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Comparison and analysis of Eu3+ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor
Materials Chemistry and Physics xxx (2015) 1e9
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Comparison and analysis of Eu3þ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor A. Yousif a, b, S. Som a, Vinod Kumar a, H.C. Swart a, * a b
Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA 9300, South Africa Department of Physics, Faculty of Education, University of Khartoum, P.O. Box 321, Postal Code 11115, Omdurman, Sudan
h i g h l i g h t s Eu3þ doped Y3Al5O12 and Y3Ga5O12 were successfully synthesized by combustion method. The luminescence properties of the material for red lighting phosphor were reported. The JuddeOfelt intensity parameters of were determined in detail. The color properties as a function of the Eu3þ concentration were performed. The color coordinates traversed in a range from the light red to deep red region.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2015 Received in revised form 21 September 2015 Accepted 29 September 2015 Available online xxx
This paper reports on the comparative investigation of the spectroscopic properties of the Y3Al5O12 (YAG) and Y3Ga5O12 (YGG) doped with Eu3þ synthesized by the combustion method. A pure phase was identified for the YGG phosphor, but one other phase was observed for the as-prepared and annealed YAG phosphor. A high intense red color emission was observed from Eu3þ doped YAG compared to the orange-red color of Eu3þ in the YGG phosphor. The optimum Eu3þ concentration was found to be higher in the YGG compared to the optimum Eu3þ concentration in the YAG. A spectroscopic investigation on the concentration quenching effect and the covalency of the EueO bond were carried out on the basis of the substitution of Ga3þ and Al3þin the garnet host. JuddeOfelt parameters, different spectral and colorimetric parameters were estimated theoretically from the experimental curves, for both the hosts to determine the better luminescence media. © 2015 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence spectroscopy Powder diffraction Crystal structure Crystal symmetry Luminescence Optical properties
1. Introduction Garnets are useful hosts for various optical applications due to a high thermal stability, hardness, and optical isotropy, good thermal conductivity particularly for lasing and solid state lighting applications [1]. Garnets are basically a combination of Ln2O3 (rare earth oxide) and M2O3 (metal oxide), where Ln ¼ Y and M ¼ Al, Ga as an example. Garnets can be described by the {A}3[B]2(C)3O12 formula, where A (Y3þ) is a dodecahedrally coordinated site with point group D2 (without an inversion center), while B (Al3þ/Ga3þ) and C (Al3þ/Ga3þ) are octahedrally and tetrahedrally coordinated sites with C3i (with an inversion center) and S4 point groups,
* Corresponding author. E-mail address: [email protected] (H.C. Swart).
respectively. Because of similar ionic radii and chemical properties, Y3þ ions can easily be replaced by other lanthanide ions (e.g., Eu3þ) in the crystal lattice, making garnets optically active materials [1,2]. As one of the most frequently used red emitting activators in rareearth ions doped luminescent materials, the trivalent europium (Eu3þ) ions mainly show characteristic emissions resulting from the transitions of 5D0, 1, 2 / 7Fj (j ¼ 4, …, 0) [3]. For decades Eu3þ has been used as an optical probe, providing spectroscopic information regarding the local site symmetry and environment within the host [4]. Therefore, the Eu3þ was selected to investigate the luminescence properties in the Y3Al5O12 (YAG) and Y3Ga5O12 (YGG) in which they have the same crystalline structure [5]. But due to the fact that, Ga ionic radius (0.62 Å) is larger than the Al ionic radius (0.53 Å) change in lattice parameter among the YAG and YGG have been reported by many researchers [6]. Moreover, the YAG have larger band gap (6.5 eV) than the band gab of YGG (5.5 eV) [6,7]. The
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Please cite this article in press as: A. Yousif, et al., Comparison and analysis of Eu3þ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.042
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covalency of the GaeO bond is higher than the AleO bond as it has been reported by Nakatsuka et al. [8] which introduce a distortion to the neighboring dodecahedra where the Eu3þ supposes to be situated [6]. All the mentioned differences between the YAG and YGG hosts leaded to the interest to investigated the luminescence properties of Eu3þ in these hosts and motivated by the need of getting environment friendly red phosphor compared with the commercial available Y2O2S:Eu3þ red phosphor [9]. The spectroscopic properties of the Eu3þ doped YAG and YGG hosts have been reported by many researchers for different purposes. Yang et al. [10] reported the enhancement of the photoluminescence (PL) by incorporated the Lithium ions in the YAG:Eu3þ phosphor. Rezende et al. [11] studied the radioluminescence enhancement in Eu3þdoped YAG phosphors by Ga substitution. Lu et al. [12] reported the investigation of synthesis and luminescence properties of microemulsion-derived YAG:Eu3þ Phosphors. While Bi et al. [13] studied the electrospinning preparation and PL properties of YAG:Eu3þ Nanobelts. On the other side, Binnemans et al. [14] investigated the optical absorption spectra of Eu3þ in YGG while Zhu et at [15] reported the luminescence spectra of the YGG:Eu3þ and the energy transfer from Bi3þ to Eu3þ. In all mentioned studies, there was no work has been reported on the JuddeOfelt parameters calculation and the comparison of luminescence properties of the Eu3þ in the YAG and YGG hosts. Several synthesized methods are already reported in literature for the preparation of garnets. The solid-state reaction and combustion method are some of the well known methods for the synthesis of garnets. The solid state reaction require extensive mechanical mixing and lengthy heat treatments at high temperatures to prevent the occurrence of intermediate phases which are the main disadvantages of the solid-state synthesis of different aluminates and gallates materials. The secondary phases such as perovskite YAlO3 or metastable Y2Al4O7 may be formed if YAG is synthesized by a solid-state reaction [16]. Recently, a wet-chemical processing method for oxide systems attracted more and more attention from the scientific community due to the considerable advantages such as good mixing of the starting materials, excellent chemical homogeneity of the final products and lower synthesis temperatures compared with the solid-state reaction. Among the wet-chemical methods, the combustion method, which have been developed and successfully used for the low-temperature production of pure phase YAG, and YGG nano-powders and related systems, is the most popular technique. The combustion method is a synthesis route involving the exothermic reaction of an oxidizer, such as metal nitrates, with an organic fuel, typically urea (CH4N2O). In this paper, the properties of Eu3þ doped YAG and YGG was studied for the as-prepared as well as 1200 C annealed samples. The effect of Eu3þ doping on the structural and luminescent properties of YAG and YGG was also investigated. The JuddeOfelt intensity parameters of Eu3þ for YAG and YGG were also determined in detail. The color properties as a function of the Eu3þ amount were performed.
(C6H8O7$H2O, analytical grade) were used as starting materials in the present study which were obtained from SigmaeAldrich Company. The materials were dissolved in diluted water under stirring and heating to obtain a mixing aqueous homogenous precursor solution. The solution was placed in a furnace preheated at 600 C. After the combustion process was completed, the obtained solid precursors were then ground. The Y3xAl5O12:Eux¼0.03 (YAG:Eu3þ) and Y3xGa5O12:Eux¼0.09 (YGG:Eu3þ) samples were selected and annealed at 1200 C for 3 h and 2 h in air, respectively. The crystalline characteristic of the product was identified by X-ray diffraction (XRD) analysis (Bruker AXS D8 ADVANCE X-ray diffractometer) using a nickel-filtered CuKa target (l ¼ 0.154056 nm). The scanning electron microscopy (SEM) of the powders was performed using a JSM-7800F microscope. The PL measurements were done at room temperature using a Cary Eclipse fluorescence spectrophotometer equipped with a xenon lamp. 3. Result and discussion 3.1. Structural analysis The YAG and YGG garnet crystal structure can be described as a network of AlO6/GaO6 octahedra and AlO4/GaO4 tetrahedra linked by shared oxygen ions at the corners of the polyhedral as presented in Fig. 1. These polyhedra are arranged in chains along the three crystallographic directions and form dodecahedral cavities which are occupied by the Y3þ ions. Thus the YAG/YGG garnet possesses three crystallographically distinct cation sites, i.e., the Al3þ/Ga3þ ions are located both in 24(d) tetrahedral S4 sites and 16(a) octahedral S6 sites with fourfold and six fold coordination, respectively, while the Y3þ ions are located in a 24(c) dodecahedral D2 site with a coordination number of 8. The O2 ions occupy the 96(h) sites with each one being a member of one tetrahedron, one octahedron, and two dodecahedra. Due to ionic size considerations in the YAG/YGG lattice, the rare earth ions are expected to predominantly enter into the distorted dodecahedral sites by replacing the Y3þ ions and to be coordinated to eight O2 ions [17,18]. The schematic representation of the unit cell of YAG and YGG after Ga substitution in the Al position is shown in Fig. 2. The Ga (ionic radius 0.62 Å) substitution in the all Al (ionic radius 0.53 Å) position will lead to change in lattice parameter for the garnet crystal structure from 12.00 to 12.28 Å [6]. As a result, the unit cell of YGG is bigger than the unit cell of YAG. The schematics of the unit cells of YAG and YGG phosphors were
2. Experimental The combustion synthesis has emerged as an important technique for the synthesis and processing of advanced ceramics, catalysts, composites, alloys, inter metallic, and nano-materials. The powder samples of {(Y1xEux)3(Al1yGay)5O12}x¼1 to 11, y¼0 and 1 were prepared by the urea-nitrate combustion synthesis technique. The aluminum nitrate (Al(NO3)3$9H2O, 99.997% pure), yttrium nitrate (Y(NO3)3$6H2O, 99.99% pure), gallium nitrate (Ga(NO3)3$xH2O, 99.999% pure), Europium nitrate pentahydrate (Eu(NO3)3$5H2O, 99.9% pure), and hydrated citric acid
Fig. 1. The arrangement of Y3þ/Eu3þ, O2, Al3þ/Ga3þ described with a polyhedral in the YAG and YGG matrix [17,18].
Please cite this article in press as: A. Yousif, et al., Comparison and analysis of Eu3þ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.042
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Fig. 2. The schematic representation of the unit cell of YAG and YGG after Ga substitution in the Al position.
drawn with the help of VESTA software in order to get some idea of the crystal structure [19]. The XRD diffraction patterns of as-prepared and annealed YAG:Eu3þ powders are presented in Fig. 3 (a) while Fig. 3 (b) showing the XRD patterns for as-prepared and annealed YGG:Eu3þ phosphors powder as well. The XRD spectra were recorded to identify the phase composition and crystal structure of the material. The XRD patterns of the as-prepared YAG:Eu3þ samples show the polycrystalline main phase of YAG which is matched with the standard YAG data base(JCPDS 79-1891). Additionally, there are also a few additional low intensities peaks which are attributed to another secondary minority phase of YAlO3 with the standard data base (JCPDS 33-0041). It is well-known that during the combustion synthesis process, the flame temperature can reached up to 1400 ± 100 C [20], from their XRD results it seem that this temperature was not enough to form pure YAG. Huh et al. [21] also reported that the pure YAG is synthesized above a combustion temperature of 1500 C. When YAG is synthesized with a stoichiometric mixture of the raw materials YAG is the last phase that formed after two other phases Y4Al2O9 and YAlO3. A broadening in lower angle sides of the XRD pattern of as-prepared YAG sample was observed. That means; these peaks are not Gaussian in shape. The asymmetry in the peak is attributed to the presence of very small crystallites existing together with the larger nanocrystals [22]. After the YAG:Eu3þ sample was annealed at 1200 C for 3 h, the secondary phase of YAlO3 has been removed. The XRD patterns for the YGG which are presented in Fig. 3 (b), reveal well-defined Bragg reflections of the crystalline powder samples and it is identical to the referenced (JCPDS 071-2151). A shift to the higher angle side is observed after annealing the sample due to the decrease in the lattice parameters of the unit cell. To estimate the effect of Eu3þ concentration on the crystal structure, the unit-cell parameters of different Eu3þ doped YAG and YGG phosphors were determined using the Program Unit Cell software [23]. The variation of lattice parameters was shown in Fig. 4. From this figure it can be seen that the lattice constant increases a very little amount with the increase in Eu3þ concentration. 3.2. Surface morphology Fig. 5 (aec) shows the SEM micrographs of the as-prepared and
Fig. 3. (a) XRD patterns of the Y3xAl5O12:Eux¼0.03 (YAG:Eu3þ) phosphor as prepared and annealed at 1200 C and the standard data for the YAG phase. (b) XRD patterns of Y3xGa5O12:Eux¼0.09 (YGG:Eu3þ) phosphor as prepared and annealed at 1200 C and standard YGG data (JCPDS 071-2151).
Fig. 4. Variation of lattice parameter with Eu3þ concentration for YaG and YGG phosphors.
1200 C annealed YAG:Eu3þ and YGG:Eu3þ phosphor powders. The particle size for both samples varies up to several tens of microns. The SEM image of the as-prepared YAG:Eu3þ phosphor is shown in
Please cite this article in press as: A. Yousif, et al., Comparison and analysis of Eu3þ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.042
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Fig. 5. SEM images of the as-prepared (a) and (b) and (c) and (d) the 1200 C annealed YAG:Eu3þ and YGG:Eu3þ phosphor powder, respectively. The inset of Fig. 5 (c) is shows an enlargement of one part of the surface.
Fig. 5 (a). The surface is smooth without the appearance of grain boundaries with several large pores in localized areas of the particles. The pores are found in materials produced using combustion synthesis owing to the phase formation during the synthesis process with great evolution of a quantity of gas, which contributes to the production of materials with high porosity, good crystallinity, and nanometric particles [24]. The as-prepared YGG:Eu3þ phosphor powder surface is shown in Fig. 5 (b). It shows particles that consist of micro and sup-micro grains with very small pores located at the grain boundaries. It is well known that the pores are formed during the escaping of the gases during the combustion reaction. Fig. 5 (c) shows the annealed YAG:Eu3þ sample, sup-micro particles in the range of 300 to 200 nm have developed on the particle's surface with some localized areas which still shows the smooth particle's surface separated from other sup-micro particles. The inset in Fig. 5 (c), shows an enlargement of one part that shows particles with micro meter sizes. Fig. 5 (d), shows the SEM of the annealed YGG:Eu3þ sample with a mixture of micro and sup-micro particles. The number of pores has decreased in the YGG with respect to the YAG. 3.3. Spectral investigation of Eu3þ in YAG and YGG host and luminescence Fig. 6 (a) exhibits the excitation and emission spectra of the asprepared and 1200 C annealed YAG:Eu3þ phosphor powder. The excitation was monitored for the 5D0 / 7F1 transition at 590 nm. Since, Eu3þ has a 4f6 configuration; it needs to gain one more electron to achieve the half-filled 4f7 configuration, which is relatively stable compared to the partially filled configurations. Thus, when Eu3þ is linked to the O2 ligand, there is a probability of electron transfer from O2 to Eu3þ to form Eu2þeO [25]. Because of the electron transfer, a broad excitation band in the range of 200e300 nm was observed centered at ~227 nm which was
assigned to the Charge transfer (CT) band. The sharp lines in the wavelength range from 295 to 420 nm were attributed to the fef transitions within 4f6 of Eu3þ configuration [26]. No change was observed in the CT band position and shape after annealed at 1200 C except for an improvement in the intensity of the peak. Upon excitation into the CTB of Eu3þ ions, the emission spectra of the as-prepared and the annealed sample are composed of several emission peaks at 591, 609, 630 and 650 nm, which correspond to the Eu3þ transitions, 5D0 / 7FJ (J ¼ 1, 2, 3, 4) respectively. In both hosts the Eu3þ transitions of 5D0 / 7F0 was not observed which agree with reported literature [10,11]. Additionally, in both emission spectra also the magnetic dipole transition 5D0 / 7F1 (MD) was found to be stronger than a hypersensitive electric dipole (ED) transition 5D0 / 7F2. It is well known that the emission intensity of MD transition is independent of the site symmetry of the Eu3þ ions, whereas the emission wavelength and intensity of the ED transition is very sensitive to the site symmetry of Eu3þ ions. According to the transition selection rules J ¼ 0 ± 1 and DS ¼ 0, MD transition (5D0 / 7F1) is allowed, and ED (5D0 / 7F2) transition is forbidden [27]. However, in some cases in which Eu3þ activators occupy sites without inversion symmetry, the parity forbiddance is not strictly maintained and the spectra resulting from the electric dipole transition appear [27]. The Y3þ ion in the cubic phase of the YAG crystal is well known as a dodecahedrally coordinated site with point group D2 (without an inversion center), while the Al3þ are in the octahedral and tetrahedral coordinated sites with C3i (with an inversion center) and S4 point groups, respectively [1]. The YAG doped Eu3þ substitutes for Y3þ, which occupy a site without inversion symmetry. As a result, the luminescent intensity of the samples concentrated mainly on the MD transition rather than the ED transition. Fig. 6 (b) represent the excitation and emission spectra of as-prepared and 1200 C annealed YGG:Eu3þ phosphor. The excitation was also monitored for the 5D0 / 7F1 transition at 590 nm. The similar behavior in the excitation spectra of YGG:Eu3þ
Please cite this article in press as: A. Yousif, et al., Comparison and analysis of Eu3þ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.042
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7
D0- F1
(a)
CTB
D0- F4
7 5
7
D0- F3
5
f-f transitions 6 3+ within 4f of Eu
5
7
200 250 300 350 400
600
650
700
Wavelength (nm)
7
D0- F4
5
7
D0- F3
f-f transitions 6 3+ within 4f of Eu
5
7
7
D0- F2
CTB
b ¼
5
PL intensity (a. u.)
Eu - O
2+
D0- F1
3+
(b)
5
As-prepared o Annealed at 1200 C
200 250 300 350 400
known that CT energy decreased with larger average distance to the surrounding anions [26]. Nakatsuka et al. reported [8] that, the substituting of all Al by Ga in YAG results in an average YeO bond length increase from 2.371 to 2.390 Å, it is attributed to the decreased in the CT energy. The CT energy is strongly related to the covalency of the Eu3þeO2 bond, it is also strongly influenced by the next nearest cation M3þ (M ¼ Y3þ, Al3þ, Ga3þ). This was supported by a previous report of Yousif et al. [6]. Here, a stronger chemical bond between EueO bond and M3þ ions is expected in the YAG compared with YGG. They also studied the luminescence behavior of Bi3þ with YAG and YGG in details and their results show that, the higher covalency in YGG is attributed to the characteristics of Ga3þ ions due to its d10 electron configuration which promotes the formation of sp3-hybrid orbitals with strong covalency. Therefore, the CTB is located at shorter wavelength YAG compared with YGG. The change in the shape and intensity of the CTB were observed after the sample was annealed at 1200 C. The behavior of Eu3þeO2 ligand can also be determined by nephelauxetic ratio (b). The b can be calculated by using the method described in [29] using the PL excitation spectra
600
650
700
Wavelength (nm) Fig. 6. (a) A comparison of excitation and emission bands spectra of as-prepared and annealed Y3xAl5O12: Eux¼0.03 powder. (b) A comparison of excitation and emission bands spectra of as-prepared and annealed Y3xGa5O12:Eux¼0.09 powder.
when the Al3þ site was occupied by a larger Ga3þ ion was observed as before except the change in the CT band as shown in Fig. 6 (b). The band width of the excitation spectrum became broader with a full width at half maxima (FWHM) of 50 nm compared to 38 nm for the YAG:Eu3þ sample. The peak position of the CTB has shifted towards longer wavelengths from ~227 to ~245 nm. It was reported in [25,26], that the spectra of the CT band are related to the different factors namely the covalency of the Eu3þeO2 bond, bond length and the coordination number of Eu3þ. The red shift in the CT band suggests the increase in the covalent character of the EueO bond. Generally, the band position can be confirmed from Jorgensen formula [28]: s ¼ [copt (X) cuncorr (M)] 30 103 cm1 where, s is the energy of the CT band, copt (X) is the optical electronegativity in Pauling's scale and the optical electronegativity of the central metal ion is denoted as cuncorr (M). However, in different crystal field environments the optical electronegativity of the ion can change. From Fig. 6 (a and b), using the CT band position values the difference in electronegativity values (c (O2) c (Eu3þ)) were calculated for both the cases as 1.47 and 1.36 for YAG and YGG phosphors respectively. The decreased in the difference in electronegativity value indicates the decreased in the ionic strength of the EueO bond and hence the covalency increased [25]. It is well
1 X ncomplex n nfreeion
(1)
where ‘n’ is the wave number of an absorption transition of Eu3þ and ‘n’ is the number of observed absorption transitions. The respective values of b in the present host matrix were observed to be increased as 0.994 and 0.991 for the YAG and YGG phosphors respectively. It is well known that the positive value of nephelauxetic ratio (<1) indicates the existence of covalent bonding between the Eu3þ ion and O2 atom. But in the present research work, b value decreased which indicates the increment of the covalency of this bond which supports our previous result. Fig. 7 represents the effect of Eu3þ concentration in the different YAG and YGG hosts. In both cases, the PL intensity first increased upto the optimize concentration and then decreased. This effect is generally known as a concentration quenching effect. The optimum doping concentrations of the Eu3þ in YAG and YGG was determined to be 3% and 11% respectively. This difference can be attributed to the difference in the unit cell or the presence of the other impurity phase in the YAG and YGG as mentioned earlier. The concentration quenching effect involves different
PL intensities of Y3Al5O12:Eu
3+
PL intensities of Y3Ga5O12:Eu
3+
*
PL intensity (a. u.)
2+
D0- F2
3+
Eu - O
5
PL intensity (a. u.)
As-prepared o Annealed at 1200 C
5
*
1
2
3
4
5
6
7
8
9
10
11
3+
Eu concentration (%) Fig. 7. PL intensity of the MD transition (590 nm) as a function of the Eu3þ dopant concentration in the as-prepared YAG and YGG matrix.
Please cite this article in press as: A. Yousif, et al., Comparison and analysis of Eu3þ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.042
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mechanisms such as defect to Eu3þ energy transfer, multiphonon assisted energy transfer by electron phonon coupling or Eu3þeEu3þ energy transfer [9]. Defect to Eu3þ energy transfer will be possible in that case when substitution of Eu3þ will create new defects due to lattice mismatch. And the energy will loose nonradiatively through those existing defects. In the present case, no lattice mismatching was observed in Eu3þ ions substitution from XRD. Also, in the observed PL spectra no such transition was observed due to defect. Therefore, defect to Eu3þ energy transfer possibility is not the case for the concentration quenching. The energy also can be transferred nonradiatively via multiphonon assisted energy transfer by electron phonon coupling. For this type of energy transfer largely spaced energy levels are required. For Eu3þ in any host as the energy distance between 5D2 and 5D3 of Eu3þ is very large ~2500 cm1. An energy transfer between the 5D2 and 5D3 levels may exist which can quench the PL emission. The nonradiative rate WNR of the multi-phonon process in Eu3þ in the YAG and YGG host were therefore estimated by using the Van Dijk and Schuurmans energy-gap equation [30]: WNR ¼ bel exp½aðDE 2Zumax Þ with the constants bel ¼ 107 s1 and a ¼ 4.5(±1) 103 cm. Here, Zumax is the energy of the active phonons. The multiphonon relaxation probability was obtained as 60 s1 for both the cases which is very small and cannot account for the very efficient quenching observed for the Eu3þ ion doped materials. The concentration quenching is therefore mainly caused by the nonradiative energy transfer among the Eu3þ ions. This type of energy transfer indicates the energy transfer between to Eu3þ ion. It is possible only when the distance between two Eu3þ ions is such that the interaction between them started to take place. As the concentration of Eu3þ in the host matrix increases, the distance between them decreases. At certain concentration the interaction between two Eu3þ started. This concentration is called as critical concentration and the distance is known as critical distance (Rc). This interaction may occur due to exchange interaction between two Eu3þ ions or due to the multipoleemultipole interaction [31]. The type of interaction mainly depends on the critical Rc between the neighboring Eu3þ ions. The Rc can be estimated by Blasse equation [32],
Rc ¼ 2
3V 4pCo N
1 3
(2)
where, Co is the optimum concentration of the activator ions, N is the number of ions per unit cell and V is the volume of the unit cell. For the YAG and YGG host the value of Rc is greater than 5 Å which indicates only the existence of the multipoleemultipole interaction for concentration quenching of Eu3þ in the YAG and YGG host. There exist different types of multipolar interaction such as dipoleedipole (ded), dipole-quadrupole (deq), quadrupoleequadrupole (qeq) interactions. The type of interaction can be derived according to the theory of Dexter [33]:
I k ¼ s C bC 3
Thus, the value of s can be calculated as 5.46 and 5.85 respectively (very close to the theoretical value 6 for the electric ded interaction), which is the main mechanism for the concentration quenching transition of Eu3þ in the prepared YAG and YGG phosphors.
3.4. Spectroscopic properties and JuddeOfelt theory The site symmetry and luminescence behavior of the Eu3þ ions in the YAG and YGG host can be well understood by determining the JuddeOfelt (J-O) parameters [34,35] Ut (t ¼ 2, 4, 6). The parameter U2 indicates the polarization and asymmetry behavior of the RE ligands. U4,6 parameters generally depend on the long range effects. The radiative behavior of RE ions in YAG/YGG host is generally known by the knowledge of these intensity parameters. Using the method described by Kodaira et al. [36] the J-O parameters can be estimated from the luminescence emission spectra. The comparative investigation of the radiative behavior of Eu3þ ions in different hosts can be possible by the detail estimation of these intensity parameters. As per Som et al. [9], the radiative emission rates were calculated as per the following equation [9,34]:
A02;4 I02;4 hn01 ¼ A01 I01 hn02;4
(4)
where the A0-J indicates radiative emission rates and the I0-J indicates the integrated emission intensities (areas below the luminescence bands) and hn0-J is the energy corresponding to transition 5 D0 / 7FJ (J ¼ 1, 2, 4). The value of the magnetic dipole radiative emission rate A0e1 is generally constant [9] and was taken as z50 s1. Thus the radiative emission rates due to the forced electric dipole transitions can be obtained from equation (4). From J-O parameters these radiative emission rates A0e2,4 can also be calculated as below [9,34]:
A0J
3 D E2 X 64p4 n02;4 e2 1 5 ðJÞ 7 ¼ c U D jU j F J 0 2;4 4pε0 J¼2;4;6 3hc3
(5)
where, c is the Lorentz local field correction factor given as function of the index of refraction n of the host c ¼ n(n2þ2)2/9. The non-zero square reduced matrix elements are taken from the reference as 〈5D0jU(2)j7F2〉2 ¼ 0.0032 and 〈5D0jU(4)j7F4〉2 ¼ 0.0023 [9,34]. Thus, using equations (4) and (5) the values of U2,4 were obtained. Using the concept of J-O parameters, the radiative properties of Eu3þ in
(3)
where, I is the emission intensity per activator ion, C is the activator concentration involved in self concentration quenching, k and b are constants for a particular interaction and s values corresponding to ded, deq and qeq interactions are 6, 8 and 10 respectively. From the slope of Eq. (3), the electric multipolar character (s) can be obtained by the slope (s/3) of the plot log (I/C) vs. log C. It can be seen that the dependence of log(I/C) on log C is linear and the slope is 1.82 and 1.95 respectively for YAG and YGG phosphors (Fig. 8).
Fig. 8. Logarithmic plot of I/C as a function of the activator concentration (C).
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intensity was taken by the area under the curve of the corresponding transition. The decrease in the U4 parameter is also observed. It implies that the efficiency for the 5D0 / 7F2 transition increases over the 5D0 / 7F4 transition. This indicates the enhancement of the red color emission. In the 5D0 / 7F1 emission, the b value and the stimulated emission cross section is higher than the other transitions, indicating that the 5D0 / 7F1 transition is the main emission of the Eu3þ in these hosts. This indicates the red emission in the materials. The calculated radiative lifetime for 5D0 level is also summarized in this Table 2. A luminescence device is characterized by the figure-of-merit (FOM) for gain defined as the product of the lifetime and the emission cross-section as follow: P ¼ st where s is the emission cross section calculated before [38]. All these spectral parameters are summarized in Tables 1 and 2.
different hosts can be calculated from the equation described elsewhere [9]. The radiative property includes the radiative transition probability (Arad), the total radiative transition probability (AT), radiative lifetime trad(jJ), the branching ratio b(jJ) and were calculated corresponding to the emission from an excited level to its lower levels as per the equations below [9,34]:
Arad ðjJ; j0 J0 Þ ¼ AJJ = AT ðjJÞ ¼
X
(6)
AJJ=
(7)
1 AT ðjJÞ
(8)
AððjJ; j0 J0 Þ AT ðjJÞ
(9)
J0
trad ðjJÞ ¼
bðjJÞ ¼
3.5. Color characteristics The color perception corresponding to luminescence of Eu3þ in YAG and YGG samples was also estimated. Color perception is a psychophysical property of the human eye, and this response can mathematically be expressed well in terms of the CIE coordinates given by the International Commission for Illumination. It involves parameters x and y to specify the chromaticity, which covers the properties hue and saturation on a two-dimensional curve, known as chromaticity diagram [1]. The chromaticity coordinates were calculated for different concentration of Eu3þ in YAG and YGG samples as shown in Fig. 9 (a) and (b) respectively and summarized in Table 2.
Generally emission transition can be of different types such as radiative and nonradiative. Radiative transition is the main reason for the light emission. The radiative transition probability for a transition jJ / j0 J0 can be defined as the chance of transition which is radiative and be calculated from the equation as Arad ðjJ; j0 J0 Þ ¼ AJJ = . In emission spectra different emission transitions may present. The totality of all radiative transition probability is expressed as AT. The ratio of each transition to the total radiative transition is known as the branching ratio of that transition. The stimulated emission cross-section (s(lp)) corresponding to any transition can be expressed as [37]:
s lp ðJ/J 0 Þ ¼
l4p 8pcn2 Dl
eff
Arad ðJ/J 0 Þ
7
Table 2 Color characteristics of YAG/YGG:Eu3þ phosphors varying Eu3þ concentration.
(10)
Phosphors
where lp is the peak wavelength and Dleff is its effective linewidth found by dividing the area of the emission band by its maximum height. The JuddeOfelt intensity parameters and the spectral parameters are presented in Table 1. The asymmetric nature of the Eu3þ ligand can be well understood by the U2 parameter whereas, U4,6 depend on the long range effects. The decrease in U2 value from YAG to YGG indicates the increase in the symmetric nature of Eu3þ in this host. This indicates the increase in covalency of the EueO bond. This is supported by the asymmetric ratio parameters. The asymmetric ratio was calculated by the ratio of the intensity of the electric dipole transition 5D0 / 7F2 to magnetic dipole transition 5D0 / 7F1. The
YAG:Eu3þ 1 mol% 3 mol% 5 mol% 7 mol% 9 mol% 11 mol% YGG:Eu3þ 1 mol% 3 mol% 5 mol% 7 mol% 9 mol% 11 mol%
Color coordinate
C.P.
(0.65, (0.67, (0.64, (0.63, (0.62, (0.63,
0.35) 0.34) 0.36) 0.37) 0.37) 0.38)
94.2 98.5 91.4 88.8 85.8 89.2
(0.61, (0.62, (0.61, (0.62, (0.62, (0.62,
0.37) 0.37) 0.38) 0.38) 0.38) 0.38)
82.9 85.8 83.3 86.3 86.3 86.3
Table 1 Comparative radiative parameters and J-O intensity parameters of YAG/YGG:Eu3þ phosphors prepared by different synthesis methods. Host
J-O intensity parameter
U2
U4
YAG 1200
0.36
0.92
YGG 1200
0.34
0.85
YAG
0.39
1
YGG
0.34
0.93
(pm2)
Transitions
A0e2,4 (s1)
A0e1 (s1)
5
e 20.8 25.5 e 19.9 23.7 e 22.7 27.9 e 19.9 26.0
50 e e 50 e e 50 e e 50 e e
At (s1)
b(%)
s (lp) (1020 cm2)
Asymmetry ratio
52 22 26 54 21 25 50 23 27 52 21 27
3.9 1.1 5.9 4.7 1.4 5.4 3.8 1.2 6.1 4.6 1.3 5.9
0.40
(pm2) D0 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5
/ / / / / / / / / / / /
7
F1 F2 7 F4 7 F1 7 F2 7 F4 7 F1 7 F2 7 F4 7 F1 7 F2 7 F4 7
96.3
93.6
100.5
95.9
0.39
0.44
0.39
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A. Yousif et al. / Materials Chemistry and Physics xxx (2015) 1e9
Fig. 9. CIE coordinates for (a) Y3xAl5O12:Eux and (b) Y3xGa5O12:Eux varying x.
The coordinates are reasonably good for different optical applications for red emission. A comparative color coordinate diagram of the as prepared YAG and YGG phosphors with the annealed one is shown in Fig. 10 and the data is summarized in Table 3. From the chromaticity diagram in the inset of Fig. 10, it can be seen that the color coordinates traverses a range from the light red to deep red region on varying the hosts from YGG to YAG. The results show that the tuning of the red emission color is possible by the change of the garnet host material. A deep red emission (0.67, 0.32) was observed for YAG:Eu3þ annealed at 1200 C, which is very much nearer to the commercial available Y2O2S:Eu3þ (0.667, 0.326) red phosphor as per NTSC system [9]. To check the purity of the emitted red color the color purity formula was adopted as [39]:
coordinates of the illuminant point. In this study, (xd, yd) ¼ (0.68, 0.32) and (xi, yi) ¼ (0.3101, 0.3162) (Illuminants C) for the dominant wavelength at 590 nm. The calculation was carried out as presented in [40] and an obtained result was summarized in Tables 2 and 3. A better color purity (98.5%) was observed in the case of the YAG phosphor. It was also observed that with annealing the color coordinate did not vary a significant amount. This indicates the stability of the emitted color with temperature. These results of color and spectral characteristics show that the YAG phosphors with more pure red chromaticity and gain can be used as a better candidate for luminescence application. This indicates that YAG phosphors could be more suitable for UV (InGaN) chips to produce pure red light.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðxs xi Þ2 þ ðys yi Þ2 ColourPurity ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 100% ðxd xi Þ2 þ ðyd yi Þ2
4. Conclusion
(11)
where (xs, ys) are the coordinates of a sample point, (xd, yd) are the coordinates of the dominant wavelength and (xi,yi) are the
The luminescence spectra of Eu3þ doped YAG and YGG powders were discussed. A red-shift of the charge-transfer band was observed, after all the Al3þ ions were substituted with Ga3þ, in the excitation spectrum of the powders. The color of Eu3þ was redder in YAG if compared to the color of Eu3þ in the YGG which was an
Fig. 10. CIE coordinates for Y3xAl5O12:Eux¼0.03 and Y3xGa5O12:Eux¼0.09.
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9
Table 3 Color characteristics of YAG/YGG:Eu3þ phosphors. Phosphors
Annealed ( C)
Tmeasured (ms)
P (1022 cm2 S)
Color coordinate
C.P.
YAG YGG YAG YGG
1200 1200 As-prepared As-prepared
10.4 10.7 9.9 10.4
4.1 4.9 3.7 4.8
(0.67, (0.62, (0.67, (0.62,
98.4 85.8 98.5 86.3
orange-red color. The optimal concentration of Eu3þ to obtain the highest intensity value of 11% in YGG compared to the 3% concentration value of Eu3þ in the YAG host attributed to the difference in the unit cell or the present of other phases in YAG.
[12] [13] [14] [15] [16] [17]
Acknowledgments This work is based on the research supported by the South African Research Chairs (84415) Initiative of the Department of Science and Technology and National Research Foundation of South Africa. The financial support from the University of the Free State is highly recognised. References [1] K. Mishra, S.K. Singh, A.K. Singh, M. Rai, B.K. Gupta, S.B. Rai, Inorg. Chem. 53 (2014) 9561. [2] S. Geller, Z. fur Kristallogr. 125 (1967) 1. [3] D. Geng, G. Li, M. Shang, C. Peng, Y. Zhang, Z.C.J. Lin, Dalton Trans. 41 (2012) 3078. [4] J.B. Gruber, U.V. Valiev, G.W. Burdick, S.A. Rakhimov, M. Pokhrel, D.K. Sadar, J. Luminescence 131 (2011) 1945. [5] Z. Nan-Fei, L. Yong-Xiang, Y. Xiao-Feng, Chin. Phys. Lett. 25 (2008) 703. [6] A. Yousif, V. Kumar, H.A.A.S. Ahmed, S. Som, L.L. Noto, O.M. Ntwaeaborwa, H.C. Swart, ECS J. Solid State Sci. Technol. 3 (11) (2014) R222. [7] C.W. Thiel, H. Cruguel, Y. Sun, G.J. Lapeyre, R.M. Macfarlaneb, R.W. Equall, R.L. Cone, J. Luminescence 94e95 (2001) 1. [8] A. Nakatsuka, A. Yoshiasa, T. Yamanaka, Acta Crystallogr. Sect. B Struct. Sci. 55 (1999) 266. [9] S. Som, A.K. Kunti, Vinod Kumar, Vijay Kumar, S. Dutta, M. Chowdhury, S.K. Sharma, J.J. Terblans, H.C. Swart, J. Appl. Phys. 115 (2014) 193101. [10] H.K. Yang, J.W. Chung, B.K. Moon, B.C. Choi, J.H. Jeong, J. Korean Phys. Soc. 52 (2008) 116. [11] M.V.S. Rezende, C.W.A. Paschoal, Opt. Mater. 46 (2015) 530.
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Comparison and analysis of Eu3þ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor A. Yousif a, b, S. Som a, Vinod Kumar a, H.C. Swart a, * a b
Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA 9300, South Africa Department of Physics, Faculty of Education, University of Khartoum, P.O. Box 321, Postal Code 11115, Omdurman, Sudan
h i g h l i g h t s Eu3þ doped Y3Al5O12 and Y3Ga5O12 were successfully synthesized by combustion method. The luminescence properties of the material for red lighting phosphor were reported. The JuddeOfelt intensity parameters of were determined in detail. The color properties as a function of the Eu3þ concentration were performed. The color coordinates traversed in a range from the light red to deep red region.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2015 Received in revised form 21 September 2015 Accepted 29 September 2015 Available online xxx
This paper reports on the comparative investigation of the spectroscopic properties of the Y3Al5O12 (YAG) and Y3Ga5O12 (YGG) doped with Eu3þ synthesized by the combustion method. A pure phase was identified for the YGG phosphor, but one other phase was observed for the as-prepared and annealed YAG phosphor. A high intense red color emission was observed from Eu3þ doped YAG compared to the orange-red color of Eu3þ in the YGG phosphor. The optimum Eu3þ concentration was found to be higher in the YGG compared to the optimum Eu3þ concentration in the YAG. A spectroscopic investigation on the concentration quenching effect and the covalency of the EueO bond were carried out on the basis of the substitution of Ga3þ and Al3þin the garnet host. JuddeOfelt parameters, different spectral and colorimetric parameters were estimated theoretically from the experimental curves, for both the hosts to determine the better luminescence media. © 2015 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence spectroscopy Powder diffraction Crystal structure Crystal symmetry Luminescence Optical properties
1. Introduction Garnets are useful hosts for various optical applications due to a high thermal stability, hardness, and optical isotropy, good thermal conductivity particularly for lasing and solid state lighting applications [1]. Garnets are basically a combination of Ln2O3 (rare earth oxide) and M2O3 (metal oxide), where Ln ¼ Y and M ¼ Al, Ga as an example. Garnets can be described by the {A}3[B]2(C)3O12 formula, where A (Y3þ) is a dodecahedrally coordinated site with point group D2 (without an inversion center), while B (Al3þ/Ga3þ) and C (Al3þ/Ga3þ) are octahedrally and tetrahedrally coordinated sites with C3i (with an inversion center) and S4 point groups,
* Corresponding author. E-mail address: [email protected] (H.C. Swart).
respectively. Because of similar ionic radii and chemical properties, Y3þ ions can easily be replaced by other lanthanide ions (e.g., Eu3þ) in the crystal lattice, making garnets optically active materials [1,2]. As one of the most frequently used red emitting activators in rareearth ions doped luminescent materials, the trivalent europium (Eu3þ) ions mainly show characteristic emissions resulting from the transitions of 5D0, 1, 2 / 7Fj (j ¼ 4, …, 0) [3]. For decades Eu3þ has been used as an optical probe, providing spectroscopic information regarding the local site symmetry and environment within the host [4]. Therefore, the Eu3þ was selected to investigate the luminescence properties in the Y3Al5O12 (YAG) and Y3Ga5O12 (YGG) in which they have the same crystalline structure [5]. But due to the fact that, Ga ionic radius (0.62 Å) is larger than the Al ionic radius (0.53 Å) change in lattice parameter among the YAG and YGG have been reported by many researchers [6]. Moreover, the YAG have larger band gap (6.5 eV) than the band gab of YGG (5.5 eV) [6,7]. The
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covalency of the GaeO bond is higher than the AleO bond as it has been reported by Nakatsuka et al. [8] which introduce a distortion to the neighboring dodecahedra where the Eu3þ supposes to be situated [6]. All the mentioned differences between the YAG and YGG hosts leaded to the interest to investigated the luminescence properties of Eu3þ in these hosts and motivated by the need of getting environment friendly red phosphor compared with the commercial available Y2O2S:Eu3þ red phosphor [9]. The spectroscopic properties of the Eu3þ doped YAG and YGG hosts have been reported by many researchers for different purposes. Yang et al. [10] reported the enhancement of the photoluminescence (PL) by incorporated the Lithium ions in the YAG:Eu3þ phosphor. Rezende et al. [11] studied the radioluminescence enhancement in Eu3þdoped YAG phosphors by Ga substitution. Lu et al. [12] reported the investigation of synthesis and luminescence properties of microemulsion-derived YAG:Eu3þ Phosphors. While Bi et al. [13] studied the electrospinning preparation and PL properties of YAG:Eu3þ Nanobelts. On the other side, Binnemans et al. [14] investigated the optical absorption spectra of Eu3þ in YGG while Zhu et at [15] reported the luminescence spectra of the YGG:Eu3þ and the energy transfer from Bi3þ to Eu3þ. In all mentioned studies, there was no work has been reported on the JuddeOfelt parameters calculation and the comparison of luminescence properties of the Eu3þ in the YAG and YGG hosts. Several synthesized methods are already reported in literature for the preparation of garnets. The solid-state reaction and combustion method are some of the well known methods for the synthesis of garnets. The solid state reaction require extensive mechanical mixing and lengthy heat treatments at high temperatures to prevent the occurrence of intermediate phases which are the main disadvantages of the solid-state synthesis of different aluminates and gallates materials. The secondary phases such as perovskite YAlO3 or metastable Y2Al4O7 may be formed if YAG is synthesized by a solid-state reaction [16]. Recently, a wet-chemical processing method for oxide systems attracted more and more attention from the scientific community due to the considerable advantages such as good mixing of the starting materials, excellent chemical homogeneity of the final products and lower synthesis temperatures compared with the solid-state reaction. Among the wet-chemical methods, the combustion method, which have been developed and successfully used for the low-temperature production of pure phase YAG, and YGG nano-powders and related systems, is the most popular technique. The combustion method is a synthesis route involving the exothermic reaction of an oxidizer, such as metal nitrates, with an organic fuel, typically urea (CH4N2O). In this paper, the properties of Eu3þ doped YAG and YGG was studied for the as-prepared as well as 1200 C annealed samples. The effect of Eu3þ doping on the structural and luminescent properties of YAG and YGG was also investigated. The JuddeOfelt intensity parameters of Eu3þ for YAG and YGG were also determined in detail. The color properties as a function of the Eu3þ amount were performed.
(C6H8O7$H2O, analytical grade) were used as starting materials in the present study which were obtained from SigmaeAldrich Company. The materials were dissolved in diluted water under stirring and heating to obtain a mixing aqueous homogenous precursor solution. The solution was placed in a furnace preheated at 600 C. After the combustion process was completed, the obtained solid precursors were then ground. The Y3xAl5O12:Eux¼0.03 (YAG:Eu3þ) and Y3xGa5O12:Eux¼0.09 (YGG:Eu3þ) samples were selected and annealed at 1200 C for 3 h and 2 h in air, respectively. The crystalline characteristic of the product was identified by X-ray diffraction (XRD) analysis (Bruker AXS D8 ADVANCE X-ray diffractometer) using a nickel-filtered CuKa target (l ¼ 0.154056 nm). The scanning electron microscopy (SEM) of the powders was performed using a JSM-7800F microscope. The PL measurements were done at room temperature using a Cary Eclipse fluorescence spectrophotometer equipped with a xenon lamp. 3. Result and discussion 3.1. Structural analysis The YAG and YGG garnet crystal structure can be described as a network of AlO6/GaO6 octahedra and AlO4/GaO4 tetrahedra linked by shared oxygen ions at the corners of the polyhedral as presented in Fig. 1. These polyhedra are arranged in chains along the three crystallographic directions and form dodecahedral cavities which are occupied by the Y3þ ions. Thus the YAG/YGG garnet possesses three crystallographically distinct cation sites, i.e., the Al3þ/Ga3þ ions are located both in 24(d) tetrahedral S4 sites and 16(a) octahedral S6 sites with fourfold and six fold coordination, respectively, while the Y3þ ions are located in a 24(c) dodecahedral D2 site with a coordination number of 8. The O2 ions occupy the 96(h) sites with each one being a member of one tetrahedron, one octahedron, and two dodecahedra. Due to ionic size considerations in the YAG/YGG lattice, the rare earth ions are expected to predominantly enter into the distorted dodecahedral sites by replacing the Y3þ ions and to be coordinated to eight O2 ions [17,18]. The schematic representation of the unit cell of YAG and YGG after Ga substitution in the Al position is shown in Fig. 2. The Ga (ionic radius 0.62 Å) substitution in the all Al (ionic radius 0.53 Å) position will lead to change in lattice parameter for the garnet crystal structure from 12.00 to 12.28 Å [6]. As a result, the unit cell of YGG is bigger than the unit cell of YAG. The schematics of the unit cells of YAG and YGG phosphors were
2. Experimental The combustion synthesis has emerged as an important technique for the synthesis and processing of advanced ceramics, catalysts, composites, alloys, inter metallic, and nano-materials. The powder samples of {(Y1xEux)3(Al1yGay)5O12}x¼1 to 11, y¼0 and 1 were prepared by the urea-nitrate combustion synthesis technique. The aluminum nitrate (Al(NO3)3$9H2O, 99.997% pure), yttrium nitrate (Y(NO3)3$6H2O, 99.99% pure), gallium nitrate (Ga(NO3)3$xH2O, 99.999% pure), Europium nitrate pentahydrate (Eu(NO3)3$5H2O, 99.9% pure), and hydrated citric acid
Fig. 1. The arrangement of Y3þ/Eu3þ, O2, Al3þ/Ga3þ described with a polyhedral in the YAG and YGG matrix [17,18].
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3
Fig. 2. The schematic representation of the unit cell of YAG and YGG after Ga substitution in the Al position.
drawn with the help of VESTA software in order to get some idea of the crystal structure [19]. The XRD diffraction patterns of as-prepared and annealed YAG:Eu3þ powders are presented in Fig. 3 (a) while Fig. 3 (b) showing the XRD patterns for as-prepared and annealed YGG:Eu3þ phosphors powder as well. The XRD spectra were recorded to identify the phase composition and crystal structure of the material. The XRD patterns of the as-prepared YAG:Eu3þ samples show the polycrystalline main phase of YAG which is matched with the standard YAG data base(JCPDS 79-1891). Additionally, there are also a few additional low intensities peaks which are attributed to another secondary minority phase of YAlO3 with the standard data base (JCPDS 33-0041). It is well-known that during the combustion synthesis process, the flame temperature can reached up to 1400 ± 100 C [20], from their XRD results it seem that this temperature was not enough to form pure YAG. Huh et al. [21] also reported that the pure YAG is synthesized above a combustion temperature of 1500 C. When YAG is synthesized with a stoichiometric mixture of the raw materials YAG is the last phase that formed after two other phases Y4Al2O9 and YAlO3. A broadening in lower angle sides of the XRD pattern of as-prepared YAG sample was observed. That means; these peaks are not Gaussian in shape. The asymmetry in the peak is attributed to the presence of very small crystallites existing together with the larger nanocrystals [22]. After the YAG:Eu3þ sample was annealed at 1200 C for 3 h, the secondary phase of YAlO3 has been removed. The XRD patterns for the YGG which are presented in Fig. 3 (b), reveal well-defined Bragg reflections of the crystalline powder samples and it is identical to the referenced (JCPDS 071-2151). A shift to the higher angle side is observed after annealing the sample due to the decrease in the lattice parameters of the unit cell. To estimate the effect of Eu3þ concentration on the crystal structure, the unit-cell parameters of different Eu3þ doped YAG and YGG phosphors were determined using the Program Unit Cell software [23]. The variation of lattice parameters was shown in Fig. 4. From this figure it can be seen that the lattice constant increases a very little amount with the increase in Eu3þ concentration. 3.2. Surface morphology Fig. 5 (aec) shows the SEM micrographs of the as-prepared and
Fig. 3. (a) XRD patterns of the Y3xAl5O12:Eux¼0.03 (YAG:Eu3þ) phosphor as prepared and annealed at 1200 C and the standard data for the YAG phase. (b) XRD patterns of Y3xGa5O12:Eux¼0.09 (YGG:Eu3þ) phosphor as prepared and annealed at 1200 C and standard YGG data (JCPDS 071-2151).
Fig. 4. Variation of lattice parameter with Eu3þ concentration for YaG and YGG phosphors.
1200 C annealed YAG:Eu3þ and YGG:Eu3þ phosphor powders. The particle size for both samples varies up to several tens of microns. The SEM image of the as-prepared YAG:Eu3þ phosphor is shown in
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Fig. 5. SEM images of the as-prepared (a) and (b) and (c) and (d) the 1200 C annealed YAG:Eu3þ and YGG:Eu3þ phosphor powder, respectively. The inset of Fig. 5 (c) is shows an enlargement of one part of the surface.
Fig. 5 (a). The surface is smooth without the appearance of grain boundaries with several large pores in localized areas of the particles. The pores are found in materials produced using combustion synthesis owing to the phase formation during the synthesis process with great evolution of a quantity of gas, which contributes to the production of materials with high porosity, good crystallinity, and nanometric particles [24]. The as-prepared YGG:Eu3þ phosphor powder surface is shown in Fig. 5 (b). It shows particles that consist of micro and sup-micro grains with very small pores located at the grain boundaries. It is well known that the pores are formed during the escaping of the gases during the combustion reaction. Fig. 5 (c) shows the annealed YAG:Eu3þ sample, sup-micro particles in the range of 300 to 200 nm have developed on the particle's surface with some localized areas which still shows the smooth particle's surface separated from other sup-micro particles. The inset in Fig. 5 (c), shows an enlargement of one part that shows particles with micro meter sizes. Fig. 5 (d), shows the SEM of the annealed YGG:Eu3þ sample with a mixture of micro and sup-micro particles. The number of pores has decreased in the YGG with respect to the YAG. 3.3. Spectral investigation of Eu3þ in YAG and YGG host and luminescence Fig. 6 (a) exhibits the excitation and emission spectra of the asprepared and 1200 C annealed YAG:Eu3þ phosphor powder. The excitation was monitored for the 5D0 / 7F1 transition at 590 nm. Since, Eu3þ has a 4f6 configuration; it needs to gain one more electron to achieve the half-filled 4f7 configuration, which is relatively stable compared to the partially filled configurations. Thus, when Eu3þ is linked to the O2 ligand, there is a probability of electron transfer from O2 to Eu3þ to form Eu2þeO [25]. Because of the electron transfer, a broad excitation band in the range of 200e300 nm was observed centered at ~227 nm which was
assigned to the Charge transfer (CT) band. The sharp lines in the wavelength range from 295 to 420 nm were attributed to the fef transitions within 4f6 of Eu3þ configuration [26]. No change was observed in the CT band position and shape after annealed at 1200 C except for an improvement in the intensity of the peak. Upon excitation into the CTB of Eu3þ ions, the emission spectra of the as-prepared and the annealed sample are composed of several emission peaks at 591, 609, 630 and 650 nm, which correspond to the Eu3þ transitions, 5D0 / 7FJ (J ¼ 1, 2, 3, 4) respectively. In both hosts the Eu3þ transitions of 5D0 / 7F0 was not observed which agree with reported literature [10,11]. Additionally, in both emission spectra also the magnetic dipole transition 5D0 / 7F1 (MD) was found to be stronger than a hypersensitive electric dipole (ED) transition 5D0 / 7F2. It is well known that the emission intensity of MD transition is independent of the site symmetry of the Eu3þ ions, whereas the emission wavelength and intensity of the ED transition is very sensitive to the site symmetry of Eu3þ ions. According to the transition selection rules J ¼ 0 ± 1 and DS ¼ 0, MD transition (5D0 / 7F1) is allowed, and ED (5D0 / 7F2) transition is forbidden [27]. However, in some cases in which Eu3þ activators occupy sites without inversion symmetry, the parity forbiddance is not strictly maintained and the spectra resulting from the electric dipole transition appear [27]. The Y3þ ion in the cubic phase of the YAG crystal is well known as a dodecahedrally coordinated site with point group D2 (without an inversion center), while the Al3þ are in the octahedral and tetrahedral coordinated sites with C3i (with an inversion center) and S4 point groups, respectively [1]. The YAG doped Eu3þ substitutes for Y3þ, which occupy a site without inversion symmetry. As a result, the luminescent intensity of the samples concentrated mainly on the MD transition rather than the ED transition. Fig. 6 (b) represent the excitation and emission spectra of as-prepared and 1200 C annealed YGG:Eu3þ phosphor. The excitation was also monitored for the 5D0 / 7F1 transition at 590 nm. The similar behavior in the excitation spectra of YGG:Eu3þ
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7
D0- F1
(a)
CTB
D0- F4
7 5
7
D0- F3
5
f-f transitions 6 3+ within 4f of Eu
5
7
200 250 300 350 400
600
650
700
Wavelength (nm)
7
D0- F4
5
7
D0- F3
f-f transitions 6 3+ within 4f of Eu
5
7
7
D0- F2
CTB
b ¼
5
PL intensity (a. u.)
Eu - O
2+
D0- F1
3+
(b)
5
As-prepared o Annealed at 1200 C
200 250 300 350 400
known that CT energy decreased with larger average distance to the surrounding anions [26]. Nakatsuka et al. reported [8] that, the substituting of all Al by Ga in YAG results in an average YeO bond length increase from 2.371 to 2.390 Å, it is attributed to the decreased in the CT energy. The CT energy is strongly related to the covalency of the Eu3þeO2 bond, it is also strongly influenced by the next nearest cation M3þ (M ¼ Y3þ, Al3þ, Ga3þ). This was supported by a previous report of Yousif et al. [6]. Here, a stronger chemical bond between EueO bond and M3þ ions is expected in the YAG compared with YGG. They also studied the luminescence behavior of Bi3þ with YAG and YGG in details and their results show that, the higher covalency in YGG is attributed to the characteristics of Ga3þ ions due to its d10 electron configuration which promotes the formation of sp3-hybrid orbitals with strong covalency. Therefore, the CTB is located at shorter wavelength YAG compared with YGG. The change in the shape and intensity of the CTB were observed after the sample was annealed at 1200 C. The behavior of Eu3þeO2 ligand can also be determined by nephelauxetic ratio (b). The b can be calculated by using the method described in [29] using the PL excitation spectra
600
650
700
Wavelength (nm) Fig. 6. (a) A comparison of excitation and emission bands spectra of as-prepared and annealed Y3xAl5O12: Eux¼0.03 powder. (b) A comparison of excitation and emission bands spectra of as-prepared and annealed Y3xGa5O12:Eux¼0.09 powder.
when the Al3þ site was occupied by a larger Ga3þ ion was observed as before except the change in the CT band as shown in Fig. 6 (b). The band width of the excitation spectrum became broader with a full width at half maxima (FWHM) of 50 nm compared to 38 nm for the YAG:Eu3þ sample. The peak position of the CTB has shifted towards longer wavelengths from ~227 to ~245 nm. It was reported in [25,26], that the spectra of the CT band are related to the different factors namely the covalency of the Eu3þeO2 bond, bond length and the coordination number of Eu3þ. The red shift in the CT band suggests the increase in the covalent character of the EueO bond. Generally, the band position can be confirmed from Jorgensen formula [28]: s ¼ [copt (X) cuncorr (M)] 30 103 cm1 where, s is the energy of the CT band, copt (X) is the optical electronegativity in Pauling's scale and the optical electronegativity of the central metal ion is denoted as cuncorr (M). However, in different crystal field environments the optical electronegativity of the ion can change. From Fig. 6 (a and b), using the CT band position values the difference in electronegativity values (c (O2) c (Eu3þ)) were calculated for both the cases as 1.47 and 1.36 for YAG and YGG phosphors respectively. The decreased in the difference in electronegativity value indicates the decreased in the ionic strength of the EueO bond and hence the covalency increased [25]. It is well
1 X ncomplex n nfreeion
(1)
where ‘n’ is the wave number of an absorption transition of Eu3þ and ‘n’ is the number of observed absorption transitions. The respective values of b in the present host matrix were observed to be increased as 0.994 and 0.991 for the YAG and YGG phosphors respectively. It is well known that the positive value of nephelauxetic ratio (<1) indicates the existence of covalent bonding between the Eu3þ ion and O2 atom. But in the present research work, b value decreased which indicates the increment of the covalency of this bond which supports our previous result. Fig. 7 represents the effect of Eu3þ concentration in the different YAG and YGG hosts. In both cases, the PL intensity first increased upto the optimize concentration and then decreased. This effect is generally known as a concentration quenching effect. The optimum doping concentrations of the Eu3þ in YAG and YGG was determined to be 3% and 11% respectively. This difference can be attributed to the difference in the unit cell or the presence of the other impurity phase in the YAG and YGG as mentioned earlier. The concentration quenching effect involves different
PL intensities of Y3Al5O12:Eu
3+
PL intensities of Y3Ga5O12:Eu
3+
*
PL intensity (a. u.)
2+
D0- F2
3+
Eu - O
5
PL intensity (a. u.)
As-prepared o Annealed at 1200 C
5
*
1
2
3
4
5
6
7
8
9
10
11
3+
Eu concentration (%) Fig. 7. PL intensity of the MD transition (590 nm) as a function of the Eu3þ dopant concentration in the as-prepared YAG and YGG matrix.
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A. Yousif et al. / Materials Chemistry and Physics xxx (2015) 1e9
mechanisms such as defect to Eu3þ energy transfer, multiphonon assisted energy transfer by electron phonon coupling or Eu3þeEu3þ energy transfer [9]. Defect to Eu3þ energy transfer will be possible in that case when substitution of Eu3þ will create new defects due to lattice mismatch. And the energy will loose nonradiatively through those existing defects. In the present case, no lattice mismatching was observed in Eu3þ ions substitution from XRD. Also, in the observed PL spectra no such transition was observed due to defect. Therefore, defect to Eu3þ energy transfer possibility is not the case for the concentration quenching. The energy also can be transferred nonradiatively via multiphonon assisted energy transfer by electron phonon coupling. For this type of energy transfer largely spaced energy levels are required. For Eu3þ in any host as the energy distance between 5D2 and 5D3 of Eu3þ is very large ~2500 cm1. An energy transfer between the 5D2 and 5D3 levels may exist which can quench the PL emission. The nonradiative rate WNR of the multi-phonon process in Eu3þ in the YAG and YGG host were therefore estimated by using the Van Dijk and Schuurmans energy-gap equation [30]: WNR ¼ bel exp½aðDE 2Zumax Þ with the constants bel ¼ 107 s1 and a ¼ 4.5(±1) 103 cm. Here, Zumax is the energy of the active phonons. The multiphonon relaxation probability was obtained as 60 s1 for both the cases which is very small and cannot account for the very efficient quenching observed for the Eu3þ ion doped materials. The concentration quenching is therefore mainly caused by the nonradiative energy transfer among the Eu3þ ions. This type of energy transfer indicates the energy transfer between to Eu3þ ion. It is possible only when the distance between two Eu3þ ions is such that the interaction between them started to take place. As the concentration of Eu3þ in the host matrix increases, the distance between them decreases. At certain concentration the interaction between two Eu3þ started. This concentration is called as critical concentration and the distance is known as critical distance (Rc). This interaction may occur due to exchange interaction between two Eu3þ ions or due to the multipoleemultipole interaction [31]. The type of interaction mainly depends on the critical Rc between the neighboring Eu3þ ions. The Rc can be estimated by Blasse equation [32],
Rc ¼ 2
3V 4pCo N
1 3
(2)
where, Co is the optimum concentration of the activator ions, N is the number of ions per unit cell and V is the volume of the unit cell. For the YAG and YGG host the value of Rc is greater than 5 Å which indicates only the existence of the multipoleemultipole interaction for concentration quenching of Eu3þ in the YAG and YGG host. There exist different types of multipolar interaction such as dipoleedipole (ded), dipole-quadrupole (deq), quadrupoleequadrupole (qeq) interactions. The type of interaction can be derived according to the theory of Dexter [33]:
I k ¼ s C bC 3
Thus, the value of s can be calculated as 5.46 and 5.85 respectively (very close to the theoretical value 6 for the electric ded interaction), which is the main mechanism for the concentration quenching transition of Eu3þ in the prepared YAG and YGG phosphors.
3.4. Spectroscopic properties and JuddeOfelt theory The site symmetry and luminescence behavior of the Eu3þ ions in the YAG and YGG host can be well understood by determining the JuddeOfelt (J-O) parameters [34,35] Ut (t ¼ 2, 4, 6). The parameter U2 indicates the polarization and asymmetry behavior of the RE ligands. U4,6 parameters generally depend on the long range effects. The radiative behavior of RE ions in YAG/YGG host is generally known by the knowledge of these intensity parameters. Using the method described by Kodaira et al. [36] the J-O parameters can be estimated from the luminescence emission spectra. The comparative investigation of the radiative behavior of Eu3þ ions in different hosts can be possible by the detail estimation of these intensity parameters. As per Som et al. [9], the radiative emission rates were calculated as per the following equation [9,34]:
A02;4 I02;4 hn01 ¼ A01 I01 hn02;4
(4)
where the A0-J indicates radiative emission rates and the I0-J indicates the integrated emission intensities (areas below the luminescence bands) and hn0-J is the energy corresponding to transition 5 D0 / 7FJ (J ¼ 1, 2, 4). The value of the magnetic dipole radiative emission rate A0e1 is generally constant [9] and was taken as z50 s1. Thus the radiative emission rates due to the forced electric dipole transitions can be obtained from equation (4). From J-O parameters these radiative emission rates A0e2,4 can also be calculated as below [9,34]:
A0J
3 D E2 X 64p4 n02;4 e2 1 5 ðJÞ 7 ¼ c U D jU j F J 0 2;4 4pε0 J¼2;4;6 3hc3
(5)
where, c is the Lorentz local field correction factor given as function of the index of refraction n of the host c ¼ n(n2þ2)2/9. The non-zero square reduced matrix elements are taken from the reference as 〈5D0jU(2)j7F2〉2 ¼ 0.0032 and 〈5D0jU(4)j7F4〉2 ¼ 0.0023 [9,34]. Thus, using equations (4) and (5) the values of U2,4 were obtained. Using the concept of J-O parameters, the radiative properties of Eu3þ in
(3)
where, I is the emission intensity per activator ion, C is the activator concentration involved in self concentration quenching, k and b are constants for a particular interaction and s values corresponding to ded, deq and qeq interactions are 6, 8 and 10 respectively. From the slope of Eq. (3), the electric multipolar character (s) can be obtained by the slope (s/3) of the plot log (I/C) vs. log C. It can be seen that the dependence of log(I/C) on log C is linear and the slope is 1.82 and 1.95 respectively for YAG and YGG phosphors (Fig. 8).
Fig. 8. Logarithmic plot of I/C as a function of the activator concentration (C).
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A. Yousif et al. / Materials Chemistry and Physics xxx (2015) 1e9
intensity was taken by the area under the curve of the corresponding transition. The decrease in the U4 parameter is also observed. It implies that the efficiency for the 5D0 / 7F2 transition increases over the 5D0 / 7F4 transition. This indicates the enhancement of the red color emission. In the 5D0 / 7F1 emission, the b value and the stimulated emission cross section is higher than the other transitions, indicating that the 5D0 / 7F1 transition is the main emission of the Eu3þ in these hosts. This indicates the red emission in the materials. The calculated radiative lifetime for 5D0 level is also summarized in this Table 2. A luminescence device is characterized by the figure-of-merit (FOM) for gain defined as the product of the lifetime and the emission cross-section as follow: P ¼ st where s is the emission cross section calculated before [38]. All these spectral parameters are summarized in Tables 1 and 2.
different hosts can be calculated from the equation described elsewhere [9]. The radiative property includes the radiative transition probability (Arad), the total radiative transition probability (AT), radiative lifetime trad(jJ), the branching ratio b(jJ) and were calculated corresponding to the emission from an excited level to its lower levels as per the equations below [9,34]:
Arad ðjJ; j0 J0 Þ ¼ AJJ = AT ðjJÞ ¼
X
(6)
AJJ=
(7)
1 AT ðjJÞ
(8)
AððjJ; j0 J0 Þ AT ðjJÞ
(9)
J0
trad ðjJÞ ¼
bðjJÞ ¼
3.5. Color characteristics The color perception corresponding to luminescence of Eu3þ in YAG and YGG samples was also estimated. Color perception is a psychophysical property of the human eye, and this response can mathematically be expressed well in terms of the CIE coordinates given by the International Commission for Illumination. It involves parameters x and y to specify the chromaticity, which covers the properties hue and saturation on a two-dimensional curve, known as chromaticity diagram [1]. The chromaticity coordinates were calculated for different concentration of Eu3þ in YAG and YGG samples as shown in Fig. 9 (a) and (b) respectively and summarized in Table 2.
Generally emission transition can be of different types such as radiative and nonradiative. Radiative transition is the main reason for the light emission. The radiative transition probability for a transition jJ / j0 J0 can be defined as the chance of transition which is radiative and be calculated from the equation as Arad ðjJ; j0 J0 Þ ¼ AJJ = . In emission spectra different emission transitions may present. The totality of all radiative transition probability is expressed as AT. The ratio of each transition to the total radiative transition is known as the branching ratio of that transition. The stimulated emission cross-section (s(lp)) corresponding to any transition can be expressed as [37]:
s lp ðJ/J 0 Þ ¼
l4p 8pcn2 Dl
eff
Arad ðJ/J 0 Þ
7
Table 2 Color characteristics of YAG/YGG:Eu3þ phosphors varying Eu3þ concentration.
(10)
Phosphors
where lp is the peak wavelength and Dleff is its effective linewidth found by dividing the area of the emission band by its maximum height. The JuddeOfelt intensity parameters and the spectral parameters are presented in Table 1. The asymmetric nature of the Eu3þ ligand can be well understood by the U2 parameter whereas, U4,6 depend on the long range effects. The decrease in U2 value from YAG to YGG indicates the increase in the symmetric nature of Eu3þ in this host. This indicates the increase in covalency of the EueO bond. This is supported by the asymmetric ratio parameters. The asymmetric ratio was calculated by the ratio of the intensity of the electric dipole transition 5D0 / 7F2 to magnetic dipole transition 5D0 / 7F1. The
YAG:Eu3þ 1 mol% 3 mol% 5 mol% 7 mol% 9 mol% 11 mol% YGG:Eu3þ 1 mol% 3 mol% 5 mol% 7 mol% 9 mol% 11 mol%
Color coordinate
C.P.
(0.65, (0.67, (0.64, (0.63, (0.62, (0.63,
0.35) 0.34) 0.36) 0.37) 0.37) 0.38)
94.2 98.5 91.4 88.8 85.8 89.2
(0.61, (0.62, (0.61, (0.62, (0.62, (0.62,
0.37) 0.37) 0.38) 0.38) 0.38) 0.38)
82.9 85.8 83.3 86.3 86.3 86.3
Table 1 Comparative radiative parameters and J-O intensity parameters of YAG/YGG:Eu3þ phosphors prepared by different synthesis methods. Host
J-O intensity parameter
U2
U4
YAG 1200
0.36
0.92
YGG 1200
0.34
0.85
YAG
0.39
1
YGG
0.34
0.93
(pm2)
Transitions
A0e2,4 (s1)
A0e1 (s1)
5
e 20.8 25.5 e 19.9 23.7 e 22.7 27.9 e 19.9 26.0
50 e e 50 e e 50 e e 50 e e
At (s1)
b(%)
s (lp) (1020 cm2)
Asymmetry ratio
52 22 26 54 21 25 50 23 27 52 21 27
3.9 1.1 5.9 4.7 1.4 5.4 3.8 1.2 6.1 4.6 1.3 5.9
0.40
(pm2) D0 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5 D0 5
/ / / / / / / / / / / /
7
F1 F2 7 F4 7 F1 7 F2 7 F4 7 F1 7 F2 7 F4 7 F1 7 F2 7 F4 7
96.3
93.6
100.5
95.9
0.39
0.44
0.39
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A. Yousif et al. / Materials Chemistry and Physics xxx (2015) 1e9
Fig. 9. CIE coordinates for (a) Y3xAl5O12:Eux and (b) Y3xGa5O12:Eux varying x.
The coordinates are reasonably good for different optical applications for red emission. A comparative color coordinate diagram of the as prepared YAG and YGG phosphors with the annealed one is shown in Fig. 10 and the data is summarized in Table 3. From the chromaticity diagram in the inset of Fig. 10, it can be seen that the color coordinates traverses a range from the light red to deep red region on varying the hosts from YGG to YAG. The results show that the tuning of the red emission color is possible by the change of the garnet host material. A deep red emission (0.67, 0.32) was observed for YAG:Eu3þ annealed at 1200 C, which is very much nearer to the commercial available Y2O2S:Eu3þ (0.667, 0.326) red phosphor as per NTSC system [9]. To check the purity of the emitted red color the color purity formula was adopted as [39]:
coordinates of the illuminant point. In this study, (xd, yd) ¼ (0.68, 0.32) and (xi, yi) ¼ (0.3101, 0.3162) (Illuminants C) for the dominant wavelength at 590 nm. The calculation was carried out as presented in [40] and an obtained result was summarized in Tables 2 and 3. A better color purity (98.5%) was observed in the case of the YAG phosphor. It was also observed that with annealing the color coordinate did not vary a significant amount. This indicates the stability of the emitted color with temperature. These results of color and spectral characteristics show that the YAG phosphors with more pure red chromaticity and gain can be used as a better candidate for luminescence application. This indicates that YAG phosphors could be more suitable for UV (InGaN) chips to produce pure red light.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðxs xi Þ2 þ ðys yi Þ2 ColourPurity ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 100% ðxd xi Þ2 þ ðyd yi Þ2
4. Conclusion
(11)
where (xs, ys) are the coordinates of a sample point, (xd, yd) are the coordinates of the dominant wavelength and (xi,yi) are the
The luminescence spectra of Eu3þ doped YAG and YGG powders were discussed. A red-shift of the charge-transfer band was observed, after all the Al3þ ions were substituted with Ga3þ, in the excitation spectrum of the powders. The color of Eu3þ was redder in YAG if compared to the color of Eu3þ in the YGG which was an
Fig. 10. CIE coordinates for Y3xAl5O12:Eux¼0.03 and Y3xGa5O12:Eux¼0.09.
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9
Table 3 Color characteristics of YAG/YGG:Eu3þ phosphors. Phosphors
Annealed ( C)
Tmeasured (ms)
P (1022 cm2 S)
Color coordinate
C.P.
YAG YGG YAG YGG
1200 1200 As-prepared As-prepared
10.4 10.7 9.9 10.4
4.1 4.9 3.7 4.8
(0.67, (0.62, (0.67, (0.62,
98.4 85.8 98.5 86.3
orange-red color. The optimal concentration of Eu3þ to obtain the highest intensity value of 11% in YGG compared to the 3% concentration value of Eu3þ in the YAG host attributed to the difference in the unit cell or the present of other phases in YAG.
[12] [13] [14] [15] [16] [17]
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Please cite this article in press as: A. Yousif, et al., Comparison and analysis of Eu3þ luminescence in Y3Al5O12 and Y3Ga5O12 hosts material for red lighting phosphor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.042