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Melilite-Structure CaYAl3O7:Eu3+ Phosphor: Structural and Optical Characteristics for Near-UV LED-Based White Light Seung Hyok Park,†,‡,∥ Kyoung Hwa Lee,§,∥ Sanjith Unithrattil,§ Ho Shin Yoon,† Ho Gyeom Jang,‡ and Won Bin Im*,§ †

Research Institute, Force4 Corp., Daechon-dong, Buk-gu, Gwangju, 500-470 Republic of Korea Department of Chemistry, Korea University, Seoul, 136-706 Republic of Korea § School of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju, 500-757 Republic of Korea ‡

ABSTRACT: A red-emitting phosphor, CaY 1‑xEu xAl3 O7 (CYA:Eu3+), was prepared by conventional solid-state reaction, and its structural properties were investigated by means of Rietveld refinement method and maximum entropy method using an X-ray source. XRD patterns confirm the tetragonal phase of CYA:Eu3+ phosphors. The photoluminescence properties of the samples were investigated for application to white light-emitting diodes (WLEDs). The emission spectrum is dominated by a red peak located at 616 nm due to the 5D0 → 7F2 electric dipole transition of Eu3+ ions. Concentration quenching occurs when the Eu 3+ concentration is about 0.35 and the critical transfer distance of the phosphor is also calculated. A white LED fabricated with blue, green, and CYA:Eu3+ red phosphor incorporated with an encapsulant in ultraviolet LEDs (λmax = 396 nm) is discussed.

1. INTRODUCTION For the past several years, white light-emitting diodes (WLEDs) have been used as solid-state lighting sources and as components of display devices due to high brightness, low energy consumption, long operation time (>100 000 h), and environmentally friendly characteristics.1−4 Several ways to assemble the white LEDs have been reported. Generally, white light can be generated by a combination of blue LED chips coated with yellow-emitting phosphors such as cerium-doped yttrium aluminum garnet (YAG:Ce3+).5,6 However, this method has several serious problems, such as a poor colorrendering index (90). However, despite that, the blending of blue, green, and red phosphor together produces poor efficiency caused by a large Stokes shift between excitation and emission in the UV-excitable phosphor.11 Therefore, it is urgent to search for novel red phosphors for NUV chip-based WLEDs. Red-emitting phosphors for white LEDs are still commercially limited to sulfide-based materials like Y2O2S:Eu3+, SrY2S4:Eu2+, and CaS:Eu2+.12 Sulfide-based phosphors have poor chemical stability and low efficiency in comparison with oxide phosphors. To overcome these problems, we have to search not only for a stable host lattice that can absorb energy © 2012 American Chemical Society

efficiently in the near-UV region but also for efficient energy transfer to the activator.13,14 Reported literature results indicate that the scheelite host lattice can effectively transfer energy to the activator ions.15 At the same time, the critical concentration of activator ions is much higher than that of conventional inorganic phosphors. In another case, phosphors with layered perovskite structures have strong direct excitation bands, and high activator concentration for energy transfer among activators is restricted to a two-dimensional activator layer.16 Motivated by these previous studies, the melilite structure, CaYAl3O7 (CYA), is examined here as a host material for Eu3+ ions. The crystal structure of CYA is a tetragonal system, and its space group is P4̅21m.17 Most studies on this compound have highlighted applications for long-lasting phosphorescence and blue mechanoluminescence.18−20 In addition, with the melilitestructure phosphors, a number of groups have been able to use a charge transfer band around a 260 nm absorption band, unlike the direct excitation band of Eu3+.17,21 However, there has been no study on CaYAl3O7 host lattice with compounds for LED application using the direct excitation band of Eu3+. In this study, we have investigated the preparation and photoluminescence (PL) of the red-emitting CaYAl3O7:Eu3+(CYA:Eu3+) phosphor and describe its structure and optical properties. WLEDs based on a combination of an Received: July 20, 2012 Revised: November 14, 2012 Published: November 29, 2012 26850

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InGaN LED chip (λmax = 396 nm) with the CYA:Eu3+, along with blue and green phosphors, have been fabricated and are discussed.

2. EXPERIMENTAL PROCEDURE 2.1. Sample Preparation. Powder samples of the general formula CaY1‑xEuxAl3O7 (CYA:Eu3+, x = 0.02, 0.04, 0.07, 0.1, 0.15, 0.2, 0.3, 0.35, and 0.4) were prepared by a solid-state reaction method. The starting materials were CaCO3 (Aldrich, 99.99%), Y2O3 (Aldrich, 99.99%), α-Al2O3 (Kojundo, 99.99%), and Eu2O3 (Aldrich, 99.99%). A stoichiometric amount of raw materials was mixed using an agate mortar and pestle for 1 h with acetone as the dispersing medium. The dried powder was then heated at various temperatures, ranging from 1300 to 1500 °C for 4 h in air at a heating rate 5 °C min−1. Finally the samples were naturally cooled to room temperature in the furnace. 2.2. Structural and Optical Properties. The structural data of the prepared samples were obtained using Cu Kα radiation (Philips X’Pert) over the angular range 10° ≤ 2θ ≤ 100° made with the General Structure Analysis System (GSAS) program.22 Additionally, to explore the electron density of the CYA:Eu3+ phosphor, the electron density was analyzed using the maximum entropy method (MEM) based on the observed structural factors obtained from Rietveld refinement. The MEM analysis was carried out with 128 × 128 × 128 pixels per lattice parameter by a program package (PRIMA) for electron density distribution calculation by the MEM.23 To investigate luminescence properties, room temperature photoluminescence (PL) spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer over the wavelength range of 200 to 750 nm. Diffuse reflectance absorption spectra were recorded using a Thermo Scientific Evolution 220 UV− visible spectrophotometer in the wavelength range of 220−550 nm. To obtain the quantum efficiency (QE), an integrating sphere was included in the Hitachi F-7000 fluorescence spectrophotometer. The QEs were corrected by subtracting a proportion of the emission, which resulted from re-excitation due to the reflected excitation source within the integrating sphere. 2.3. Fabrications of Prototype WLEDs. Prototype LED devices were fabricated by applying an intimate mixture of the CYA:Eu3+, blue and green phosphors, and transparent silicone resin on an InGaN LED (λmax = 396 nm). For electroluminescence measurements, discrete LEDs grown on m-plane GaN were placed on silver headers, and gold wires were attached for electrical operation. The device was then encapsulated in a phosphor/silicone mixture, with the mixture placed directly on the headers, and then cured at 70 and 150 °C for 1 h. After packaging was completed, the device with phosphor was measured in an integrating sphere under DC bias forward conditions.

Figure 1. (a and b) Unit cell representation of the crystal structure of CaYAl3O7 (CYA). Blue, black, red, and orange spheres represent Ca, Y, Al, and O atoms, respectively. (c) Coordination geometry of (Ca/ Y)O8 is depicted.

or Ba; B is La, Gd, or Y; and C is Al or Ga. In the unit cell (Z = 2) of CYA, Eu and Y atoms occupy only the 4e positions. Half of the 4e sites are expected to be filled by Ca atoms, and the other half by Y/Eu atoms. Ca/Y sites corresponding to polyhedra with 8-coordinations are likely to be substituted by Eu ions. The cell also contains 2a and 4e sites occupied by Al atoms. Oxygen atoms are distributed over the 2c, 4e, and 8f sites, denoted as O(1), O(2), and O(3), respectively. In the unit cell of CYA, the compounds consist of fivemembered rings with AlO45‑ tetrahedra unit linked at each corner, and the Ca2+/Y3+ ions are placed at the centers of these rings as shown in Figure 1a. CYA consists of alternating cationic (Ca/Y)2 and corner-sharing tetrahedral anionic Al3O7 layers and features 5-fold tunnels that accommodate the eightcoordinate Ca2+/Y3+ as chains of cations, as shown in Figure 1b. These alternating layers of Ca2+/Y3+ (or Eu3+) are very important in the development of highly efficient phosphors using the direct excitation band from the excited 5D0 level to the 7FJ (J = 0−6) levels of the 4f 6 configuration in Eu3+.26 Figure 2 displays the results of the Rietveld refinement of the X-ray diffraction (XRD) data profiles of CaY0.65Eu0.35Al3O7, obtained with 3.61% of Rwp and 3.339 goodness-of-fit parameter (χ2). We begin with the description of structural and optical properties of CaY0.65Eu0.35Al3O7, which represents the optimal composition of all synthesized samples. Hereafter, the abbreviation CYA:Eu3+ refers to the specific composition CaY0.65Eu0.35Al3O7. The diffraction pattern of the sample was very similar to that of the initial model of CYA. From the Rietveld refinement results, no impurity phases were identified in the sample, regardless of the Eu content. The cell parameters were a,b = 7.6989(2) Å and c = 5.0583(2) Å as listed in Table 1. Tables 2 and 3 list the refined structural parameters and the selected bond distances of CYA:Eu3+ obtained from the structural refinement.

3. RESULTS AND DISCUSSION 3.1. Phase Characterization and XRD Analysis. Figure 1 displays the unit cell representation of CaYAl3O7 (CYA), which structurally belongs to the melilite structure group (tetragonal, P4̅21m, S.G. no. 113).24 There have been several reports on rare-earth-doped ABC3O7 compounds, which have been widely investigated as important optical materials, especially as laser materials and phosphors.21,25 The melilite group is composed of minerals of the general formula ABC3O7, where A is Ca, Sr, 26851

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has been reported to have many advantages in obtaining further structural information from the diffraction data independency of a structural model.28−30 From the Rietveld refinement result, the observed structural factors for 162 reflections derived from the Rietveld refinement were used as initial structural factors. The MEM analysis was carried out in 128 × 128 × 128 pixels per lattice parameter. In MEM analysis, the final converged reliability factor (RWF) was 2.61% for the CYA:Eu3+ powder sample, which was good enough to determine the charge densities of the constituent atoms. The final reliable R factors of MEM electron density are defined by ∑ R WF =

∑

− FMEM|

1 |F | σ2 0

(1)

where F0 is the observed structure factor, σ is their estimated standard deviation, FMEM is a structure factor estimated by the MEM analysis, and the summation is conducted over the reflections analyzed by the MEM.29 Figure 3 shows (a) the unit cell of the CYA:Eu3+ crystal structure with an electron isosurface image and (b) an electron density contour map on the (002) plane as obtained by the MEM, respectively. From calculating the electron density of CYA:Eu3+, the electron density between Ca2+/Y3+ (or Eu3+) atoms occupying the 4e site was seen to have a somewhat nonspherical shape, unlike what is observed around all other sites in the unit cell. As discussed before, the Ca2+/Y3+ (or Eu3+) ions in the melilite structure are randomly distributed on the 4e sites, with divalent and trivalent ions. As a consequence, a number of sites, resulting from antisite ordering, yield defects with a deviation of the charge state. Recently, Kwak et al.21 systematically investigated the formation of defects in CYA:Eu3+ by electron spin resonance and thermoluminescence analyses. These defect centers may play an important role in the optical properties of the CYA:Eu3+ phosphor.25,31 Additionally, large thermal parameters on the O(2) and O(3) sites can be ascribed to the antisite ordering by Ca2+/Y3+ (or Eu3+) ions in the melilite structure (see Table 2). Although the antisite ordering resulted in large thermal parameters on the O(2) and O(3) sites, we are in the process of acquiring microscopic structural properties of these using neutron diffraction, which should reveal a more accurate mechanism for the antisite ordering. The optimum Eu3+ concentration in the study was 0.35 for Ca2+/Y3+ sites, which is such a small amount in comparison with other elements in the host lattice. Also, a phosphor is polycrystalline and contains some defects such as vacancies, dislocations, etc., inside the crystal. Thus, the

Figure 2. Rietveld refinement of the powder X-ray diffraction profile of CaY0.65Eu0.35Al3O7. Data (points) and fit (lines), the difference profile, and expected reflection positions are displayed.

Table 1. Rietveld Refinement and Crystal Data for CaY0.65Eu0.35Al3O7a formula radiation type 2θ range (degree) T/K symmetry space group a, b/Å c/ Å volume/ Å3 Z Rp Rwp χ2

1 |F σ2 0

CaY0.65Eu0.35Al3O7 Cu Kα 10−100 295 tetragonal P4̅21m 7.6989(2) 5.0583(2) 299.82(8) 2 2.37% 3.61% 3.339

a

The numbers in parentheses are the estimated standard deviations of the last significant figure.

In order to explore the local structure of the CYA:Eu3+ phosphor, the maximum entropy method (MEM) was used for the CYA:Eu3+ powder sample with an X-ray source at room temperature. Generally, the energy transfer among the Eu3+ ions located at the lanthanide site is also affected significantly by the local structure around the site.26,27 As one of the versatile methods to investigate local structure, MEM analysis

Table 2. Refined Structural Parameters for CaY0.65Eu0.35Al3O7 Obtained from the Rietveld Refinement Using X-ray Powder Diffraction Data at Room Temperaturea atom

Wyckoff symbol

x

y

z

gb

100 × Uisoc/Å2

Ca Y Eu Al(1) Al(2) O(1) O(2) O(3)

4e 4e 4e 2a 4e 2c 4e 8f

0.3394(1) 0.3394(1) 0.3394(1) 0 0.1443(2) 1/2 0.1426(1) 0.0917(2)

0.1605(1) 0.1605(1) 0.1605(1) 0 0.3556(1) 0 0.3573(2) 0.1592(2)

0.5118(1) 0.5118(1) 0.5118(1) 0 0.9598(2) 0.1868(1) 0.3004(2) 0.8041(1)

0.5 0.325 0.175 1.0 1.0 1.0 1.0 1.0

0.72(1) 0.72(1) 0.72(1) 0.25(2) 0.76(2) 0.40(1) 1.76(2) 1.28(2)

The numbers in parentheses are the estimated standard deviations of the last significant figure. CaY0.65Eu0.35Al3O7. bConstraint on occupancy: g(Ca) + g(Y) + g(Eu) = 1.0. cConstraint on isotropic thermal factor: Uiso(Ca) = Uiso(Y) = Uiso (Eu). a

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Table 3. Selected Interatomic Distances and Bond Angles for CaY0.65Eu0.35Al3O7 at Room Temperature CaY0.65Eu0.35Al3O7 distance (Å) Ma−O(1) M−O(2) M−O(2) M−O(3) M−O(3) a

polyhedron 2.3998(1) 2.3951(2) 2.5240(1) 2.4133(2) 2.8728(2)

distance (Å)

X2 X2 X2

angle (degree)

Al(1)−O(3)

tetrahedral coordination 1.7277(1) X4

Al(2)−O(1) Al(2)−O(2) Al(2)−O(3)

1.7384(2) 1.7230(2) 1.7524(2)

X2

109.2(1) 110.0(1) 114.6(1) 102.2(1) 117.0(1) 101.3(1)

X4 X2 X2 X2

M: Ca/Y (or Eu).

Generally, charge transfer (CT) transitions occur when a valence electron is transferred from the ligand toward the unoccupied orbitals of the metallic cation, namely, the excitation energy transfer from AlO45‑ groups to Eu3+ ions via Y3+ ions. In this study, the 260 nm absorption band is assigned to a charge transfer transition in the Eu3+−O2‑ bond: an electron jumps from O2−(2p6) to the empty orbital of 4f for Eu3+26 (see Figure 4). In the CYA:Eu3+, the excitation energy

Figure 4. (a) Relative emission intensity (5D0 → 7F2) as a function of Eu3+ substitution x (b) excitation and emission spectra of the CaY0.65Eu0.35Al3O7 under 394 nm excitation source.

Figure 3. (a) MEM isosurface of the electron density depicted within the unit cell of CYA:Eu3+. (a) Projection down the a axis of the unit cell and (b) projection down the c axis, showing the plane containing the 4e site with (Ca/Y (or Eu)) with O(2). Panel b shows the (002) plane of the structure showing a map of the MEM electron density. The scale bar for coloring of the map is depicted alongside, as a percentage of the strongest peaks in the MEM electron density.

transfer from AlO45‑ groups absorption to Eu3+ was probably realized by exchange processes as argued by Dexter.32 The probability of energy transfer required both the wave function overlap and the energy overlap. In this experiment, an incident X-ray during the measurement works as an excitation source for activators and the host lattice, like with scintillator phosphors. A trace of electron transfer in the CT band can be observed in the electron density distribution calculated by MEM analysis as shown in Figure 3, and an especially narrow electron density distribution is observed between Ca2+/Y3+ (or Eu3+) and O(2) elements. We applied the MEM to XRD data for CYA:Eu3+, and successfully visualized the CT band in CYA:Eu3+. 3.2. Optical Properties of CYA:Eu3+ Phosphor. Figure 4a shows the relative emission intensity as a function of Eu3+ concentration x. For CaY1‑xEuxAl3O7, the intensity increases

CYA:Eu3+ we designed in this study could be synthesized even though the defects are formed. In addition, as a charge compensator, alkali metals such as Li+, Na+, and K+ can sometimes be added to the host lattice. Generally speaking, the charge compensation can give positive/negative effects to a phosphor depending on the phosphor type. In this study, we could not obtain enhanced results by adding Li+ charge compensators in terms of PL intensity. In future studies, substitutions of Na+ and K+ as charge compensators in the title compound are anticipated to improve the phosphor efficiency, while healing the defects. 26853

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with increasing Eu3+ concentration up to x = 0.35 and decreases thereafter. The PL intensity was found to decrease for a concentration of doped Eu3+ greater than x = 0.35, which is attributed to the well-known concentration quenching effect. The concentration quenching occurs because of the energy transfer from one activator to another. The critical distance for energy transfer (Rc) in CYA:Eu3+ can be calculated from the structural parameters with unit cell volume (V) and the number of total Eu3+ sites per unit cell (N), together with the critical concentration (Xc).33 ⎡ 3V ⎤1/3 R c ≈ 2⎢ ⎥ ⎣ 4πXcN ⎦

(2)

For the CYA:Eu3+ system, N = 2, V = 299.939 Å3, and Xc = 0.35. The critical transfer distance of Eu3+ in CYA:Eu3+ is determined to be ∼9 Å. In particular, melilite-structure CYA has a high Eu3+ concentration without concentration quenching compared to other host materials. In the CYA structure (see Figure 1), the structure of CYA has two-dimensional layer alternating cationic (Ca/Y)2 and anionic Al3O7. Thus, the energy transfer between Eu3+−Eu3+ should be restricted to the two-dimensional cationic layer. Berdowski and Blasse34 reported that the interaction between the Eu3+ ions in twodimensional compounds is due to a superexchange interaction, which can describe compounds that have a high critical value of concentration quenching with low-dimensional compounds.16 Figure 4b indicates the excitation and emission spectra of the optimized CYA:Eu3+ phosphor under a 394 nm excitation at room temperature. The excitation spectrum shows a broad band and several sharp lines corresponding to the 4f 6 transitions of Eu3+ ions. These were ascribed to the transitions from the ground state (7F0) to each excited state of Eu3+. On the other hand, the broad band with a maximum at about 260 nm is due to the CT transition from O2‑ ions to Eu3+ ions. Emission lines of Eu3+ ion correspond to transitions from the excited 5D0 level to the 7FJ (J = 0−6) levels of the 4f 6 configuration. The ratio of the 5D0 → 7F1 and 5D0 → 7F2 transitions indicates that the sites have approximately no inversion symmetry.26 As a result, the intensity of emission assigned to the electric dipole transition of 5D0 → 7F2 is much stronger than that of emission assigned to the magnetic dipole transition of 5D0 → 7F1. As shown in Figure 5, the PL spectrum of CaY1‑xEuxAl3O7 (x = 0.02−0.4) phosphors under a 394 nm excitation are dominated by the transition of 5D0 → 7F2, regardless of the activator concentration. Figure 6 shows the PL spectrum of CYA:Eu3+ phosphor under a 394 nm excitation compared to that of the commercial red-emitting phosphor, CaAlSiN 3:Eu 2+, under 450 nm. Although the CaAlSiN3:Eu2+ phosphor showed about 158% wider area than that of the CYA:Eu3+ phosphor, due to the sharp emission line of Eu3+, the CYA:Eu3+ phosphor has 181% stronger emission intensity than the CaAlSiN3:Eu2+ phosphor. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of CYA:Eu3+ were (0.6303, 0.3607) which were closer to ideal white point compared to those of the CaAlSiN3:Eu2+ phosphor (0.6145, 0.3815). Thus, CYA:Eu3+ has better color purity than the CaAlSiN3:Eu2+. The quantum efficiency (QE) was measured using a excitation source at 394 nm. The QE from CYA:Eu3+ was obtained as ∼52% at room temperature. The diffuse reflectance absorption spectra of CYA and CYA:Eu3+ are shown in Figure 7. In the Eu3+ undoped phase,

Figure 5. PL spectra (λex = 394 nm) of CaY1‑xEuxAl3O7 phosphor samples prepared for various concentration of Eu3+: (x = 0.02, 0.04, 0.07, 0.1, 0.15, 0.2, 0.3, 0.35, and 0.4).

Figure 6. PLE spectra and PL spectra of CaY0.65Eu0.35Al3O7 phosphor (λex = 394 nm), compared to the PL spectrum of a commercial CaSiAlN3:Eu2+ phosphor (λex = 450 nm).

CYA exhibits no absorption. However, CYA:Eu3+ consists of a broad band at around 260 nm due to the charge transfer and absorption line of the f−f transition of the Eu3+ ions. The body color of this phosphor was weak-red, unlike the CYA host lattice. Figure 8 shows the temperature quenching characteristics of the commercial Sr2SiO4:Eu2+ (Force4 Corp.), CaAlSiN3:Eu2+ (Force4 Corp.) and CYA:Eu3+ phosphors in the temperature range from room temperature to 200 °C. As the temperature increases from RT to 200 °C, the PL intensities of CYA:Eu3+ decreased by 6 and 8% of the initial PL intensity, corresponding to 150 and 200 °C, respectively, as shown in Figure 8. Compared with the commercial yellow-emitting phosphor, the thermal quenching properties of the CYA:Eu3+ showed more stability, but the CaAlSiN3:Eu2+ phosphor was the most stable in this feasibility test. 3.3. Fabrication of Prototype White LEDs. Figure 9 shows electroluminescence (EL) spectra and CIE chromaticity 26854

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Figure 7. UV−visible diffuse reflection spectra of CaY0.65Eu0.35Al3O7 and CaYAl3O7.

Figure 9. Luminescence of the InGaN LED + phosphor, under different forward bias currents (indicated): InGaN (λmax = 396 nm) + CYA:Eu3+ + blue, green phosphor (commercial).

method and investigated their structural and optical properties by maximum entropy method (MEM) assisted by Rietveld refinement and optical measurements. From the MEM calculation with an X-ray source, the charge transfer band and defects resulting from antisite ordering were seen in the electron density distribution. Through an investigation of the luminescent properties of the CYA:Eu3+ phosphor, the critical transfer distance for this phosphor is 9 Å. Applying CYA:Eu3+ (x = 0.35) on InGaN LEDs (λmax = 396 nm), we obtain WLEDs outputting 3.2 lm/W at 20 mA, with a color rendering index of 77 and a color temperature of 9200 K.

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Figure 8. Temperature-dependent emission intensities of the commercial Sr2SiO4:Eu2+ phosphor, red-emitting CaSiAlN3:Eu2+ phosphor (Force4 Corp.) and CaY0.65Eu0.35Al3O7 phosphor.

AUTHOR INFORMATION

Corresponding Author

*Tel: +82-62-530-1715. Fax: +82-62-530-1699. E-mail: [email protected].

coordinates the device fabricated with the blue BaMgAl10O17:Eu2+, green Sr2SiO4:Eu2+ (commercial), and CYA:Eu3+ phosphor on an InGaN LED (λmax = 396 nm) under different forward bias currents in the range of 20 to 100 mA as indicated. LEDs were operated at a voltage ranging from 3.1 to 3.6 V. From the observed CIE chromaticity coordinates (0.26, 0.35) under 20 mA, we obtained a white LED with a color rendering index of about 77 and a luminous efficacy of 3.2 lm/W. Different LED performance, maximum wavelength, ratio of epoxy resin to phosphor powder as well as the use of mixing agents might be responsible for the white LED performances, and it is anticipated that these properties can be further improved through optimization.

Author Contributions ∥

Contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation. This work was also supported by the Materials and Components Research and Development funded by the Ministry of Knowledge Economy (MKE, Korea).

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4. CONCLUSIONS We have successfully synthesized CaY 0.65 Eu 0.35 Al 3 O 7 (CYA:Eu3+) powder samples using a solid-state reaction

REFERENCES

(1) Schubert, E. F.; Kim, J. K. Science 2005, 308 (5726), 1274−1278.

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dx.doi.org/10.1021/jp307192y | J. Phys. Chem. C 2012, 116, 26850−26856