases. Furthermore, with decreasing size of M2+ the τ value was found to be smallest for CaAl2B2O7:Eu2+, as compared to those for MAl2B2O7:Eu2+ with M = Sr and Ba. Rationalizations for the observed variation in decay lifetime require further investigations.
4. Conclusions In clonclusion, our observations confirm that Blasse’s prediction regarding the crystal-field dependent 4f65d1 energy level for Eu2+ is also applicable in ceramic aluminoborate hosts. We have also demonstrated that systematic tuning of the host compositions through isovalent substitution can effectively modify the emissive characteristics of SrAl2B2O7:Eu2+ phase.
440
430
83
Acknowledgment 400 0.0
0.2
0.4
0.6
x or y
0.8
1.0
Figure 4. λem maxima as a function of M content for (Sr,M)Al2B2O7:Eu2+ phases
The research was supported by the National Science Council (NSC) of R.O.C. under Contract No.NSC89-2113-M-009-024. The Instruments Center of NSC in Hsinchu is acknowledged for lifetime measurements.
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References [1] [2] [3] [4]
Chang K. S.; Keszler A, Mater. Res. Bull. 1998 33, 299. Lucas, F.; Jaulmes, S.; Quarton, M.; LeMercier T.; Guillen, F.; Fouassier, C. J. Solid State Chem. 2000, 150, 404. Machida, K.; Adachi G.; Shiokawa, J. J. Lumin. 1979, 21, 101. Blasse, G. in Bartolo, B. O., Luminescence of Inorganic Solids, Plenum Press, New York, U.S.A. 1978; pp 463-464.
Manuscript Received: Apr. 1, 2002 and Accepted: May 6, 2002
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