Narrow-Band Green-Emitting Phosphor ... - ACS Publications

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Chemistry of Materials

Narrow-Band Green-Emitting Phosphor Ba2LiSi7AlN12:Eu2+ with High Thermal Stability Discovered by a Single Particle Diagnosis Approach Takashi Takeda*, Naoto Hirosaki, Shiro Funahshi, Rong-Jun Xie Sialon Unit, National Institute for Materials Science, Tsukuba 305-0044, Japan ABSTRACT: The narrow-band green-emitting phosphor Ba2LiSi7AlN12:Eu2+ was discovered by analyzing a single particle in a powder mixture, which we call the Single Particle Diagnosis Approach. Single crystal X-ray diffraction (XRD) analysis of the particle revealed that Ba2LiSi7AlN12:Eu2+ crystallizes in the Pnnm space group (No. 58) with a = 14.0941 Å, b = 4.8924 Å, c = 8.0645 Å, and Z = 2. The crystal structure is composed of a corner-sharing (Si,Al)N4 corrugated layer and edge-sharing (Si,Al)N4 and LiN4 tetrahedra. Ba(Eu) occupies the one-dimensional channel in a zigzag manner. The luminescence properties were also measured using a single crystalline particle. Ba2LiSi7AlN12:Eu2+ shows a green luminescence peak at approximately 515 nm with a narrow full-width at half-maximum (FWHM) of 61 nm. It shows high quantum efficiency (QE) of 79 % with 405-nm excitation and a small decrease of luminescence intensity, even at 300 °C.

Introduction The use of white-light-emitting diodes (white LEDs) is now rapidly spreading to several applications (lighting, display backlight, car headlamps, etc.). A white LED is composed of blue LED and a phosphor. The combination of luminescence from the phosphor excited by LED and emission from the LED produce white light. The phosphor is a key material for governing the color characteristics of white LEDs, including color rendering in the lighting and color reproduction in the backlight. The high luminescence intensity of phosphor contributes to the high efficiency of white LEDs. Although the conventional white LED phosphor, YAG:Ce, has a high luminescence intensity, the luminescence spectrum is not suitable for high-color-rendering white LEDs used for lighting, and the matching to the color filter is not good in the backlight. Alternative phosphors have been investigated. Some nitride and oxynitride phosphors ((Ca1-xSrx)AlSiN3:Eu2+1, (Ba1-xSrx)2Si5N8:Eu2+2, Cam/2Si12-m-nAlm+nOnN16-n:Eu2+3, and Si6-zAlzOzN8-z:Eu2+4) were found to have excellent luminescence properties that would improve color rendering and matching, and they have been commercialized. New phosphors are still required to 1) obtain highly varied emission spectra in terms of peak position and peak width to produce various types of white LED, 2) contribute to the coming change in the emission wavelength of LEDs (near-UV LED), and 3) generate a high-power LED, which suffer from more predominant thermal quenching of luminescence. Recently, a new red phosphor was reported, and it has attracted much attention due to its narrow-band-emitting spectrum which makes it suited to several applications.5-7 A new green phosphor is also a type of required phosphor, as well as the red phosphor.

The oxynitride green phosphor Si6-zAlzOzN8-z:Eu2+ has a narrow full-width at half-maximum (FWHM), which is suitable for backlight applications.8 However, the doping amount of Eu2+ possible in Si6-zAlzOzN8-z is limited, and the luminescence intensity is not high despite the high internal quantum efficiency (QE).4,9 In (Ba,Sr)2SiO4:Eu2+ oxide green phosphor, the luminescence shows severe thermal quenching.10 Although other green phosphors have been reported11-15, new green phosphors are required for the improvement of white LEDs. In the synthesis of phosphors, a small amount of a luminescent center is doped into a host material to obtain luminescence. Because the 5d–4f emission of Eu2+/Ce3+ has allowed transition probability, Eu2+/Ce3+-doped phosphors are a main target to develop new phosphors for LED applications. In those phosphors, the luminescence property is strongly affected by the coordination environment of the luminescent center. That is to say, the usage of different host materials leads to new phosphors. The search for new phosphors can be achieved using two approximate classes of methods. One is to investigate known crystal structures that are suitable for luminescent center doping and are not studied to be host crystals.1-4 The other is to find new host materials by analyzing single crystals5-7,11,13-18 or powder products12,19,20. In the single crystal analysis, a large-size crystal (typically larger than 100 μm in all dimensions) is necessary to determine the crystal structure. Growing the crystal is a time-intensive process, especially for materials with high melting temperatures or no liquid phase. In the powder process, it is necessary to at least synthesize the new material as a single phase to solve the crystal structure. Even if the powder consists of a single phase of the new material, the complicated crystal structure is difficult to solve (e.g., crystal

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structures with large lattice parameters and low symmetry or crystal structures containing disorder). Recently, we have developed a new efficient method to find new phosphors from a powder synthesized by a standard process.21 We have named the new method “Single Particle Diagnosis Approach”. Even if the powder product is not a single phase, by considering each (isolated) particle, it can be treated as a single phase and a single crystal. We choose the single particle from the product mixture and analyze its crystal structure and the luminescence properties. The most cutting-edge commercial single crystal X-ray diffraction (XRD) apparatus can solve the crystal structure down to 5~10 μm in the laboratory. We have established an optical measurement system for one particle (emission and excitation spectra, temperature dependence, and QE). The merits of this method are summarized as follows: 1) It is not necessary to synthesize the new phosphor as a single phase powder. 2) It is not necessary to grow a large crystal of the new phosphor. 3) The true luminescence properties of the given composition are obtained because the single crystal has no compositional or structural distributions as found in the powder sample. 4) The small amount of a new phosphor (including unintended new phosphors) in a powder sample is not overlooked because all produced particles are candidates. Because the luminescence property is highly dependent on the crystal structure and composition, it is not difficult to select a particle from among many candidate particles by using the luminescence as a clue. Here, we describe a new green phosphor, Ba2LiSi7AlN12:Eu2+, discovered by the Single Particle Diagnosis Approach. It shows high luminescence intensity with a narrow FWHM and high thermal stability.

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structure was solved by direct methods implemented in SHELXL-97.23 Refinement of the crystal structure was carried out with anisotropic displacement parameters for all atoms by full-matrix least-squares calculation on F2 in SHELXL-2013.23 The elemental analysis was conducted using a scanning electron microscope (Hitachi Hightechnology, SU1510) equipped with an energy dispersive spectroscopy instrument (Bruker AXS, XFlash SDD) operated at 15 kV. The Li/Si ratio was analyzed by laser ablation inductively coupled plasma mass spectrometry (LAICP-MASS) with He as the carrier gas (CETAC,

LSX213F2 and ThermoFisher, iCAP Qc) using LiSi2N3 (laboratory made), NIST1834, and BCR126A as the standard materials. To determine the valence of Eu, a microbeam X-ray absorption fine structure (XAFS) analysis was carried out at the BL37XU beamline of the SPring8 synchrotron radiation facility (Hyogo, Japan).24 The luminescence of one phosphor particle was measured using a microspectroscopic method.21 The monochromatized and focused light from a Xe lamp (Otsuka electronics, QE2100) was applied to the particle, and the luminescence from the particle was observed by a spectrometer (Otsuka electronics, MCPD7700) through a microscope (Olympus, BX51M). The calibration of the instrument was performed with a Lambertian diffuser (Labsphere, Spectralon). In the temperature dependence measurement from room temperature to 300 °C, the particle was positioned on a heater stage (Linkam Scientific Instruments, THMS600). An xyz stage was used to cancel the shift resulting from the sample movement caused by the temperature. For the quantum efficiency measurement, the particle was placed in a 1-in integrated half sphere (Otsuka electronics, HalfMoon) and irradiated by a 405-nm laser (Thorlabs, S1FC405) through a focusing lens. The particle’s absorption was obtained by subtracting the spectrum of the particle and BaSO4 from the reflection spectrum of BaSO4 (without the particle). The internal QE was obtained by the following equation: ݅ܳ‫= ܧ‬

Experimental Section The starting materials of Ba3N2 (Cerac, 99.7%), EuN (laboratory made), Si3N4 (Ube, E10), AlN (Tokuyama, Egrade), and Li3N (Kojundo Chemical, 2N) were mixed in a cation molar ratio of Ba:Eu:Si:Al:Li = 0.80:0.20:0.58:6.42:3.00 in a globe box under nitrogen atmosphere. The mixture was filled in a boron nitride crucible and fired in a nitrogen atmosphere of 1.0 MPa at 1800 °C for 2 h (Fujidempa Kogyo, FVPHR-R-10, FRET40). The experiments after the synthesis were all carried out in air. The product was irradiated by 370-nm UV LED light, and luminescent particles were selected under microscopic observation. The selected particles were mounted at the top of a glass capillary with glue, and the single crystal XRD data of the single particle were collected using a diffractometer (Bruker-AXS, SMART APEX II Ultra) with Mo Kα radiation (λ = 0.71073 Å) and multilayer focusing mirrors as a monochromator operated at 50 kV and 50 mA. The absorption corrections were applied using the multiscan procedure SADABS.22 The crystal

‫ ׬‬λ ∙ ܲ(ߣ)݀ߣ ‫ߣ ׬‬ሼ‫ )ߣ(ܧ‬− ܴ(ߣ)ሽ݀ߣ

where E(λ)/hν, R(λ)/hν, and P(λ)/hν are the number of photons in the excitation, reflectance, and emission spectra of the phosphor, respectively.25 The decay property was measured with a time-controlled single photon counting (TCSPC) technique. A pulsed LED of 370 nm and a photon counter (Horiba Jobin Yvon, Nano-LED and TBX) were coupled to microscope (Olympus, BX51M). A low-energy pass filter (420 nm) was used to block the incident light. In the powder synthesis of the new phosphor, the above starting materials were mixed based on the analyzed composition and fired at 1700 °C and 1800 °C. The product was characterized by a powder XRD instrument (Rigaku, SmartLab CuKα1 (λ = 1.5406 Å)) and a powder spectrofluorometer (Otsuka electronics, QE1100). Result and Discussion


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Synthesis and Crystal structure We have surveyed a variety of Ba:Eu:Si:Al:Li cation compositions and identified a new green phosphor using a composition of Ba:Eu:Si:Al:Li = 0.80:0.20:0.58:6.42:3.00. The product obtained using this composition was a mixture of many luminescent particles, as shown in Figure 1a. Out of the many luminescent particles, the green luminescent particle shown in Figure 1b was found to be a new phosphor. It was formed as a plate shape with dimensions of 46 μm × 31 μm × 11 μm, which make it suitable for analysis by conventional XRD. Although some tiny particles, which probably consist of the same new green phosphor, are present, their effect on the characterization results will be small. Other luminescent particles with different emission colors were identified as BaSi7N10:Eu2+ (blue emission)26, Ba2Si5N8:Eu2+ (red emission)2, and 2+ 27 BaSi6N8:Eu (blue emission) based on single crystal XRD analysis. The actual composition will likely deviate from the nominal composition because of the introduction other elements (Al, Li, and O). Here, we use the nominal composition to simplify the analysis. The same convention is used with regard to the impurities in the powder synthesis discussed later. Table 1. Crystallographic Data of Ba2LiSi7AlN12:Eu2+ Formula mass / g·mol-1

675.53

Crystal system

Orthorhombic

Space group

Pnnm (No.58)

Cell parameters / Å V / Å3 Density / g·cm

a=14.0993(2),

b=4.89670(10),

c=8.07190(10) 557.28(15)

−3

4.048

Z

2

Crystal size / mm

3

0.046 x 0.031 x 0.011

Temperature / K

295

Color

Clear light yellow

Diffractometer

Bruker APEXII CCD area detector

Radiation type

Mo Kα (λ = 0.71073 Å)

Scan mode

ω scan

Abs correction

Multiscan (SADABS)

µ / mm-1

8.182

2θmax / deg

45.28

Measured reflections

31576

Independent reflections

2441

Observed reflections

2214

Tmin, Tmax

0.6758, 0.7489

R[F2 > 2σ(F2)], wR(F2) ∆ρmax, ∆ρmin / e Å S

-3

0.0149, 0.0380

The single crystal XRD analysis showed that the new phosphor has an orthorhombic unit cell of a = 14.0993(2) Å, b = 4.89670(10) Å, and c = 8.07190(10) Å with the Pnnm space group (No. 58). The crystal structure that was finally obtained is depicted in Figure 2. Because of the similar ionic radii, Si/Al were constrained to occupy the same site, as seen in other Si/Al compounds. Si/Al occupies the tetrahedral site (blue tetrahedron), and Ba occupies the site coordinated by eleven nitrogen atoms (green polyhedron). The structural details are presented later. In the initial stage, the refinement was carried out without introducing Li. In the difference Fourier synthesis, there was residual electron density (5.67 e Å-3) at an edgesharing tetrahedral site (red tetrahedron). Ba occupation at the tetrahedral site was rejected due to its large size. The model of Si/Al occupation at the tetrahedral site showed a negative isotropic atomic displacement parameter, suggesting that a much lighter element is present. By positioning Li at the tetrahedral site, a reasonable value was obtained for the anisotropic atomic displacement parameter. The bond valence sum at the Li site was calculated to be 1.0.28 From the site multiplicity and occupancy of each site, the composition was determined to be (Ba,Eu)2Li(Si,Al)8N12. Considering the electrical neutrality, the Si/Al ratio was determined to be 7:1. This value corresponds well with the cation ratio obtained by EDS analysis (Ba:Eu:Si:Al = 1.8:0.2:6.9:1.1). Eu was restricted to occupy the Ba site in the above analysis. However, in the difference Fourier synthesis, there was obvious residual electron density (6.33 e Å-3) near the Ba site. Eu was positioned at the site where the electron density was observed, and the atomic coordinates and atomic displacement parameter of Ba and Eu were independently refined. The reliability factor decreased from 2.12 % to 1.49 % for R1 and from 5.26 % to 3.80 % for wR2 and the refined occupancy parameter (Ba:Eu = 0.925:0.075) was close to that of the EDS analysis (Ba:Eu = 0.9:0.1). The ionic radius of Ba2+ is larger than that of Eu2+, and the stable position for Eu2+ in the large polyhedron should be slightly different from that of Ba2+. Such a shift is observed in other nitride and oxynitride materials.30,31 Using the high-resolution diffraction data, it is possible to distinguish the dopant site from the host structure site. The Ueq at Ba/Eu sites is larger than that at Si/Al sites. Ba/Eu occupies the large polyhedron site formed by the Si(Al)N4 tetrahedral network with strong covalent bonding. This may be the reason for the large Ueq of Ba/Eu. The large Ueq of the Ba site has been reported in other Ba-Si-N and Ba-Si-N-O compounds.32,33

1.894, -1.094 1.047

Table 2. Atomic Coordinates, Occupancies, and Isotropic Atomic Displacement Parameters of Ba2LiSi7AlN12:Eu2+



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Atom

x

y

Z

Ueq / Å2

Occupancy

Ba1

0.10198(2)

0.30128(7)

0.5

0.01243(6)

0.925(6)

Eu1

0.1038(4)

0.266(3)

0.5

0.0309(11)

0.075(6)

Si1

0.18162(2)

0.27738(6)

1

0.00363(5)

0.875

Si2

0.31414(2)

0.29054(4)

0.69891(3)

0.00438(4)

0.875

Si3

0

0

0.84130(4)

0.00558(5)

0.875

Al1

0.18162(2)

0.27738(6)

1

0.00363(5)

0.125

Al2

0.31414(2)

0.29054(4)

0.69891(3)

0.00438(4)

0.125

Al3

0

0

0.84130(4)

0.00558(5)

0.125

Li1

0

0.5

1.1097(8)

0.0157(9)

0.5

N1

0.06367(7)

0.17581(18)

1

0.00581(12)

1

N2

0.19892(7)

0.62812(19)

1

0.00647(12)

1

N3

0.07207(6)

-0.20073(15)

0.72214(10)

0.01088(11)

1

N4

0.23128(5)

0.12621(14)

1.17378(9)

0.00730(9)

1

Finally, the composition was assigned as (Ba0.925Eu0.075)2LiSi7AlN12. The crystallographic data and the atomic coordinates and isotropic displacement parameters are shown in Tables 1 and 2, respectively. The anisotropic atomic displacement parameters and selected interatomic distances are listed in the supporting information. The LA-ICP-MASS analysis clearly showed the presence of Li in the particle, and the Li:Si ratio was determined to be 1.4:7.0. The difference from the estimated composition is ascribed to the low accuracy of the Si content value because of N2 contamination in the carrier gas. Si/Al occupy the tetrahedral site (blue tetrahedron), and Li occupies the independent tetrahedral site (red tetrahedron), as shown in Figure 2. The Li occupancy is 0.5, and either of the sites in the edge-sharing tetrahedra are occupied by Li. Vertex-sharing (Si,Al)N4 tetrahedra form a corrugated layer (marked A) along the c axis direction. Edge-sharing (Si,Al)N4 tetrahedra and edge-sharing LiN4 tetrahedra alternately align along the b direction and form a pillar (marked B). The corrugated layer (A) and pillar (B) form a large one-dimensional channel along the b direction. Ba occupies the one-dimensional channel in a zigzag manner along the b direction. There is only one crystallographic site for Ba. Ba is coordinated by eleven N atoms, and the distance between Ba and N ranges from 2.93 Å to 3.32 Å with an average distance of 3.12 Å. The BaN11 polyhedra are linked by face-sharing, and the distance between Ba atoms is 3.49 Å. Eu occupies the position that is 0.2-Å away from the Ba site and emits green luminescence. Eu is also coordinated by eleven N atoms, and the distance between Eu and N ranges from 2.86 Å to 3.24 Å with an average distance of 3.12 Å. Luminescence properties

The emission and excitation spectra from one particle of Ba2LiSi7AlN12:Eu2+ phosphor are shown in Figure 3. The emission spectrum has a peak at approximately 515 nm with a FWHM of 61 nm (2280 cm−1). The excitation spectrum spans 350 nm to 450 nm. This is attributed to the allowed electronic transition from 4f65d1 to 4f7 of Eu2+. Although the FWHM is relatively wide compared with that of very narrow green-emitting phosphor Si6-zAlzOzN82+ (55 nm), it is fairly narrow for a Eu2+ emitting phosz:Eu phor and is suitable for backlight applications. The Commission International de l’Eclairage (CIE) chromaticity coordinates based on the emission (x = 0.24 and y = 0.61) are outside of the triangle of the sRGB standard. The blue-green color gamut will be extensively expanded by this phosphor. In the powder sample, the FWHM will be reduced because the higher energy range of the emission spectrum is absorbed by other Ba2LiSi7AlN12:Eu2+ particles.21 The coexistence of Eu2+ and Eu3+ is sometimes observed, even if there is no sharp red emission of the Eu3+ f-f transition in the luminescence spectra.34 To determine the valence of the doped Eu, microbeam XAFS analysis of Eu L3 edge was carried out in the fluorescence mode. The beamsize is below the particle size. The X-ray absorption near edge structure (XANES) region is shown in Figure 4. Although the S/N was low, there was a clear absorption corresponding to Eu2+ at approximately 6972 eV. Additionally, there seems to be a shoulder in the high energy region. High S/N data are necessary to discuss the presence of Eu3+. The temperature dependence of the emission spectra, peak intensity and integrated intensity are shown in Figure 5. The peak intensity gradually decreased with temperature, and the peak intensities at 200 °C and 300 °C were 84 % and 76 % of the intensity observed at RT, respectively. At 200 °C and 300 °C, the integrated intensities


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were 95% and 91% of that at room temperature, respectively. This characteristic is important for the application of high-power LED, which suffer from more predominant thermal quenching. The peak top was shifted to shorter wavelengths by 4 nm, and the spectra became broader, especially in the shorter wavelength region, as the temperature increased. The decrease in the peak intensity is canceled by the broadening of the spectra. The degree of structure condensation expressed by the ratio between the framework cation (Li,Si,Al) and anion (N) is 0.75. This is a high value for an Si-N-based material like Si3N4. The rigid condensed structure of this material is one reason for the high thermal stability of this phosphor.35 The QE and decay were also measured using one particle of the Ba2LiSi7AlN12:Eu2+ phosphor. Both absorption and emission are clearly recognized from the single particle, as shown in Figure 6. The spectral intensity was converted to photon number, and the internal QE by 405-nm excitation was found to be 79 %. The decay curve shown in Figure 7 was analyzed by two exponentials, and the decay times were 0.41 µs (44%) and 0.75 µs (56%). The crystallographic site for Eu is one in the Ba2LiSi7AlN12 structure. Some static structural disorder (oxygen incorporation, Si/Al inhomogeneous distribution, Li deficiency, or Eu occupation at the Ba site) may be present. Powder synthesis The crystal structure and luminescence properties of Ba2LiSi7AlN12:Eu2+ were elucidated from one particle. Subsequently, the synthesis of Ba2LiSi7AlN12:Eu2+ powder was attempted. The mixed starting materials corresponding to the 5% Eu-doped composition, (Ba0.95Eu0.05)2LiSi7AlN12, was fired at the same temperature. Compared to the nominal composition, the product was contaminated with a large amount of impurity phases, and the ratio of the Ba2LiSi7AlN12:Eu2+ phase was found to approximately 50 % by profile fitting analysis of the XRD pattern. The Li component is easily evaporated in the high-temperature synthesis, and the composition of the product thus deviates from the target composition. Hence, the Li content was increased to twice the nominal composition, and the synthesis temperature was lowered to 1700 °C. Figure 8 shows the XRD pattern of the product. The ratio of the Ba2LiSi7AlN12:Eu2+ phase increased to 80%, and the impurity phases were Ba2Si5N8:Eu2+ and BaSi6N8:Eu2+. In the luminescence spectra, in addition to the green luminescence from Ba2LiSi7AlN12:Eu2+, red emission from Ba2Si5N8:Eu2+ was clearly observed, as shown in the inset of Figure 8. The FWHM of the green luminescence (66 nm) was broader than that of the single particle. This may be attributed to the overlapping of the luminescence of the impurity phases and the compositional and structural distributions of Ba2LiSi7AlN12:Eu2+ particles. Further detailed composition studies and investigations of the experimental conditions (starting composition, synthesis temperature, holding time, elevating speed, etc.) will be necessary to obtain a single phase powder of Ba2LiSi7AlN12:Eu2+ with a narrow FWHM.

Conclusions In this paper, a new green nitride phosphor, Ba2LiSi7AlN12:Eu2+, was discovered by analyzing a particle from the powder product generated using the Ba:Eu:Si:Al:Li = 0.80:0.20:0.58:6.42:3.00 composition. The new phosphor showed a green luminescence peak at approximately 515 nm with a FWHM of 61 nm, and the internal QE by 405-nm excitation was 79 %. This phosphor showed only a small decrease in luminescence intensity even at 300 °C. This new green phosphor will be a promising candidate for backlight applications, in which narrowband-emitting is required, and high-power LEDs, which suffer from more predominant thermal quenching. Because the analyzed particle is a single crystal with high crystallinity, the luminescence properties will be close to the highest values obtained from the composition and structure. However, there is room for improvement. The Eu concentration is one important parameter that controls the luminescence properties (emission wavelength, peak intensity, and thermal quenching). Sr substitution at the Ba site can change the coordination structure around Eu. Both the Si/Al and N/O ratios are adjustable. Such compositional and structural change will lead to improved luminescence properties. In the powder synthesis, some impurity phases were observed. Ba2Si5N8 was discovered during the early stages of nitridosilicate research36 and is easily formed as an impurity phase in the synthesis of Ba-Si-containing nitride phosphors. The optimization of the experimental conditions is necessary to obtain Ba2LiSi7AlN12:Eu2+ phosphor as a single phase.

ASSOCIATED CONTENT Supporting Information. Anisotropic displacement parameters, selected interatomic distances, and X-ray crystallographic information file (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

ACKNOWLEDGMENT We thank K. Nakajima (NIMS) for help with the experiments. The synchrotron radiation experiments were performed at the BL37XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2014B1075).

REFERENCES (1) Uheda, K.; Hirosaki, N.; Yamamoto, Y.; Naoto, A.; Nakajima, T.; Yamamoto, H. Luminescence Properties of a


Chemistry of Materials 2+

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Red Phosphor, CaAlSiN3:Eu for White Light-Emitting Diodes. Electrochem. Solid State Lett. 2006, 9, H22-H25. (2) Li, Y. Q.; van Steen, J. E. J.; van Krevel, J. W. H.; Botty, G.; Delsing, A. C. A.; Disalvo, F. J.; de With, G.; Hintzen, H. T. 2+ Luminescence properties of red-emitting M2Si5N8:Eu (M = Ca, Sr, Ba) LED conversion phosphors. J. Alloys Compd. 2006, 417, 273-279. (3) Xie, R. –J.; Hirosaki, N.; Sakuma, K.; Yamamoto, Y.; Mi2+ tomo, M. Eu -doped Ca-α-SiAlON: A yellow phosphor for white light-emitting diodes. Apl. Phys. Lett. 2004, 84, 54045406. (4) Hirosaki, N.; Xie, R. -J.; Kimoto, K.; Sekiguchi, T.; Yamamoto,Y.; Suehiro, T.; Mitomo, M. Characterization and 2+ properties of green-emitting β-SiAlON:Eu powder phosphors for white light-emitting diodes. Appl. Phys. Lett. 2005, 86, 211905. (5) Pust, P.; Weiler, V.; Hecht, C.; Tucks, A.;, Wochnik, A. S.; Henß, A. K.; Wiechert, D.; Scheu, C.; Schmidt, P. J.; 2+ Schnick, W. Narrow-band red-emitting Sr[LiAl3N4]:Eu as a next-generation LED-phosphor material. Nature Mat. 2014, 13, 891-896. (6) Schmiechen, S.; Schneider, H.; Wagatha, P.; Hecht, C.; Schmidt, P. J.; Schnick, W. Toward New Phosphors for Application in Illumination-Grade White pc-LEDs: The Nitrid3+ 2+ omagnesosilicates Ca[Mg3SiN4]:Ce , Sr[Mg3SiN4]:Eu , and Eu[Mg3SiN4]. Chem. Mater. 2014, 24, 2712−2719. (7) Pust, P.; Wochnik, A. S.; Baumann, E.; Schmidt, P. J; 2+ Wiecher, D.; Scheu, C.; Schnick, W. Ca[LiAl3N4]:Eu -A Narrow-Band Red-Emitting Nitridolithoaluminate. Chem. Mater. 2014, 24, 3544−3548. (8) Xie, R. -J; Hirosaki, N.; Takeda, T. Wide Color Gamut Backlight for Liquid Crystal Displays Using Three-Band Phosphor-Converted White Light-Emitting Diodes. Appl. Phys. Express. 2009, 2, 022401. (9) Xie, R. -J; Hirosaki, N.; Li, H. -L.; Li, Y. Q.; Mitomo, M. 2+ Synthesis and Photoluminescence Properties of β-sialon:Eu 2+ (Si6−zAlzOzN8−z:Eu ). Electrochem. Soc. 2007, 154, J314−J319. (10) Kim, J. S.; Park, Y. H.; Kim, S. M. Emission color varia2+ tion of M2SiO4:Eu (M=Ba, Sr, Ca) phosphors for lightemitting diode. Solid State Comm. 2005, 133, 187-190. (11) Mueller-Mach, R; Mueller, G.; Krames, M. R.; Höppe, H. A.; Stadler, F.; Schnick, W.; Juestel, T.; Schmidt, P. Highly efficient all-nitride phosphor-converted white light emitting diode. Phys. Stat. Sol. (a) 2005, 202, 1727–1732. (12) Mikami, M.; Shimooka, S.; Uheda, K.; Imura, H.; Kijima, N. New Green Phosphor Ba3Si6O12N2:Eu for White LED: Crystal Structure and Optical Properties. Key Eng. Mat. 2009, 403, 11-14. (13) Fukuda, Y.; Ishida, K.; Mitsuishi, I.; Nunoue, S. Lumi2+ nescence Properties of Eu -Doped Green-Emitting Sr-Sialon Phosphor and Its Application to White Light-Emitting Diodes. Appl. Phys. Express. 2009, 2, 012401. (14) Oeckler, O.; Kechele, J. A.; Koss, H.; Schmidt, P. J.; 2+ Schnick W. Sr5Al5+xSi21-xN35-xO2+x:Eu (x≈0)—A Novel Green Phosphor for White-Light pcLEDs with Disordered Intergrowth Structure. Chem. Euro. J. 2009, 15, 5311-5319. (15) Shioi, K.; Michiue, Y.; Hirosaki, N.; Xie, R. -J.; Takeda, T.; Matsushita, Y.; Tanaka, M.; Li, Y. Q. Synthesis and photo2+ luminescence of a novel Sr-SiAlON:Eu blue-green phosphor 2+ (Sr14Si68-sAl6+sOsN106−s:Eu (s≈7)). J. Alloys and Compds. 2011, 332–337.

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(16) Hintze, F.; Hummel, F.; Schmidt, P. J.; Wiechert, D.; 2+ Schnick, W. Ba3Ga3N5-A Novel Host Lattice for Eu -Doped Luminescent Materials with Unexpected Nitridogallate Substructure. Chem. Mater. 2012, 24, 402−407. (17) Kechele, J. A.; Hecht, C.; Oeckler, O.; auf der Gunne, J. S.; Schmidt, P. J.; Schnick, W. Ba2AlSi5N9-A New Host Lattice 2+ for Eu -Doped Luminescent Materials Comprising a Nitridoalumosilicate Framework with Corner- and EdgeSharing Tetrahedra. Chem. Mater. 2009, 21, 1288−1295. (18) Hoppe, H. A.; Lutz, H.; Morys, P.; Schnick, W.; Seil2+ meier, A. J. Luminescence in Eu -doped Ba2Si5N8: fluorescence, thermoluminescence, and upconversion. Phys. Chem. Solids. 2000, 61, 2001−2006. (19) Son, K. H.; Singh, S. P.; Sohn, K. S. Discovery of novel phosphors for use in light emitting diodes using heuristics optimization-assisted combinatorial chemistry. J. Mater. Chem. 2012, 22, 8505−8511. (20) Park, W. B.; Shin, N.; Hong, K. P.; Pro, M.; Sohn, K. S. A New Paradigm for Materials Discovery: Heuristics-Assisted Combinatorial Chemistry Involving Parameterization of Material Novelty. Adv. Funct. Mater. 2012, 22, 2258−2266. (21) Hirosaki, N.; Takeda, T.; Funahashi, A.; Xie. R. –J. Discovery of New Nitridosilicate Phosphors for Solid State Lighting by the Single-Particle-Diagnosis Approach. Chem. Mat. 2014, 26, 4280. (22) Sheldrick, G. M. SADABS, v. 2: Multi-Scan Absorption Correction; Bruker-AXS: WA, 2012. (23) Sheldrick, G. M. A short history of SHELX. Acta Cryst. 2008, A64, 112–122. (24) Terada Y.; Tanida H.; Uruga T.; Takeuchi A.; Suzuki Y.; Goto S. High-Resolution X-ray Microprobe Using a Spatial Filter and Its Application to Micro-XAFS Measurements. AIP Conf. Proc. 2000, 1365, 172-175. (25) Ohkubo, K; Nakagawa, Y. Quantum Efficiency Measurement of Lamp Phosphors in Accordance with Radiometric Standards. J. Light & Vis. Env. 2013, 37, 16-27. (26) Li, Y. Q.; Delsing, A. C. A.; Metslaar, R.; de With G.; Hintzen, H. T. Photoluminescence properties of rare-earth activated BaSi7N10. J. Alloys Compd. 2009, 487, 28-33. (27) Shioi, K.; Hirosaki, N.; Xie, R. -J.; Takeda, T.; Li, Y. Q. 2+ Luminescence properties of SrSi6N8:Eu . J. Mater. Sci. 2008, 43, 5659−5661. (28) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Crystallogr. 1985, B41, 244-247. (29) Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272-1276. (30) Huppertz, H.; Schnick, W. Synthesis and Crystal Structure of BaEu(Ba0.5Eu0.5)YbSi6N11. Z. anorg. allg. Chem. 1998, 624, 371-374. (31) Stadler, F; Oeckler, O.; Schnick, W. Synthese, Kristallstruktur und festkörper-NMR-spektroskopische Untersuchung neuer Oxonitridosilicate der Mischkristallreihe Ba4xCaxSi6N10O. Z. Anorg. Allg. Chem. 2006, 632, 54-58. (32) Huppertz, H.; Schnick, W. Edge-sharing SiN4 Tetrahedra in the Highly Condensed Nitridosilicate BaSi7N10. Chem. Eur. J. 1997, 3, 259-252. (33) Stadler, F.; Kraut,R.; Oeckler, O.; Schmid, S.; Schnick, W. Synthesis, Crystal Structure and Solid-State NMR Spectroscopic Investigation of the Oxonitridosilicate BaSi6N8O. Z. Anorg. Allg. Chem. 2005, 631, 1773-1778.


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(34) Shioi, K.; Hirosaki, N.; Xie, R. -J.; Takeda, Li, Y. Q. Synthesis, crystal structure and photoluminescence of Eu-αSiAlON. J. Alloys and Compds. 2010, 504 579–584. (35) Brgoch, J.; , DenBaars , S. P.; Seshadri, R. Proxies from 3+ Ab Initio Calculations for Screening Efficient Ce Phosphor Hosts. J. Phys. Chem. C 2013, 117, 17955−17959.

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Table of Contents: Graphics


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Figure 1. Photographs of (a) a product generated from Ba:Eu:Si:Al:Li = 0.80:0.20:0.58:6.42:3.00 and (b) the selected green luminescent particle. Both images were collected under UV light. 106x90mm (300 x 300 DPI)



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Figure 1. Photographs of (a) a product generated from Ba:Eu:Si:Al:Li = 0.80:0.20:0.58:6.42:3.00 and (b) the selected green luminescent particle. Both images were collected under UV light. 86x60mm (300 x 300 DPI)


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Figure 2. Crystal structure of Ba2LiSi7AlN12:Eu2+ (a) from the [010] direction and (b) from the [001] direction. Blue and red tetrahedra are (Si,Al)N4 and LiN4, respectively. Green polyhedra are BaN11. Green spheres are Ba, and white spheres are N. These images were drawn with the program VESTA.29 101x76mm (300 x 300 DPI)



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Figure 2. Crystal structure of Ba2LiSi7AlN12:Eu2+ (a) from the [010] direction and (b) from the [001] direction. Blue and red tetrahedra are (Si,Al)N4 and LiN4, respectively. Green polyhedra are BaN11. Green spheres are Ba, and white spheres are N. These images were drawn with the program VESTA.29 253x141mm (300 x 300 DPI)


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Figure 3. Emission (λex = 400 nm) and excitation (λem = 515 nm) spectra of one Ba2LiSi7AlN12:Eu2+ particle. 68x59mm (300 x 300 DPI)



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Figure 4. Eu L3 XANES spectrum of one Ba2LiSi7AlN12:Eu2+ particle. The positions of Eu2+ and Eu3+ are indicated by ticks. 143x123mm (144 x 144 DPI)


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Figure 5. Temperature dependence of the emission spectra of one Ba2LiSi7AlN12:Eu2+ particle. The peak intensity (open circle) and integrated intensity (filled circle) are shown in the inset. 68x59mm (300 x 300 DPI)



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Figure 6. Absorption and emission spectra of one Ba2LiSi7AlN12:Eu2+ particle for QE measurement by 405nm excitation. 68x59mm (300 x 300 DPI)


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Figure 7. Observed (green dot) and fitted (black line) decay curves of one Ba2LiSi7AlN12:Eu2+ particle by the excitation of 370-nm pulsed light. The region below 0.4 µs was excluded in the analysis due to the background from the instrument. 68x59mm (300 x 300 DPI)



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Figure 8. XRD patterns of (a) the powder product synthesized from an Li-rich composition at 1700 °C and (b) the simulation based on the structural parameters of the single crystal XRD analysis. The peak positions of the impurity phases Ba2Si5N8 and BaSi6N8 are shown as red and blue ticks, respectively. The emission spectrum of the product excited by 370 nm is depicted in the inset. 78x68mm (300 x 300 DPI)


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Narrow-Band Green-Emitting Phosphor ... - ACS Publications

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