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## Creator

Shota Takemura, [Yukinori Koyama](https://orcid.org/0000-0002-7090-4430), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), [Shiro Funahashi](https://orcid.org/0000-0002-9381-3603), [Naoto Hirosaki](https://orcid.org/0000-0001-9218-9557), Hidekazu Ikeno, [Takashi Takeda](https://orcid.org/0000-0003-2510-4562)

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[Narrow-Band Emitting Phosphor Na2Cs2Sr(B9O15)2:Eu2+ Discovered from Local Structure Similarity with Sulfate Phosphor](https://mdr.nims.go.jp/datasets/5345f984-56e2-4da0-bb55-f27ab8fd6166)

## Fulltext

Template for Electronic Submission to ACS JournalsNarrow-Band Emitting Phosphor Na2Cs2Sr(B9O15)2:Eu2+ Discovered from Local Structure Similarity with Sulfate PhosphorShota Takemuraa, Yukinori Koyamab, Takayuki Nakanishic, Shiro Funahashic, Naoto Hirosakic, Hidekazu Ikenod, and Takashi Takeda*ca. Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo, 669-1330, Japan.b. Research and Services Division of Materials Data and Integrated System, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044, Japan.c. Luminescent Materials Group, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044, Japan.d. Department of Materials Science, Graduate School of Engineering, Osaka Metropolitan University, Sakai, Osaka, 599-8570, Japan.Corresponding AuthorTakashi Takeda, TAKEDA.Takashi@nims.go.jpABSTRACT Narrow-band emitting phosphors are required to improve performance of phosphor-converted light-emitting diodes. Here, we found a new narrow-band emitting phosphor Na2Cs2Sr(B9O15)2:Eu2+ using the local structure similarity with a known narrow-band emitting phosphor. In a 2D scatter plot of the structure dissimilarity between the local structures, Sr site in Na2Cs2Sr(B9O15)2 was located near the Ba site of the known narrow-band emitting sulfate phosphor BaSO4:Eu2+ with a distorted local structure. We synthesized Na2Cs2Sr(B9O15)2:Eu2+ and characterized the luminescence properties by microspectroscopy. Na2Cs2Sr(B9O15)2:Eu2+ has a violet luminescence peaked at approximately 417 nm and the narrow full width at half maximum is narrow as 26 nm (1497 cm-1).TOC GRAPHICSKEYWORDSPhotoluminescence, Crystal structure, t-SNE visualization, Materials informatics, MicrospectroscopyPhosphor-converted light-emitting diodes (pc-LEDs) composed of phosphors and blue LEDs have realized white light with high efficiency and been applied to general lighting and the backlight of liquid crystal display (LCD).1,2 The emission properties of phosphors (peak wavelength and full width at half maximum (FWHM)) dominates the spectra of pc-LEDs. To enlarge the color gamut of the display, narrow-band emitting phosphors are necessary.3 Commercial narrow-band emitting phosphors for backlighting are β-SiAlON:Eu2+ (Si6-xAlzOzN8-z:Eu2+) as a green phosphor4-6 and K2SiF6:Mn4+ as a red phosphor.7 An even wider color gamut is necessary to satisfy the BT.2020 standard for 8K televisions. Therefore, much narrower-band emitting phosphors are required.Because the 5d−4f emission of Eu2+ has allowed transition probability and the ground state 8S7/2 is degenerated with almost no splitting, the emission shows a single peak. Eu2+ is suitable as an luminescent center for a narrow-band emitting phosphor. A very narrow-band red emission was observed in the nitride phosphor SrLiAl3N4:Eu2+.8 This phosphor has a UCr4C4-type crystal structure9, and the local structures of the Sr-sites, which are the substitution sites for Eu2+, are similar to a cubic structure. Triggered by the discovery of SrLiAl3N4:Eu2+, UCr4C4-type structure with a cube-like local structure has attracted much attention and new narrow-band emitting nitride, oxynitride, oxide phosphors have been discovered (SrMg3SiN4:Eu2+, CaLiAl3N4:Eu2+, Sr[Li2Al2O2N2]:Eu2+, RbLi(Li3SiO4)2:Eu2+, RbNa(Li3SiO4)2:Eu2+, CsKNaLi(Li3SiO4)4:Eu2+).10-15 It is empirically known that the FWHM of the emission spectrum relates to the local structure around Eu2+ in the phosphor. However, the FWHM cannot be explained by ligand-field theory and the ideal local structure with the narrowest FWHM remains theoretically unknown. Data-driven approaches are applied to the exploration of new narrow-band emitting phosphors. Crystal structures having a local structure similar to a local structure of known narrow-band emitting phosphors have been selected from the structure database. Kim et al. selected crystal structures having cube-like local structures by qualitatively screening the crystal structures in Inorganic Crystal Structure Database (ICSD)16 using cubic structure as a reference local structure.17. To expand the search area and efficiently discover new phosphors, it is necessary to adopt various local structures as a reference local structure and quantitatively evaluate the similarity between local structures.We proposed a quantitative dissimilarity measure of local structures in inorganic crystals using the Wasserstein distance, which is often used in data science.18 In this method, the local structure is defined as a geometric polyhedron consisting of the central ion and its surrounding ligands. The obtained Wasserstein distance between local structures indicates the minimum deformation to change the local polyhedral structure into the other local polyhedral structure. The smaller the Wasserstein distance value, the more similar the structures. We applied this method to the reference local structure of the Ba-site having the flat rectangular structure in BaSi2O2N2:Eu2+ (λem = 535 nm, FWHM = 1920 cm–1)19 and discovered new narrow-band emitting phosphor K2ZnP2O7 :Eu2+. The K2ZnP2O7 :Eu2+ particle showed blue emission peak at approximately 440 nm with a narrow FWHM of 30 nm (1549 cm−1).20 The flat rectangular structure is on the extension of a cube-like structure. They can be viewed as high symmetry structures. On the other hand, the narrow-band emission is observed from the distorted local structure and the local structure with different coordination number. In the present study, we applied this method to the local structure of the sulfate phosphor BaSO4:Eu2+ as shown in Figure 1(a) and found new narrow-band emitting phosphor Na2Cs2Sr(B9O15)2:Eu2+. Figure 1. (a) Local structure of the Ba site of BaSO4. (b) Local structure of the Sr site of Na2Cs2Sr(B9O15)2. (c) Crystal structure of Na2Cs2Sr(B9O15)2. Green, Blue, yellow polyhedra are SrO12, CsO12, and NaO6, respectively. Gray spheres are B, and red spheres are O.BaSO4:Eu2+ shows narrow-band emission (λem = 375 nm, FWHM = 1693cm-1).21 Eu2+ occupies the Ba site in the crystal structure and is coordinated by twelve oxygen atoms. The Ba-O interatomic distances range from 2.790 Å to 3.314 Å and the Ba site has Cs symmetry. The distorted polyhedron is largely different from a cuboctahedron, which has the highest symmetry among the 12-coordinated structure and can be viewed as a representative. Oxide, oxynitride, nitride phosphors are practical LED phosphors. As reference materials, it is not necessary to limit to those materials. Any materials can be reference candidates if they have narrow-band Eu2+ emission. Using this structure as a reference local structure, we extracted similar local structures from crystal structures in the ICSD. Some compositions (i.e. metallic compounds) are excluded because they are not suitable as host material. To extract a local structure from a crystal structure, it is necessary to determine the coordination number. The coordination numbers were determined using the CrystalNN method.22 The compositions in which automatic acquisition of the coordination number is difficult are also excluded (i.e. partially occupied crystal structures). As the substitution site for Eu2+, alkali metal (Na, K, Rb, Cs), alkaline-earth metal (Ca, Sr, Ba), rare earth (Y, La, Lu) are selected since the ionic radius of Eu2+ is close to those of those ions. From the remaining ionic crystals, 12 coordinated local structures consisting of the central ion and ligands were extracted. The number of extracted local structures are following (Na:98, K:456, Rb:331, Cs:681, Ca:81, Sr:344, Ba:1018, Y:4, La:112, Lu:4). The Wasserstein distance was calculated for all pairs using the SciPy package.23 Then we visualized the distributions of all the local structures as a scatterplot on a 2-dimensional (2D) plane using the t-distributed stochastic neighbor embedding (t-SNE) method.24 The procedure detail is shown in the previous paper.18,20Figure 2 shows the scatterplot of all data. Each data point indicates one local structure. If several local structures are extracted from one crystal structure, several points regarding the crystal structure are shown in the scatterplot. Figure 2. 2D t-SNE plot of 12 coordinated local structures.At upper right, cuboctahedral structures cluster together. The region in the neighborhood of the reference local structure the Ba site of BaSO4 is magnified. Because the local structure of the Ba site of BaSO4 and the Sr site of SrSO4 are almost identical, they are so closely pointed. Although many points are situated near the Ba site of BaSO4, most of the crystals having those local structures contain transition metal, radioactive element or toxic element, and they are unsuitable for Eu2+ phosphor hosts. On the other hand, the same chemical species occupies several crystallographically different sites in a crystal structure in some crystals. They were also excluded to avoid emission broadening by overlapping from multiple Eu2+ sites. The crystal having single substitutional site for Eu2+ is preferred for narrow-band emission except for the case multiple sites are resemble as in SrLiAl3N4:Eu2+.After eliminating these points, we found the Sr site of Na2Cs2Sr(B9O15)2 borate25 is positioned near the Ba site. Although there are three possible substitution sites (Na, Cs, Sr) for Eu2+ in the structure, Eu2+ will preferentially occupy the Sr site from the similarity of ionic radii between Sr2+ (rVIII = 1.26 Å) and Eu2+ (rVIII = 1.25 Å ).26 The local structure of the Sr site and crystal structure of Na2Cs2Sr(B9O15)2 are shown in Figure 1b and 1c. Different from the characteristic local structures like cube, it is rather difficult to visually judge the similarity between the Ba site in BaSO4 and the Sr site in Na2Cs2Sr(B9O15)2. The quantitative similarity evaluation is very useful. We selected Na2Cs2Sr(B9O15)2 as a host material for Eu2+ narrow-band emitting phosphor.Na2Cs2Sr(B9O15)2:Eu2+ was synthesized by a solid-state method. Although Na2Cs2Sr(B9O15)2 was recognized from the powder X-ray diffraction pattern of the powder product, it was contaminated with large amount of impurity phases as illustrated in Figure S1. PL spectrum of the powder product showed narrow violet emission peaked at 417 nm and FWHM of 26 nm (1497 cm−1) excited by 290 nm as illustrated in Figure S2. As the product was a mixture, it is not clear that the observed luminescence is due to Na2Cs2Sr(B9O15)2:Eu2+. Therefore, a violet luminescent particle as shown in Figure 3 was picked up and its crystal structure and PL properties were analyzed.27,28Figure 3. Single particle of the Na2Cs2Sr(B9O15)2:Eu2+ phosphor emitting violet color under UV irradiation.Single crystal XRD analysis of the particle showed that the crystal structure has a monoclinic unit cell with a=8.4804(3) Å, b=8.5380(3) Å, c=17.2077(5) Å, β=92.487(3)° and P21/c space group (No.14). The refined structural parameters are shown in Tables S1 and S2, and they are in good agreement with the literature.25 The emission and excitation spectra of the Na2Cs2Sr(B9O15)2:Eu2+ particle are shown in Figure 4a. Figure 4. (a) Emission (blue line, λex = 297 nm) and excitation (black line, λem = 417 nm) spectra from a single particle of Na2Cs2Sr(B9O15)2:Eu2+. (b) Observed (black dot) and fitted (blue line) decay curves of one Na2Cs2Sr(B9O15)2:Eu2+ particle upon exciting with 345 nm pulsed light. (c) Temperature dependence of the emission spectra of one Na2Cs2Sr(B9O15)2:Eu2+ particle excited by 370 nm light. The peak intensity (filled circle), integrated intensity (open circle) and FWHM (open square) are shown in the inset.The emission spectrum had a peak at approximately 417 nm with a FWHM of 26 nm (1497 cm-1). The emission spectrum of the particle was same with the powder result. The narrow-band violet emission is derived from Na2Cs2Sr(B9O15)2:Eu2+. The excitation spectrum spanned from 280-350 nm. The luminescence decay was measured as shown in Figure 4b. It was well fitted by one exponential and background. The decay time was 0.73 μs and corresponds to Eu2+ 5d -4f emission. Both narrow-band emission and single exponential decay support that the narrow-band violet emission is due to the single Eu2+ site.The temperature dependence of luminescence is important because the temperature of LED increases during the operation. The temperature dependence of the emission spectra, peak in-tensity and integrated intensity are shown in Figure 4c. The peak intensity decreased with temperature and the peak intensity at 100 °C, 200 °C and 300 °C were 88 %, 72 % and 57 % of the intensity of RT, respectively. In the integrated intensity, they were 95 %, 85 % and 74 % of RT at 100 °C, 200 °C and 300 °C, respectively. While, the FWHM gradually increased with temperature.In conclusion, we extracted Na2Cs2Sr(B9O15)2 from ICSD as a host crystal of the Eu2+ narrow-band emitting phosphor based on quantitative evaluation of dissimilarity between local structures using the Ba site of BaSO4 as the reference local structure. From the powder product, we found a violet luminescent Na2Cs2Sr(B9O15)2:Eu2+ particle. It had luminescence peak at approximately 417 nm with the FWHM of 26 nm (1497 cm−1). The FWHM is comparable to other reported well-known UCr4C4-type narrow-band emitting phosphors. We used a distorted coordination structure as a reference local structure. By adopting other local structure of known narrow-band emitting phosphor as a reference local structure, new narrow-band phosphor will be discovered. Here we focus on the local structure of the substitution site for Eu2+. The emission spectrum is not only determined by the local structure. Other physical/chemical and geometrical parameters can be additional parameters for the discovery of narrow-band emitting phosphors. EXPERIMENTAL METHODSStarting materials of NaHCO3 (Aldrich, 99.7%), Cs2CO3 (Kojundo Chemical, 5N), SrCO3 (Aldrich, 3N), Eu2O3 (Shin-etsu Chemical, 3N) and H3BO3 (FUJIFILM Wako Pure Chemical, 99.5%) were mixed at a Na:Cs:Sr:Eu:B ratio of 2:2:0.95:0.05:18. The mixture was fired in air at 600 °C and fired again in a tube furnace at 600 °C for 5 hours under a reducing atmosphere (5% H2/N2). The product was characterized by powder XRD (Rigaku, Smartlab, Cu Kα1 radiation) and powder spectrofluorometer (JASCO, FP8600).The single crystal XRD data of the particle were collected using a diffractometer (Rigaku, XtaLab Synergy-R-custom) with Mo Kα radiation (λ = 0.71073 Å). The data were integrated and corrected for absorption using CrysAlisPro. The crystal structure was solved and refined with SHELX.29,30 The excitation and emission spectra of the particle were obtained using a spectrometer (Otsuka electronics, MCPD7700) through a microscope (Olympus, BX51M) under excitation of 150 W Xenon light (Otsuka electronics, QE2100). To obtain the temperature dependence of emission spectrum, the temperature of the particle was controlled from room temperature to 300 °C by a heater stage (Linkam Scientific Instruments, THMS600). The luminescence decay was measured by a streak camera (Hamamatsu Photonics, C14831-110) under excitation of a 365 nm pulsed laser through a microscope (Olympus, BX51M).ASSOCIATED CONTENTSupporting Information. XRD pattern and PL spectrum of the powder product, Crystallographic Data and Atomic Coordinates, Occupancies, and Isotropic Atomic Displacement Parameters of Na2Cs2SrB18O30:Eu2+NotesThe authors declare no competing financial interests.ACKNOWLEDGMENTThis work was supported by JST, the Core Research for Evolutional Science and Technology [JPMJCR19J2]. REFERENCES(1) K. Bando, K.; Noguchi, Y.; Sakamoto, K.;  Shimizu, Y. Development and Application of High-Brightness white LEDs, in: Tech. Digest. Phosphor Res. Soc. 264th Meeting, November 29, 1996.(2) Nakamura, S.; Fasol, G.  The Blue Laser Diode: GaN Based Light Emitters and Lasers; Springer, 1997.(3) 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 2019, 2, 022401.(4) Hirosaki, N.; Xie, R. J.; Kimoto, K. 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