The variety of emission color Eu 2+ doped barium silicate phosphors for LEDs, Ba 4 Si 6 O 16 :Eu 2+ and Ba 5 Si 8 O 21 :
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The variety of emission color Eu2+ doped barium silicate phosphors for LEDs, Ba4Si6O16:Eu2+ and Ba5Si8O21:Eu2+ T. Ishigaki1, K. Sato2, S. Kamei2, K. Uematsu3, K. Toda1,2, M. Sato1,3 1 Center for Transdiciplinary Research, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, JAPAN 2 Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-no-cho, Niigata 950-2181, JAPAN 3 Department of Chemistry and Chemical Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, JAPAN ABSTRACT Blue-green emitted barium silicate phosphors, Ba4Si6O16:Eu2+ and Ba5Si8O21:Eu2+, were prepared by a conventional solid-state reaction. In these hosts, the corner-sharing [SiO4] tetrahedral formed a chain framework structure. These silicate phosphors can be efficiently excited by n-UV light, yielding an intense blue-green emission. Under excitation by near UV light, the emission bands are broader than that of the Ba2SiO4:Eu2+ phosphor. INTRODUCTION Since the investigation of the blue and n-UV LEDs, there is an interest in developing white LEDs for lighting applications. Due to the wide absorption and emission band, the f-d transition of Eu2+ activated silicates seem suitable for use as lighting phosphors. Actually, Eu2+ activated silicates and oxynitrides are already being used in commercial LED lighting. The alkaline-earth silicates phosphor compounds, particularly the barium containing compounds, have been widely investigated. In the first report, Barry and Blasse et al. reported divalent europium in barium orthosilicate in 1968 [1, 2]. Eu2+ doped barium silicates emitted under UV excitation, for example, Ba3SiO5:Eu2+ (λem = 504~566 nm or 560~590 nm) [3, 4], Ba2SiO4:Eu2+ (λem = 504~510 nm) [1,2,5–11], BaSiO3:Eu2+ (λem ~550 nm) [6], Ba2Si3O8:Eu2+ (λem ~485 nm) [2,6], and BaSi2O5:Eu2+ (λem ~ 520 nm) [1, 6]. The Ba2SiO4:Eu2+ phosphor has a short decay time and high luminescence characteristics under long wavelength ultraviolet excitation. The conventional Ba2SiO4:Eu2+ phosphor was prepared by a solid-state reaction at 1200 ºС for 4 h in a flowing mixture of 20 vol.% H2 / N2 reductive atmosphere gas [12]. Though Ba2Si3O8 and Ba5Si8O21were reported as stable phases in the phase diagram, there are few reports of them as phosphor materials. In this study, we focused on Ba4Si6O16 (= Ba2Si3O8) and Ba5Si8O21. Generally in silicates, the corner-sharing [SiO4] tetrahedral is arranged in chains. Three corner-sharing [SiO4] single chains are linked into a zweier triple chain in Ba4[Si6O16]. In Ba5[Si8O21], four chains make a zweier quadruple chain [13, 14]. (Figures 1 - 3). Such polyhedral framework-formed phosphor host compounds tend to have good thermal and chemical stabilities [15]. Silicates have strong and rigid characteristics of partly covalent Si–O bond structures [16], which are expected to avoid thermal degradation of emission. This is because the light degradation begins to occur above the temperature of 80ºC for emitting LEDs [17].
Figure 1. [SiO4] multiple chain models for Ba4Si6O16
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