- Email: [email protected]
Ca2+-substitution effects in Ba9-xCaxAl2Si6O24:Ce3+, Na+ phosphors
Optical Materials 99 (2020) 109548
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Ca2þ-substitution effects in Ba9-xCaxAl2Si6O24:Ce3þ, Naþ phosphors Sungjun Yang, Sangmoon Park * Division of Energy and Chemical Engineering, Major in Energy & Applied Chemistry, Silla University, Busan, 46958, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: X-ray diffraction Cell parameters Phosphors Quantum efficiency
The Ca2þ ion was substituted for a Ba2þ ion in Ba9Al2Si6O24 host lattice using a solid-state reaction. The solidsolubility of Ba9-xCaxAl2Si6O24 compounds was characterized using X-ray powder diffraction (XRD) for x ¼ 0 to x ¼ 9 according to the Hume-Rothery rule. The obtained Ba9-xCaxAl2Si6O24 hosts between x ¼ 2 and x ¼ 5 were examined in order to index the peak positions as a single-phase orthosilicate, which has a trigonal unit-cell. The structure of Ba6Ca3Al2Si6O24 was characterized using synchrotron XRD. The photoluminescence excitation, emission spectra, and scanning electron microscopy (SEM) data of intense blue Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors were monitored. Furthermore, a comparison of the quantum efficiency of the obtained phosphors and sodium salicylate was performed.
1. Introduction Y3Al5O12 (YAG) is a popular optical host material owing to its excellent chemical stability and optical isometric properties [1–6]. When a Nd3þ ion is doped in the YAG lattice, YAG:Nd can act as an active laser material, which emits light with a wavelength of 1064 nm [3,4]. Furthermore, when a Ce3þ ion is doped in the YAG structure, YAG: Ce can act as a predominant luminescent material, which emits intense yellow colors in the emission spectra. This yellow phosphor has been widely used in blue-emitting InGaN light-emitting diode chips, as an essential white light, for various applications [5,6]. Yttrium aluminum garnet (Y3Al5O12) consists of Y3Al(I)2Al(II)3O12, where Y3þ, Al(I)3þ, and Al(II)3þ cations occupy 12-, 8-, and 4-coordinated sites in the garnet structure, respectively [2,6]. YAG, which has a cubic crystal system with Ia3d symmetry, belongs to the class of orthosilicate structures. A unique feature of orthosilicate compounds is that the tetrahedral Si4þ poly hedron is isolated in the structure and does not share O atoms [7,8]. A garnet compound, as an orthosilicate structure, can be represented as the perfect phase of Ca3Sc2Si3O12, which consists of dodecahedral Ca2þ, octahedral Sc3þ, and tetrahedral Si4þ sites [9,10]. A good orthosilicate luminescent host, (Ba3Ba1.5ScSi3O12)2, was intensively studied in prior research [11,12]. The orthosilicate phase is composed of 9-, 10-, and 12-coordinated Ba2þ, 8-coordinated Sc3þ, and 4-coordinated Si4þ sites. The cubic phase of the perfect garnet Ca3Sc2Si3O12 was transformed into a trigonal orthosilicate structure with R3H due to the out-range of the cation radius after substituting Ca2þ ions with Ba2þ ions. When the larger Y3þ ions were replaced with smaller Sc3þ ions in the Ba9Sc2Si6O24
host lattice, the trigonal crystal system and the three different cation sites were retained [13–17]. Subsequently, the phosphors of Ce3þ or Eu2þ-doped in Ba6Ca3YAlSi6O24 host structure via an energy-transfer mechanism were studied [14,15]. In this study, Ba9-xCaxAl2Si6O24 (for x ¼ 0 to x ¼ 9) hosts and Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (for x ¼ 2 to x ¼ 5) phosphors were prepared at high temperature. The orthosilicate Ba6Ca3Al2Si6O24 host structure was characterized using synchrotron X-ray diffraction (XRD). The luminescence property and particle morphology of Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (for x ¼ 2 to x ¼ 5) phosphors are studied. The relative quantum efficiencies of the phos phors were compared with the one of sodium salicylate based on the integrated emission. 2. Experimental Samples of Ba9-xCaxAl2Si6O24 (x ¼ 0–9) and Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 (x ¼ 2–5) were prepared by mixing the appropriate stoichiometric amounts of BaCO3 (Alfa 99.8%), CaCO3 (Alfa 99.5%), Al2O3 (Alfa 99.95%), SiO2 (Alfa 99.5%), CeO2 (Alfa 99.9%), and Na2CO3 (Alfa 99.5%) with an agate mortar-pestle and subsequently heating at � 950 and 1150 � C for 6 h with a heating rate of 5 C/min, respectively, under the atmosphere using 8%H2/92%Ar. Phase identification was exploited using a Shimadzu XRD-6000 powder diffractometer (CuKααradiation). High-resolution synchrotron X-ray powder diffraction data were collected at PLS-II 6D UNIST-PAL beamline of Pohang Accelerator Laboratory (PAL) [18]. The structure of Ba6Ca3Al2Si6O24 with unit-cell parameters was determined by using the Rietveld
* Corresponding author. E-mail address: [email protected] (S. Park). https://doi.org/10.1016/j.optmat.2019.109548 Received 17 August 2019; Received in revised form 23 October 2019; Accepted 15 November 2019 Available online 9 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
S. Yang and S. Park
Optical Materials 99 (2020) 109548
refinement program Rietica. UV spectroscopy to measure the photo luminescence excitation and emission spectra of the blue luminescent materials was done using spectrofluoro-meters (Sinco Fluoromate FS-2). Sodium salicylate (Alfa 99%) was used as a standard phosphor to calculate the relative quantum yield of the obtained phosphors. The emission spectra of the samples were relatively measured with the standard phosphor; furthermore, the spectroscopy measurement system (Xenon lamp) was warmed up for 30 min before using it at room tem perature. The scanning electron microscopy (SEM, Hitachi, TM3030plus) was performed for the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (for x ¼ 2 to x ¼ 5) phosphor.
DAB ¼
rA
rB rB
(1)
� 100%
where DAB is the radius % difference and rA and rB are the ionic radii of A and B metal ions, respectively. When Al3þ ions were substituted with Y3þ ions in the Ba9Y2Si6O24 host lattice, an approximately 50% differ ence in the ionic radius between Y3þ (r ¼ 0.9 Å for 6 CN) and Al3þ (r ¼ 0.535 Å for 6 CN) can be calculated. Therefore, the Ba2þ ions were sequentially replaced by Ca2þ ions to attune the size factor for the Al3þ substitutions, which show a 20% difference in the ionic radius between Ba2þ (r ¼ 1.47 Å for 9 CN) and Ca2þ (r ¼ 1.18 Å for 9 CN). As a result, the single phase of Ba9-xCaxAl2Si6O24, based on an AlO6 octahedral poly hedron from the Ba9Y2Si6O24 structure, could be successfully prepared when two Ca2þ ions (x ¼ 2) were substituted with Ba2þ ions. The limited solid-solution solubility of the Ba9-xCaxAl2Si6O24 compounds are pre sented (for x ¼ 2 to x ¼ 5) in Fig. 1 (i)–(l) and Table 1. The multi-phased Ca2SiO4, CaSiO3, and Ca2Al2SiO7 occurred at x ¼ 6 to x ¼ 9 in the Ba9-
3. Results and discussion Fig. 1 (a)–(f) show XRD patterns for the calculated Ba9Sc2Si6O24 (Inorganic Crystal Structure Database (ICSD) 75175), Ba2SiO4 (ICSD 6246), Ca2SiO4 (ICSD 81097), Ca2SiO4 (ICSD 39006), Ca2Al2SiO7 (ICSD 27427), and CaSiO3 (ICSD 20571), respectively. The XRD patterns of the Ba9-xCaxAl2Si6O24 (for x ¼ 2 to x ¼ 5) compounds in Fig. 1 (i)–(l) depict the single-phase host lattices, without any obvious impurities, indexed in a trigonal unit cell. Furthermore, the XRD patterns of Ba9-xCax Al2Si6O24 (x ¼ 0, 1, 6, 7, 8, 9) formulas appear in multi-phases as shown in Fig. 1 (g)–(h) and (m)-(p). When Y3þ (r ¼ 0.9 Å for 6 Coordination Number (CN)) ions were replaced by Al3þ (r ¼ 0.535 Å for 6 CN) ions in Ba9Y2Si6O24, the solid solubility could be determined by appropriate size-factors between the (Ba2þ, Y3þ) and (Ca2þ, Al3þ) ions in order to minimize the lattice strain [19,20]. When the Al3þ ions fully occupied the Y3þ site of the Ba9Y2Si6O24 host lattice, the impurity phase of Ba2SiO4 was observed as shown in Fig. 1 (g). Furthermore, when a smaller Ca2þ ion was substituted with Ba2þ ion in the Ba9Al2Si6O24 composition to reduce the lattice tension, the impurity phase of Ba2SiO4, as well as (Ba, Ca)9Al2Si6O24, were monitored as shown in Fig. 1 (h). The unlimited solid solubility can be satisfied by the size factor, crystal structure, electronic valence, and electronegativity according to the Hume-Rothery rule [19,20]. If the metal cations of compounds have equivalent valence and similar electronegativity in the same crystal system, the ionic size of the cations can be a crucial factor. If the ionic radius is less than a 15% difference, the unlimited solid solution can possibly be achieved. The radius percent difference is calculated by the following equation:
Table 1 The different phases according to the limited solid-solution solubility of the Ba9xCaxAl2Si6O24 compounds (for x ¼ 0 to x ¼ 9). x
Fomula
x¼ 9 x¼ 8 x¼ 7 x¼ 6 x¼ 5 x¼ 4 x¼ 3 x¼ 2 x¼ 1 x¼ 0
Ca9Al2Si6O24
→
Phases
Ba1Ca8Al2Si6O24
→
Ca2SiO4 (ICSD 39006) þ CaSiO3 (ICSD 20571) þ Ca2Al2SiO7 (ICSD 27427) Ca2SiO4 (ICSD 81097) þ Ca2Al2SiO7 (ICSD 27427)
Ba2Ca7Al2Si6O24
→
Ca2SiO4 (ICSD 81097)
Ba3Ca6Al2Si6O24
→
(Ba(Ca))9Al2Si6O24 þ Ca2SiO4 (ICSD 81097)
Ba4Ca5Al2Si6O24
→
Ba4Ca5Al2Si6O24
Ba5Ca4Al2Si6O24
→
Ba5Ca4Al2Si6O24
Ba6Ca3Al2Si6O24
→
Ba6Ca3Al2Si6O24
Ba7Ca2Al2Si6O24
→
Ba7Ca2Al2Si6O24
Ba8Ca1Al2Si6O24
→
Ba2SiO4 (ICSD 6246) þ (Ba(Ca))9Al2Si6O24
Ba9Al2Si6O24
→
Ba2SiO4 (ICSD 6246)
Fig. 1. XRD patterns for the calculated (a) Ba9Sc2Si6O24, (b) Ba2SiO4, (c) Ca2SiO4 (orthorhombic), (d) Ca2SiO4 (monoclinic), (e) Ca2Al2SiO7, and (f) CaSiO3 and the observed (g)–(p) Ba9-xCaxAl2Si6O24 (x ¼ 0 to x ¼ 9) structures. 2
S. Yang and S. Park
Optical Materials 99 (2020) 109548
xCaxAl2Si6O24
composition as shown in Fig. 1 (m)–(p). A gradual shift in the positions of the various Bragg reflections to higher angles was observed because the smaller Ca2þ ions (r ¼ 1.18 Å for 9 CN) could occupy the Ba2þ sites (r ¼ 1.47 Å for 9 CN) in the Ba9-xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) structure as shown in Fig. 1 (i) – (m). Fig. 2 and Table 2 show the cell parameters, including cell volumes of the Ba9-xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) host lattice. As depicted by the XRD patterns of the com pounds, the a and c lattice parameters and cell volumes decrease with the increase in Ca2þ substitution. The decrease in the c axis was doubly reduced in comparison to the a axis from Ba7Ca2Al2Si6O24 (a ¼ 9.9893 (6) Å, c ¼ 22.179(2) Å) to Ba4Ca5Al2Si6O24 (a ¼ 9.9382(2) Å, c ¼ 21.964 (2) Å). There are two Ba2þ and one Ca2þ cation sites in the Ba6Ca3Al2 Si6O24 structure, which has a trigonal unit-cell as shown in Fig. 3 (a). The smaller Ca2þ cation prefers to occupy the low-coordinated site. Thus, Ba(1), Ca(2), and Ba(3) can be located at 12, 9, and 11-coordi nated O atoms in the Ba6Ca3Al2Si6O24 orthosilicate host lattices. Fig. 3 (b) shows the fit obtained by the structure refinement from the syn chrotron XRD data, when Ca2þ ions fully occupy the 9-coordinated cation site in the Ba6Ca3Al2Si6O24 host lattice at room temperature. Tables 3 and 4 summarize the Rietveld refinement data of the Ba6Ca3Al2Si6O24 structure. The a and c cell parameters with the unit-cell volume of Ba6Ca3Al2Si6O24 are a ¼ 9.9659(2) Å and c ¼ 22.0199(8) Å. The lattice parameters for the complete substitution of Al3þ ions in the Ba6Ca3Al2Si6O24 host lattice are decreased in comparison to those for the partial substitution of Al3þ ions in the Ba6Ca3YAlSi6O24 host (a ¼ 9.9848(2) Å and c ¼ 22.1216(8) Å), and this observation has been previously reported [18]. Ba(1), Ca(2), and Ba(3) occupy Wyckoff sites 3a, 6c, and 18f, respectively. Both Ba6Ca3Al2Si6O24 and Ba6Ca3YAl Si6O24 host lattices have three different metal cation sites, correspond ing to Ba(1), Ca(2), and Ba(3), located at 12, 9, and 11-coordinated O atoms with Al–O6 (or (Y, Al)–O6) octahedral and Si–O4 tetrahedral linkages [18]. The (Y, Al)–O6 octahedral polyhedron is composed of four different Y–O (2.92329(6) Å and 1.92082(3) Å) and Al–O (2.42957 (4) Å and 2.41183(5) Å) bond distances in Ba6Ca3YAlSi6O24, which were significantly distorted through the (Ba(3)O11)-(SiO4)-(Y,AlO6)-( SiO4)-(Ba(3)O11) linkages as shown in Fig. 3 (c). In contrast, the Al–O6 octahedra in the Ba6Ca3Al2Si6O24 structure consist of two interatomic Al–O bond distances (2.44256(3) Å and 2.21315(3) Å) as shown in Fig. 3 (c) and Table 5. When the luminescent centers of the Ce3þ ions are doped in both Ba6Ca3YAlSi6O24 and Ba6Ca3Al2Si6O24 structures, the Ce3þ ions can occupy the Ba2þ and Ca2þ sites, instead of the Y3þ and Al3þ sites due to the distorted and insufficient space for the Ce3þ ion. Although the Ba2þ and Ca2þ sites can be replaced with Ce3þ in the Ba6-2pCa3CepNapYAlSi6O24 and Ba6Ca3-2pCepNapYAlSi6O24 lattices, the substitution of Ce3þ ions was selectively increased in the Ba2þ site or both Ba2þ and Ca2þ sites. As a result, the formulae were finally derived
Table 2 The unit-cell parameters of the Ba9-xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) structures. (a) x ¼ 2 (b) x ¼ 3 (c) x ¼ 4 (d) x ¼ 5
a (Å)
c (Å)
V (Å3)
9.9893(6) 9.9528(5) 9.9410(4) 9.9382(2)
22.179(2) 22.050(1) 21.978(1) 21.964(2)
1916.6(2) 1891.6(2) 1880.9(2) 1878.7(2)
Fig. 3. (a) Synchrotron XRD pattern of Ba6Ca3Al2Si6O24 structure and calcu lated XRD patterns of BaSiO3 (ICSD 6245), (b) the structure of Ba6Ca3Al2Si6O24 host lattice, and (c) the top views of the Ba6Ca3YAlSi6O24 and Ba6Ca3Al2 Si6O24 structures.
Fig. 2. The cell parameters and cell volumes of the Ba9-xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) structure. 3
S. Yang and S. Park
Optical Materials 99 (2020) 109548
Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors is shifted from 372 to 433 nm when the excitation peak was used at maximum. The Ce3þ activator can occupy three independent cation sites in the (Ba, Ca)9Al2Si6O24 host structure. In Fig. 4(b), two excitation peaks occur centered approximately 287 and 323 nm in the Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 (x ¼ 2) phosphor. In contrast, three main excitation peaks centered on 287, 323, and 367 nm with the increase of Ca2þ contents in the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 3 to x ¼ 5) phosphors are noticeably observed. When the smaller Ca2þ ions, particularly in the Ba6Ca3Al2Si6O24 host lattice, are fully located in the 9-coordinated cation site, the Ce3þ ions can be substituted with Ba2þ ion sites in the Ba5.8Ce0.1Na0.1Ca3Al2Si6O24 phosphors. It is known that the Ce3þ ion is located in the 11-coordinated Ba(3) site because the Ba(3) ion is effec tively linked by Al–O6 octahedra and Si–O4 tetrahedra in the host structure, whereas the Ba(1) ion occupies the interlayer gap of Al–O6 and Si–O4 [18]. The maximum intensity peaks of the blue-emission bands, centered from 421 to 434 nm for Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5), were monitored with the increase of Ca2þ ions as shown in Fig. 4(b). When the Ca2þ ions were substituted in the Ba8.8-xCax Ce0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors, the crystal field splitting of the 5d level of Ce3þ, in the case of calcium rich phosphors, shifts the emission band in a lower energy region with a gradual increase of the blue-emission intensity. The intense blue emission of the sodium salic ylate was used as a standard to estimate the quantum efficiency (QE) compared with the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors. The Naþ ion was used as a charge compensator as well as a flux in the phosphors, which achieved about 14% emission enhancement compared to the case with no Naþ ions in the phosphors. A relative QE of the obtained phosphors was calculated by following equation [22–24].
Table 3 Rietveld refinement and crystal data for Ba6Ca3Al2Si6O24. Formula
Ba6Ca3Al2Si6O24
Radiation type λ (Å) synchrotron 2θ range (deg), Temperature (K) Crystal system Space group
0.66 6–42 295 trigonal R3
Lattice parameter (Å)
a ¼ 9.9659 (2) c ¼ 22.0199 (8) V ¼ 1893.98 (8) 9.76 14.04 0.79
Volume (Å3) Rp Rwp
χ2
Table 4 Refined atomic coordinated coordinates (x, y, z), Wyckoff positions, isotropic temperature factors (Biso), and occupancies (SOF) of Ba6Ca3Al2Si6O24. Atom
Wyckoff position
x
y
z
Biso
SOF
Ba1 Ca2 Ba3 Al Si O1 O2 O3 O4
3a 6c 18f 6c 18f 18f 18f 18f 18f
0.0000 0.3333 0.0024 0.0000 0.3379 0.3575 0.4884 0.0102 0.1729
0.0000 0.6670 0.6683 0.0000 0.0017 0.1009 0.1425 0.1793 0.5032
0.0000 0.0007 0.1099 0.1722 0.0675 0.0165 0.0883 0.1164 0.1135
22.77 1.25 1.45 7.80 3.21 7.60 5.27 6.89 4.08
1 1 1 1 1 1 1 1 1
Table 5 Selected interatomic distances for Ba6Ca3Al2Si6O24.
Relative QE ¼ Standard QE x
Atom
distance (Å)
atom
distance (Å)
Ba1–O1 (x6) Ba1–O3 (x6) Ca2–O1 Ca2–O1 Ca2–O1 Ca2–O2 Ca2–O2 Ca2–O2 Ca2–O4 Ca2–O4 Ca2–O4
3.20346(5) 3.15446(7) 2.80307(5) 2.80117(5) 2.80705(5) 2.68878(5) 2.68868(5) 2.69232(5) 2.96347(7) 2.96368(7) 2.96072(6)
Ba3–O1 Ba3–O1 Ba3–O2 Ba3–O2 Ba3–O2 Ba3–O3 Ba3–O3 Ba3–O3 Ba3–O4 Ba3–O4 Ba3–O4
2.92049(9) 3.21209(4) 2.82135(5) 3.04880(5) 3.32817(9) 2.81235(5) 2.96450(5) 2.79718(7) 2.89154(5) 2.83115(5) 2.95104(6)
Al–O2 (x3) Al–O3 (x3)
2.44256(3) 2.21315(3)
Si–O1 Si–O2 Si–O3 Si–O4
1.44365(3) 1.52471(2) 1.86449(2) 1.98206(2)
ze2 r4 ; 6R5
(3)
Sodium salicylate was employed as a standard phosphor and its ab solute QE of 58% was used. Approximately 10% error can be estimated because the amount of the reflected light from the obtained sample and the sodium salicylate was neglected in the equation [22–24]. Based on the integrated emission intensity at a 365 nm excitation, the relative QEs were calculated as 1, 17, 42, and 77% at x ¼ 2 to x ¼ 5 in Ba8.8-xCax Ce0.1Na0.1Al2Si6O24 phosphors, respectively. Furthermore, the relative QE of the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 phosphor (x ¼ 5) was consid erably enhanced to 105% from 77% when the large particles of the phosphor were retained by a 25 μm sieve. Fig. 4 (c) shows the SEMs of the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors. When the Ca2þ ion was increased from x ¼ 2 to x ¼ 5 in the Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 phosphors, the particle size clearly increased by growth of the grain boundary with the enhancement of blue emission lights. The precursor of calcium carbonate critically affects the higher crystallinity of the annealed phosphors due to their lower melting temperature, which leads to a decrease in the eutectic point, compared to that of the barium carbonate.
as Ba6-2pCa3CepNapYAlSi6O24 and (Ba,Ca)9-2pCepNapYAlSi6O24 in a prior study [18]. In this study, when the Ce3þ and Naþ ions were substituted by Ba2þ sites, Ca2þ ions were gradually added in the Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors (S1). Fig. 4 (a) shows that the PL spectra of Ba8.8-xCaxCe0.1Na0.1Si6O24 (x ¼ 2 to x ¼ 5) phosphors exhibit blue emissions, which is attributed to the d-f Ce3þ transition. When the smaller Ca2þ ions were substituted with the Ba2þ ion in the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (for x ¼ 2 to x ¼ 5) host lattice, the bond length of Ce–O would decrease and finally the crystal splitting was ex pected to increase, which resulted in the red shift shown in Fig. 4(a). The crystal field splitting (Dq) was calculated using the following formula: Dq ¼
integrated emission of samples integrated emission of standard
4. Conclusions Ba9-xCaxAl2Si6O24 (x ¼ 0 to x ¼ 9) hosts were successfully prepared using a solid-state reaction at 1100 � C. The single phase of the Ba9xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) host was characterized as the trigonal unit cell (R3H). The Ba6Ca3Al2Si6O24 structure was refined with the lattice parameters of a ¼ 9.9659(2) Å and c ¼ 22.0199(8) Å using synchrotron XRD. The efficient blue emission, quantum efficiency, and morphology of the Ba8.8-x CaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors were achieved. The maximum luminescent intensity of Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 (x ¼ 5) phosphors, approximately 105%, was achieved with particles larger than 25 μm.
(2)
where Dq is the magnitude of the 5d energy level separation, R is the bond distance between the central ion and its ligands, z is the anionic charge, e is the charge of the electron, and r is the radius of the 5d wave function [20,21]. The center of the emission spectra in 4
S. Yang and S. Park
Optical Materials 99 (2020) 109548
Fig. 4. (a) The normalized photoluminescence emission spectra, (b) the excitation and emission spectra, and (c) the SEMs of Ba8.8-xCaxCe0.1Na0.1Si6O24 (x ¼ 2 to ¼ 5) phosphors.
5
S. Yang and S. Park
Optical Materials 99 (2020) 109548
Declaration of competing interest
[6] L. Havl� ak, J. B� arta, M. Buryi, V. Jarý, E. Mih� okov� a, V. Laguta, P. Boh� a�cek, M. Nikl, Eu2þ stabilization in YAG structure: optical and electron paramagnetic resonance study, J. Phys. Chem. C 120 (2016) 21751–21761. [7] L.H. Wang, L.F. Schneemeyer, R.J. Cava, T. Siegrist, A new barium scandium silicate: Ba9Sc2(SiO4)6, J. Solid State Chem. 113 (1994) 211–214. [8] Y. Kim, S. Park, Preparation and luminescent properties of Eu-substituted barium–yttrium orthosilicate phosphors, Opt. Mater. 36 (2013) 458–462. [9] Y. Shimomura, T. Honma, M. Shigeiwa, T. Akai, K. Okamoto, N. Kijima, Photoluminescence and crystal structure of green-emitting Ca3Sc2Si3O12 : Ce3 þ phosphor for white light emitting diodes, J. Electrochem. Soc. 154 (2007) J35–J38. [10] Y. Chen, K.W. Cheah, M. Gong, Low thermal quenching and high-efficiency Ce3þ, Tb3þ-co-doped Ca3Sc2Si3O12 green phosphors for white light-emitting diodes, J. Lumin. 131 (2011) 1589–1593. [11] T. Nakano, Y. Kawakami, K. Uematsu, T. Ishigaki, K. Toda, M. Sato, Novel Ba–Sc–Si-oxide and oxynitride phosphors for white LED, J. Lumin. 129 (2009) 1654–1657. [12] L. Bian, F. Du, S. Yang, Q. Ren, Q.L. Liu, Crystal structure and near-ultraviolet photoluminescence properties of Ba9Sc2Si6O24:Ce3þ, Naþ, J. Lumin. 137 (2013) 168–172. [13] S. Lee, S. Park, Preparation and luminescent properties of Tb3þ and Tb3þ–Ce3þ doped Ba9Y2Si6O24 phosphors, J. Lumin. 143 (2013) 215–218. [14] Y. Kim, S. Park, Eu2þ, Mn2þ co-doped Ba9Y2Si6O24 phosphors based on near-UVexcitable LED lights, Mater. Res. Bull. 49 (2014) 469–474. [15] S. Park, Ce3þ–Mn2þ cooperative Ba9Y2Si6O24 orthosilicate phosphors, Mater. Lett. 135 (2014) 59–62. [16] H. Yun, S.-H. Kim, S. Park, Bi3þ, Eu3þ-doped Ba9Y2Si6O24 phosphors based on the site-selected substitution, Opt. Mater. 72 (2017) 571–577. [17] H. Yun, Y. An, S. Park, Influences of substituting Naþ ions in Eu2þ, Mn2þ doped Ba9Y2Si6O24 phosphors, Displays 50 (2017) 1–6. [18] S. Yang, T.J. Shin, S. Park, Luminescent-center tuning for Ba6Ca3YAlSi6O24:Ce3þ, Naþ orthosilicate phosphors, J. Alloy. Comp. 777 (2019) 572–577. [19] D.R. Askeland, P.P. Fulay, W.J. Wright, The Science and Engineering of Materials, Cengage Learning, 2010. [20] Z. Xia, J. Zhou, Z. Mao, Near UV-pumped green-emitting Na3(Y,Sc)Si3O9:Eu2þ phosphor for white-emitting diodes, J. Mater. Chem. C 1 (2013) 5917–5924. [21] J. Brgoch, C.K.H. Borg, K.A. Denault, A. Mikhailovsky, S.P.D. Baars, R. Seshadri, An efficient, thermally stable cerium-based silicate phosphor for solid state white lighting, Inorg. Chem. 52 (2013) 8010–8016. [22] L.S. Rohwer, J.E. Martin, Measuring the absolute quantum efficiency of luminescent materials, J. Lumin. 115 (2005) 77–90. [23] S. Park, T. Vogt, Defect monitoring and substitutions in Sr3-xAxAlO4F (A ¼ Ca, Ba) oxyfluoride host lattices and phosphors, J. Phys. Chem. C 114 (2010) 11576–11583. [24] D.-S. Xing, M.-L. Gong, X.-Q. Qiu, D.-J. Yang, K.-W. Cheah, A bluish green barium aluminate phosphor for PDP application, Mater. Lett. 60 (2006) 3217–3220.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education, Science and Technology (NRF2018R1D1A3B07048543) and the BB21 þ project in 2018. Authors thank to Prof. T.J. Shin for synchrotron experiments at 6D UNIST-PAL beamline, which is supported in part by MSIT, POSTECH, and UNIST Central Research Facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109548. References [1] D. Michalik, M. Sopickalizer, J. Plewa, T. Pawlik, Application of mechanochemical processing to synthesis of YAG:Ce garnet powder, Arch. Metall. Mater. 56 (2011) 1257–1264. [2] Z. Xia, A. Meijerink, Ce3þ-Doped garnet phosphors: composition modification, luminescence properties and applications, Chem. Soc. Rev. 46 (2017) 275–299. [3] I. Shoji, S. Kurimra, Y. Sato, T. Taira, Optical properties and laser characteristics of highly Nd3þ-doped Y3Al5O12 ceramics, Appl. Phys. Lett. 77 (2000) 939. � Lazarevic, V. Radojevic, A. Milutinovic, M. Romcevic, [4] S. Kostic, Z.Z. � Romcevic, A. Valcic, Study of structural and optical properties of YAG and Nd: N.Z. YAG single crystals, Mater. Res. Bull. 63 (2015) 80–87. [5] S. Ye, F. Xiao, Y.X. Parn, Y.Y. Ma, Q.Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties, Mater. Sci. Eng. R 71 (2010) 1–34.
6
Optical Materials 99 (2020) 109548
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Ca2þ-substitution effects in Ba9-xCaxAl2Si6O24:Ce3þ, Naþ phosphors Sungjun Yang, Sangmoon Park * Division of Energy and Chemical Engineering, Major in Energy & Applied Chemistry, Silla University, Busan, 46958, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: X-ray diffraction Cell parameters Phosphors Quantum efficiency
The Ca2þ ion was substituted for a Ba2þ ion in Ba9Al2Si6O24 host lattice using a solid-state reaction. The solidsolubility of Ba9-xCaxAl2Si6O24 compounds was characterized using X-ray powder diffraction (XRD) for x ¼ 0 to x ¼ 9 according to the Hume-Rothery rule. The obtained Ba9-xCaxAl2Si6O24 hosts between x ¼ 2 and x ¼ 5 were examined in order to index the peak positions as a single-phase orthosilicate, which has a trigonal unit-cell. The structure of Ba6Ca3Al2Si6O24 was characterized using synchrotron XRD. The photoluminescence excitation, emission spectra, and scanning electron microscopy (SEM) data of intense blue Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors were monitored. Furthermore, a comparison of the quantum efficiency of the obtained phosphors and sodium salicylate was performed.
1. Introduction Y3Al5O12 (YAG) is a popular optical host material owing to its excellent chemical stability and optical isometric properties [1–6]. When a Nd3þ ion is doped in the YAG lattice, YAG:Nd can act as an active laser material, which emits light with a wavelength of 1064 nm [3,4]. Furthermore, when a Ce3þ ion is doped in the YAG structure, YAG: Ce can act as a predominant luminescent material, which emits intense yellow colors in the emission spectra. This yellow phosphor has been widely used in blue-emitting InGaN light-emitting diode chips, as an essential white light, for various applications [5,6]. Yttrium aluminum garnet (Y3Al5O12) consists of Y3Al(I)2Al(II)3O12, where Y3þ, Al(I)3þ, and Al(II)3þ cations occupy 12-, 8-, and 4-coordinated sites in the garnet structure, respectively [2,6]. YAG, which has a cubic crystal system with Ia3d symmetry, belongs to the class of orthosilicate structures. A unique feature of orthosilicate compounds is that the tetrahedral Si4þ poly hedron is isolated in the structure and does not share O atoms [7,8]. A garnet compound, as an orthosilicate structure, can be represented as the perfect phase of Ca3Sc2Si3O12, which consists of dodecahedral Ca2þ, octahedral Sc3þ, and tetrahedral Si4þ sites [9,10]. A good orthosilicate luminescent host, (Ba3Ba1.5ScSi3O12)2, was intensively studied in prior research [11,12]. The orthosilicate phase is composed of 9-, 10-, and 12-coordinated Ba2þ, 8-coordinated Sc3þ, and 4-coordinated Si4þ sites. The cubic phase of the perfect garnet Ca3Sc2Si3O12 was transformed into a trigonal orthosilicate structure with R3H due to the out-range of the cation radius after substituting Ca2þ ions with Ba2þ ions. When the larger Y3þ ions were replaced with smaller Sc3þ ions in the Ba9Sc2Si6O24
host lattice, the trigonal crystal system and the three different cation sites were retained [13–17]. Subsequently, the phosphors of Ce3þ or Eu2þ-doped in Ba6Ca3YAlSi6O24 host structure via an energy-transfer mechanism were studied [14,15]. In this study, Ba9-xCaxAl2Si6O24 (for x ¼ 0 to x ¼ 9) hosts and Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (for x ¼ 2 to x ¼ 5) phosphors were prepared at high temperature. The orthosilicate Ba6Ca3Al2Si6O24 host structure was characterized using synchrotron X-ray diffraction (XRD). The luminescence property and particle morphology of Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (for x ¼ 2 to x ¼ 5) phosphors are studied. The relative quantum efficiencies of the phos phors were compared with the one of sodium salicylate based on the integrated emission. 2. Experimental Samples of Ba9-xCaxAl2Si6O24 (x ¼ 0–9) and Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 (x ¼ 2–5) were prepared by mixing the appropriate stoichiometric amounts of BaCO3 (Alfa 99.8%), CaCO3 (Alfa 99.5%), Al2O3 (Alfa 99.95%), SiO2 (Alfa 99.5%), CeO2 (Alfa 99.9%), and Na2CO3 (Alfa 99.5%) with an agate mortar-pestle and subsequently heating at � 950 and 1150 � C for 6 h with a heating rate of 5 C/min, respectively, under the atmosphere using 8%H2/92%Ar. Phase identification was exploited using a Shimadzu XRD-6000 powder diffractometer (CuKααradiation). High-resolution synchrotron X-ray powder diffraction data were collected at PLS-II 6D UNIST-PAL beamline of Pohang Accelerator Laboratory (PAL) [18]. The structure of Ba6Ca3Al2Si6O24 with unit-cell parameters was determined by using the Rietveld
* Corresponding author. E-mail address: [email protected] (S. Park). https://doi.org/10.1016/j.optmat.2019.109548 Received 17 August 2019; Received in revised form 23 October 2019; Accepted 15 November 2019 Available online 9 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
S. Yang and S. Park
Optical Materials 99 (2020) 109548
refinement program Rietica. UV spectroscopy to measure the photo luminescence excitation and emission spectra of the blue luminescent materials was done using spectrofluoro-meters (Sinco Fluoromate FS-2). Sodium salicylate (Alfa 99%) was used as a standard phosphor to calculate the relative quantum yield of the obtained phosphors. The emission spectra of the samples were relatively measured with the standard phosphor; furthermore, the spectroscopy measurement system (Xenon lamp) was warmed up for 30 min before using it at room tem perature. The scanning electron microscopy (SEM, Hitachi, TM3030plus) was performed for the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (for x ¼ 2 to x ¼ 5) phosphor.
DAB ¼
rA
rB rB
(1)
� 100%
where DAB is the radius % difference and rA and rB are the ionic radii of A and B metal ions, respectively. When Al3þ ions were substituted with Y3þ ions in the Ba9Y2Si6O24 host lattice, an approximately 50% differ ence in the ionic radius between Y3þ (r ¼ 0.9 Å for 6 CN) and Al3þ (r ¼ 0.535 Å for 6 CN) can be calculated. Therefore, the Ba2þ ions were sequentially replaced by Ca2þ ions to attune the size factor for the Al3þ substitutions, which show a 20% difference in the ionic radius between Ba2þ (r ¼ 1.47 Å for 9 CN) and Ca2þ (r ¼ 1.18 Å for 9 CN). As a result, the single phase of Ba9-xCaxAl2Si6O24, based on an AlO6 octahedral poly hedron from the Ba9Y2Si6O24 structure, could be successfully prepared when two Ca2þ ions (x ¼ 2) were substituted with Ba2þ ions. The limited solid-solution solubility of the Ba9-xCaxAl2Si6O24 compounds are pre sented (for x ¼ 2 to x ¼ 5) in Fig. 1 (i)–(l) and Table 1. The multi-phased Ca2SiO4, CaSiO3, and Ca2Al2SiO7 occurred at x ¼ 6 to x ¼ 9 in the Ba9-
3. Results and discussion Fig. 1 (a)–(f) show XRD patterns for the calculated Ba9Sc2Si6O24 (Inorganic Crystal Structure Database (ICSD) 75175), Ba2SiO4 (ICSD 6246), Ca2SiO4 (ICSD 81097), Ca2SiO4 (ICSD 39006), Ca2Al2SiO7 (ICSD 27427), and CaSiO3 (ICSD 20571), respectively. The XRD patterns of the Ba9-xCaxAl2Si6O24 (for x ¼ 2 to x ¼ 5) compounds in Fig. 1 (i)–(l) depict the single-phase host lattices, without any obvious impurities, indexed in a trigonal unit cell. Furthermore, the XRD patterns of Ba9-xCax Al2Si6O24 (x ¼ 0, 1, 6, 7, 8, 9) formulas appear in multi-phases as shown in Fig. 1 (g)–(h) and (m)-(p). When Y3þ (r ¼ 0.9 Å for 6 Coordination Number (CN)) ions were replaced by Al3þ (r ¼ 0.535 Å for 6 CN) ions in Ba9Y2Si6O24, the solid solubility could be determined by appropriate size-factors between the (Ba2þ, Y3þ) and (Ca2þ, Al3þ) ions in order to minimize the lattice strain [19,20]. When the Al3þ ions fully occupied the Y3þ site of the Ba9Y2Si6O24 host lattice, the impurity phase of Ba2SiO4 was observed as shown in Fig. 1 (g). Furthermore, when a smaller Ca2þ ion was substituted with Ba2þ ion in the Ba9Al2Si6O24 composition to reduce the lattice tension, the impurity phase of Ba2SiO4, as well as (Ba, Ca)9Al2Si6O24, were monitored as shown in Fig. 1 (h). The unlimited solid solubility can be satisfied by the size factor, crystal structure, electronic valence, and electronegativity according to the Hume-Rothery rule [19,20]. If the metal cations of compounds have equivalent valence and similar electronegativity in the same crystal system, the ionic size of the cations can be a crucial factor. If the ionic radius is less than a 15% difference, the unlimited solid solution can possibly be achieved. The radius percent difference is calculated by the following equation:
Table 1 The different phases according to the limited solid-solution solubility of the Ba9xCaxAl2Si6O24 compounds (for x ¼ 0 to x ¼ 9). x
Fomula
x¼ 9 x¼ 8 x¼ 7 x¼ 6 x¼ 5 x¼ 4 x¼ 3 x¼ 2 x¼ 1 x¼ 0
Ca9Al2Si6O24
→
Phases
Ba1Ca8Al2Si6O24
→
Ca2SiO4 (ICSD 39006) þ CaSiO3 (ICSD 20571) þ Ca2Al2SiO7 (ICSD 27427) Ca2SiO4 (ICSD 81097) þ Ca2Al2SiO7 (ICSD 27427)
Ba2Ca7Al2Si6O24
→
Ca2SiO4 (ICSD 81097)
Ba3Ca6Al2Si6O24
→
(Ba(Ca))9Al2Si6O24 þ Ca2SiO4 (ICSD 81097)
Ba4Ca5Al2Si6O24
→
Ba4Ca5Al2Si6O24
Ba5Ca4Al2Si6O24
→
Ba5Ca4Al2Si6O24
Ba6Ca3Al2Si6O24
→
Ba6Ca3Al2Si6O24
Ba7Ca2Al2Si6O24
→
Ba7Ca2Al2Si6O24
Ba8Ca1Al2Si6O24
→
Ba2SiO4 (ICSD 6246) þ (Ba(Ca))9Al2Si6O24
Ba9Al2Si6O24
→
Ba2SiO4 (ICSD 6246)
Fig. 1. XRD patterns for the calculated (a) Ba9Sc2Si6O24, (b) Ba2SiO4, (c) Ca2SiO4 (orthorhombic), (d) Ca2SiO4 (monoclinic), (e) Ca2Al2SiO7, and (f) CaSiO3 and the observed (g)–(p) Ba9-xCaxAl2Si6O24 (x ¼ 0 to x ¼ 9) structures. 2
S. Yang and S. Park
Optical Materials 99 (2020) 109548
xCaxAl2Si6O24
composition as shown in Fig. 1 (m)–(p). A gradual shift in the positions of the various Bragg reflections to higher angles was observed because the smaller Ca2þ ions (r ¼ 1.18 Å for 9 CN) could occupy the Ba2þ sites (r ¼ 1.47 Å for 9 CN) in the Ba9-xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) structure as shown in Fig. 1 (i) – (m). Fig. 2 and Table 2 show the cell parameters, including cell volumes of the Ba9-xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) host lattice. As depicted by the XRD patterns of the com pounds, the a and c lattice parameters and cell volumes decrease with the increase in Ca2þ substitution. The decrease in the c axis was doubly reduced in comparison to the a axis from Ba7Ca2Al2Si6O24 (a ¼ 9.9893 (6) Å, c ¼ 22.179(2) Å) to Ba4Ca5Al2Si6O24 (a ¼ 9.9382(2) Å, c ¼ 21.964 (2) Å). There are two Ba2þ and one Ca2þ cation sites in the Ba6Ca3Al2 Si6O24 structure, which has a trigonal unit-cell as shown in Fig. 3 (a). The smaller Ca2þ cation prefers to occupy the low-coordinated site. Thus, Ba(1), Ca(2), and Ba(3) can be located at 12, 9, and 11-coordi nated O atoms in the Ba6Ca3Al2Si6O24 orthosilicate host lattices. Fig. 3 (b) shows the fit obtained by the structure refinement from the syn chrotron XRD data, when Ca2þ ions fully occupy the 9-coordinated cation site in the Ba6Ca3Al2Si6O24 host lattice at room temperature. Tables 3 and 4 summarize the Rietveld refinement data of the Ba6Ca3Al2Si6O24 structure. The a and c cell parameters with the unit-cell volume of Ba6Ca3Al2Si6O24 are a ¼ 9.9659(2) Å and c ¼ 22.0199(8) Å. The lattice parameters for the complete substitution of Al3þ ions in the Ba6Ca3Al2Si6O24 host lattice are decreased in comparison to those for the partial substitution of Al3þ ions in the Ba6Ca3YAlSi6O24 host (a ¼ 9.9848(2) Å and c ¼ 22.1216(8) Å), and this observation has been previously reported [18]. Ba(1), Ca(2), and Ba(3) occupy Wyckoff sites 3a, 6c, and 18f, respectively. Both Ba6Ca3Al2Si6O24 and Ba6Ca3YAl Si6O24 host lattices have three different metal cation sites, correspond ing to Ba(1), Ca(2), and Ba(3), located at 12, 9, and 11-coordinated O atoms with Al–O6 (or (Y, Al)–O6) octahedral and Si–O4 tetrahedral linkages [18]. The (Y, Al)–O6 octahedral polyhedron is composed of four different Y–O (2.92329(6) Å and 1.92082(3) Å) and Al–O (2.42957 (4) Å and 2.41183(5) Å) bond distances in Ba6Ca3YAlSi6O24, which were significantly distorted through the (Ba(3)O11)-(SiO4)-(Y,AlO6)-( SiO4)-(Ba(3)O11) linkages as shown in Fig. 3 (c). In contrast, the Al–O6 octahedra in the Ba6Ca3Al2Si6O24 structure consist of two interatomic Al–O bond distances (2.44256(3) Å and 2.21315(3) Å) as shown in Fig. 3 (c) and Table 5. When the luminescent centers of the Ce3þ ions are doped in both Ba6Ca3YAlSi6O24 and Ba6Ca3Al2Si6O24 structures, the Ce3þ ions can occupy the Ba2þ and Ca2þ sites, instead of the Y3þ and Al3þ sites due to the distorted and insufficient space for the Ce3þ ion. Although the Ba2þ and Ca2þ sites can be replaced with Ce3þ in the Ba6-2pCa3CepNapYAlSi6O24 and Ba6Ca3-2pCepNapYAlSi6O24 lattices, the substitution of Ce3þ ions was selectively increased in the Ba2þ site or both Ba2þ and Ca2þ sites. As a result, the formulae were finally derived
Table 2 The unit-cell parameters of the Ba9-xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) structures. (a) x ¼ 2 (b) x ¼ 3 (c) x ¼ 4 (d) x ¼ 5
a (Å)
c (Å)
V (Å3)
9.9893(6) 9.9528(5) 9.9410(4) 9.9382(2)
22.179(2) 22.050(1) 21.978(1) 21.964(2)
1916.6(2) 1891.6(2) 1880.9(2) 1878.7(2)
Fig. 3. (a) Synchrotron XRD pattern of Ba6Ca3Al2Si6O24 structure and calcu lated XRD patterns of BaSiO3 (ICSD 6245), (b) the structure of Ba6Ca3Al2Si6O24 host lattice, and (c) the top views of the Ba6Ca3YAlSi6O24 and Ba6Ca3Al2 Si6O24 structures.
Fig. 2. The cell parameters and cell volumes of the Ba9-xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) structure. 3
S. Yang and S. Park
Optical Materials 99 (2020) 109548
Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors is shifted from 372 to 433 nm when the excitation peak was used at maximum. The Ce3þ activator can occupy three independent cation sites in the (Ba, Ca)9Al2Si6O24 host structure. In Fig. 4(b), two excitation peaks occur centered approximately 287 and 323 nm in the Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 (x ¼ 2) phosphor. In contrast, three main excitation peaks centered on 287, 323, and 367 nm with the increase of Ca2þ contents in the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 3 to x ¼ 5) phosphors are noticeably observed. When the smaller Ca2þ ions, particularly in the Ba6Ca3Al2Si6O24 host lattice, are fully located in the 9-coordinated cation site, the Ce3þ ions can be substituted with Ba2þ ion sites in the Ba5.8Ce0.1Na0.1Ca3Al2Si6O24 phosphors. It is known that the Ce3þ ion is located in the 11-coordinated Ba(3) site because the Ba(3) ion is effec tively linked by Al–O6 octahedra and Si–O4 tetrahedra in the host structure, whereas the Ba(1) ion occupies the interlayer gap of Al–O6 and Si–O4 [18]. The maximum intensity peaks of the blue-emission bands, centered from 421 to 434 nm for Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5), were monitored with the increase of Ca2þ ions as shown in Fig. 4(b). When the Ca2þ ions were substituted in the Ba8.8-xCax Ce0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors, the crystal field splitting of the 5d level of Ce3þ, in the case of calcium rich phosphors, shifts the emission band in a lower energy region with a gradual increase of the blue-emission intensity. The intense blue emission of the sodium salic ylate was used as a standard to estimate the quantum efficiency (QE) compared with the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors. The Naþ ion was used as a charge compensator as well as a flux in the phosphors, which achieved about 14% emission enhancement compared to the case with no Naþ ions in the phosphors. A relative QE of the obtained phosphors was calculated by following equation [22–24].
Table 3 Rietveld refinement and crystal data for Ba6Ca3Al2Si6O24. Formula
Ba6Ca3Al2Si6O24
Radiation type λ (Å) synchrotron 2θ range (deg), Temperature (K) Crystal system Space group
0.66 6–42 295 trigonal R3
Lattice parameter (Å)
a ¼ 9.9659 (2) c ¼ 22.0199 (8) V ¼ 1893.98 (8) 9.76 14.04 0.79
Volume (Å3) Rp Rwp
χ2
Table 4 Refined atomic coordinated coordinates (x, y, z), Wyckoff positions, isotropic temperature factors (Biso), and occupancies (SOF) of Ba6Ca3Al2Si6O24. Atom
Wyckoff position
x
y
z
Biso
SOF
Ba1 Ca2 Ba3 Al Si O1 O2 O3 O4
3a 6c 18f 6c 18f 18f 18f 18f 18f
0.0000 0.3333 0.0024 0.0000 0.3379 0.3575 0.4884 0.0102 0.1729
0.0000 0.6670 0.6683 0.0000 0.0017 0.1009 0.1425 0.1793 0.5032
0.0000 0.0007 0.1099 0.1722 0.0675 0.0165 0.0883 0.1164 0.1135
22.77 1.25 1.45 7.80 3.21 7.60 5.27 6.89 4.08
1 1 1 1 1 1 1 1 1
Table 5 Selected interatomic distances for Ba6Ca3Al2Si6O24.
Relative QE ¼ Standard QE x
Atom
distance (Å)
atom
distance (Å)
Ba1–O1 (x6) Ba1–O3 (x6) Ca2–O1 Ca2–O1 Ca2–O1 Ca2–O2 Ca2–O2 Ca2–O2 Ca2–O4 Ca2–O4 Ca2–O4
3.20346(5) 3.15446(7) 2.80307(5) 2.80117(5) 2.80705(5) 2.68878(5) 2.68868(5) 2.69232(5) 2.96347(7) 2.96368(7) 2.96072(6)
Ba3–O1 Ba3–O1 Ba3–O2 Ba3–O2 Ba3–O2 Ba3–O3 Ba3–O3 Ba3–O3 Ba3–O4 Ba3–O4 Ba3–O4
2.92049(9) 3.21209(4) 2.82135(5) 3.04880(5) 3.32817(9) 2.81235(5) 2.96450(5) 2.79718(7) 2.89154(5) 2.83115(5) 2.95104(6)
Al–O2 (x3) Al–O3 (x3)
2.44256(3) 2.21315(3)
Si–O1 Si–O2 Si–O3 Si–O4
1.44365(3) 1.52471(2) 1.86449(2) 1.98206(2)
ze2 r4 ; 6R5
(3)
Sodium salicylate was employed as a standard phosphor and its ab solute QE of 58% was used. Approximately 10% error can be estimated because the amount of the reflected light from the obtained sample and the sodium salicylate was neglected in the equation [22–24]. Based on the integrated emission intensity at a 365 nm excitation, the relative QEs were calculated as 1, 17, 42, and 77% at x ¼ 2 to x ¼ 5 in Ba8.8-xCax Ce0.1Na0.1Al2Si6O24 phosphors, respectively. Furthermore, the relative QE of the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 phosphor (x ¼ 5) was consid erably enhanced to 105% from 77% when the large particles of the phosphor were retained by a 25 μm sieve. Fig. 4 (c) shows the SEMs of the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors. When the Ca2þ ion was increased from x ¼ 2 to x ¼ 5 in the Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 phosphors, the particle size clearly increased by growth of the grain boundary with the enhancement of blue emission lights. The precursor of calcium carbonate critically affects the higher crystallinity of the annealed phosphors due to their lower melting temperature, which leads to a decrease in the eutectic point, compared to that of the barium carbonate.
as Ba6-2pCa3CepNapYAlSi6O24 and (Ba,Ca)9-2pCepNapYAlSi6O24 in a prior study [18]. In this study, when the Ce3þ and Naþ ions were substituted by Ba2þ sites, Ca2þ ions were gradually added in the Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors (S1). Fig. 4 (a) shows that the PL spectra of Ba8.8-xCaxCe0.1Na0.1Si6O24 (x ¼ 2 to x ¼ 5) phosphors exhibit blue emissions, which is attributed to the d-f Ce3þ transition. When the smaller Ca2þ ions were substituted with the Ba2þ ion in the Ba8.8-xCaxCe0.1Na0.1Al2Si6O24 (for x ¼ 2 to x ¼ 5) host lattice, the bond length of Ce–O would decrease and finally the crystal splitting was ex pected to increase, which resulted in the red shift shown in Fig. 4(a). The crystal field splitting (Dq) was calculated using the following formula: Dq ¼
integrated emission of samples integrated emission of standard
4. Conclusions Ba9-xCaxAl2Si6O24 (x ¼ 0 to x ¼ 9) hosts were successfully prepared using a solid-state reaction at 1100 � C. The single phase of the Ba9xCaxAl2Si6O24 (x ¼ 2 to x ¼ 5) host was characterized as the trigonal unit cell (R3H). The Ba6Ca3Al2Si6O24 structure was refined with the lattice parameters of a ¼ 9.9659(2) Å and c ¼ 22.0199(8) Å using synchrotron XRD. The efficient blue emission, quantum efficiency, and morphology of the Ba8.8-x CaxCe0.1Na0.1Al2Si6O24 (x ¼ 2 to x ¼ 5) phosphors were achieved. The maximum luminescent intensity of Ba8.8-xCaxCe0.1 Na0.1Al2Si6O24 (x ¼ 5) phosphors, approximately 105%, was achieved with particles larger than 25 μm.
(2)
where Dq is the magnitude of the 5d energy level separation, R is the bond distance between the central ion and its ligands, z is the anionic charge, e is the charge of the electron, and r is the radius of the 5d wave function [20,21]. The center of the emission spectra in 4
S. Yang and S. Park
Optical Materials 99 (2020) 109548
Fig. 4. (a) The normalized photoluminescence emission spectra, (b) the excitation and emission spectra, and (c) the SEMs of Ba8.8-xCaxCe0.1Na0.1Si6O24 (x ¼ 2 to ¼ 5) phosphors.
5
S. Yang and S. Park
Optical Materials 99 (2020) 109548
Declaration of competing interest
[6] L. Havl� ak, J. B� arta, M. Buryi, V. Jarý, E. Mih� okov� a, V. Laguta, P. Boh� a�cek, M. Nikl, Eu2þ stabilization in YAG structure: optical and electron paramagnetic resonance study, J. Phys. Chem. C 120 (2016) 21751–21761. [7] L.H. Wang, L.F. Schneemeyer, R.J. Cava, T. Siegrist, A new barium scandium silicate: Ba9Sc2(SiO4)6, J. Solid State Chem. 113 (1994) 211–214. [8] Y. Kim, S. Park, Preparation and luminescent properties of Eu-substituted barium–yttrium orthosilicate phosphors, Opt. Mater. 36 (2013) 458–462. [9] Y. Shimomura, T. Honma, M. Shigeiwa, T. Akai, K. Okamoto, N. Kijima, Photoluminescence and crystal structure of green-emitting Ca3Sc2Si3O12 : Ce3 þ phosphor for white light emitting diodes, J. Electrochem. Soc. 154 (2007) J35–J38. [10] Y. Chen, K.W. Cheah, M. Gong, Low thermal quenching and high-efficiency Ce3þ, Tb3þ-co-doped Ca3Sc2Si3O12 green phosphors for white light-emitting diodes, J. Lumin. 131 (2011) 1589–1593. [11] T. Nakano, Y. Kawakami, K. Uematsu, T. Ishigaki, K. Toda, M. Sato, Novel Ba–Sc–Si-oxide and oxynitride phosphors for white LED, J. Lumin. 129 (2009) 1654–1657. [12] L. Bian, F. Du, S. Yang, Q. Ren, Q.L. Liu, Crystal structure and near-ultraviolet photoluminescence properties of Ba9Sc2Si6O24:Ce3þ, Naþ, J. Lumin. 137 (2013) 168–172. [13] S. Lee, S. Park, Preparation and luminescent properties of Tb3þ and Tb3þ–Ce3þ doped Ba9Y2Si6O24 phosphors, J. Lumin. 143 (2013) 215–218. [14] Y. Kim, S. Park, Eu2þ, Mn2þ co-doped Ba9Y2Si6O24 phosphors based on near-UVexcitable LED lights, Mater. Res. Bull. 49 (2014) 469–474. [15] S. Park, Ce3þ–Mn2þ cooperative Ba9Y2Si6O24 orthosilicate phosphors, Mater. Lett. 135 (2014) 59–62. [16] H. Yun, S.-H. Kim, S. Park, Bi3þ, Eu3þ-doped Ba9Y2Si6O24 phosphors based on the site-selected substitution, Opt. Mater. 72 (2017) 571–577. [17] H. Yun, Y. An, S. Park, Influences of substituting Naþ ions in Eu2þ, Mn2þ doped Ba9Y2Si6O24 phosphors, Displays 50 (2017) 1–6. [18] S. Yang, T.J. Shin, S. Park, Luminescent-center tuning for Ba6Ca3YAlSi6O24:Ce3þ, Naþ orthosilicate phosphors, J. Alloy. Comp. 777 (2019) 572–577. [19] D.R. Askeland, P.P. Fulay, W.J. Wright, The Science and Engineering of Materials, Cengage Learning, 2010. [20] Z. Xia, J. Zhou, Z. Mao, Near UV-pumped green-emitting Na3(Y,Sc)Si3O9:Eu2þ phosphor for white-emitting diodes, J. Mater. Chem. C 1 (2013) 5917–5924. [21] J. Brgoch, C.K.H. Borg, K.A. Denault, A. Mikhailovsky, S.P.D. Baars, R. Seshadri, An efficient, thermally stable cerium-based silicate phosphor for solid state white lighting, Inorg. Chem. 52 (2013) 8010–8016. [22] L.S. Rohwer, J.E. Martin, Measuring the absolute quantum efficiency of luminescent materials, J. Lumin. 115 (2005) 77–90. [23] S. Park, T. Vogt, Defect monitoring and substitutions in Sr3-xAxAlO4F (A ¼ Ca, Ba) oxyfluoride host lattices and phosphors, J. Phys. Chem. C 114 (2010) 11576–11583. [24] D.-S. Xing, M.-L. Gong, X.-Q. Qiu, D.-J. Yang, K.-W. Cheah, A bluish green barium aluminate phosphor for PDP application, Mater. Lett. 60 (2006) 3217–3220.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education, Science and Technology (NRF2018R1D1A3B07048543) and the BB21 þ project in 2018. Authors thank to Prof. T.J. Shin for synchrotron experiments at 6D UNIST-PAL beamline, which is supported in part by MSIT, POSTECH, and UNIST Central Research Facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109548. References [1] D. Michalik, M. Sopickalizer, J. Plewa, T. Pawlik, Application of mechanochemical processing to synthesis of YAG:Ce garnet powder, Arch. Metall. Mater. 56 (2011) 1257–1264. [2] Z. Xia, A. Meijerink, Ce3þ-Doped garnet phosphors: composition modification, luminescence properties and applications, Chem. Soc. Rev. 46 (2017) 275–299. [3] I. Shoji, S. Kurimra, Y. Sato, T. Taira, Optical properties and laser characteristics of highly Nd3þ-doped Y3Al5O12 ceramics, Appl. Phys. Lett. 77 (2000) 939. � Lazarevic, V. Radojevic, A. Milutinovic, M. Romcevic, [4] S. Kostic, Z.Z. � Romcevic, A. Valcic, Study of structural and optical properties of YAG and Nd: N.Z. YAG single crystals, Mater. Res. Bull. 63 (2015) 80–87. [5] S. Ye, F. Xiao, Y.X. Parn, Y.Y. Ma, Q.Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties, Mater. Sci. Eng. R 71 (2010) 1–34.
6