Crystallographic and Luminescent Characteristics of New Blue Phosphor (Sr,Ba)Al2Si3O4N4:Eu2+
Authors: Yoshimatsu Ryo , Funahashi Shiro , Yamada Suzuya , Hirosaki Naoto , Ishigaki Tadashi , Ohmi Koutoku 
Published: February 2020

Recently, nitrides and oxynitrides have received a great deal of attention as phosphor materials for white LED (WLED) lighting application, since these are thermally and mechanically stable owing to a strong lattice consisting of point-shared and/or edge-shared (Si,Al)(N,O)4tetrahedral network.
In this paper, new oxynitride-based phosphor materials (Sr,Ba)Al2Si3O4N4:Eu2+ are proposed as the WLED phosphor for near-UV LED excitation. The crystal structure of SrAl2Si3O4N4 is monoclinic system and belongs to space group P21
(No.4) [1]. By using the single phase crystal samples, lattice parameters have been accurately determined as a = 7.2516(5) Å, b = 9.3434(5) Å, c = 10.8761(5) Å, β = 104.489(1)°, V = 713.45(7) Å3 [1]. The substructure in this crystal consists of (Si,Al)(N,O)4 tetrahedra and Sr2+ ions occupy the channel formed from its tetrahedrons. The crystallographic and luminescent characteristics are investigated for the (Sr,Ba)Al2Si3O4N4:Eu2+samples having different Ba content.
The (Sr1-xBax)Al2Si3O4N4:Eu2+ phosphor samples were synthesized at 1400 ~ 1700 ¢ªC in N2 at atmospheric pressure for a few hours. SrCO3, BaCO3, Al2O3, Si3N4 and Eu2O3 were used as source materials. The Ba content x was varied from 0 to 1. In all xrange, almost the single crystal phase of the solid solution compound was obtained.
Figure 1 shows the photoluminescence (PL) and PL excitation spectra of the SrAl2Si3O4N4:Eu2+ and Bal2Si3O4N4:Eu2+ [2]. The measurements were performed at room temperature under near UV excitation at 405 nm. Both samples show a broad blue luminescence attributed to the 4f65d to 4f7 internal transition in Eu2+ centers. The peak wavelength is 473 nm for the SrAl2Si3O4N4:Eu2+. The wavelength shifts to 465 nm by replacing Sr with Ba. The PL excitation bands lie in UV and near-UV region below 450 nm for both samples. The result indicates that (Sr,Ba)Al2Si3O4N4:Eu2+phosphors are a promising blue phosphor for WLED for near UV-LED excitation.
Figure 2 shows dependences of the internal quantum efficiency (I.Q.E.) on the Ba content x. The I.Q.E. drastically increases with increasing x and reaches 0.90 at x = 0.5. The thermal quenching characteristics were investigated. In Fig. 2, the relative PL intensity measured at 200°C to that at 30°C (I200/I30) is shown. The relative intensity increases with increasing xas well as I.Q.E..
From the accurate determination of crystal structure analysis in these samples, it was found that the lattice structure changes from monoclinic to orthorhombic at around x of 0.25. Since the lattice symmetry of orthorhombic crystal is higher than that of monoclinic one, it is considered that the inherent lattice distortion is relaxed by the crystal phase transition, resulting in improvement of luminescent characteristics such as I.Q.E. and thermal quenching. In oxy-nitride materials, MSi2O2N2:Eu2+ (M = Ca, Sr, Ba) phosphor and have been investigated in detail on luminescent characteristics [3,4]. In the reference [3], it is reported that the efficiency of SrSi2O2N2:Eu2+ and BaSi2O2N2:Eu2+ is 91% and 76%, respectively. The crystal structure of Sr0.5Ba0.5Si2O2N2:Eu2+ was determined as triclinic with the space group P1(No.1) [5]. Additionally, the crystal structure of Ba(Si,Al)5(N,O)8 with space group A21ma(No.36) has been reported by W. B. Park et al.. [2] However, the systematic investigation regarding the relations between the space group and luminescent efficiency and thermal quenching characteristics have not been discussed yet. At the meeting, we will discuss the relation with considering the lattice distortion and/or defects, which may strongly affect the I.Q.E. and thermal quenching.

References
[1] Hirosaki et al., PCT WO 2013/069693.
[2] Woon Bae Park et al., J. Am. Ceram. Soc., 136(2014) 2363.
[3] Volker Bachmann et al., Chem. Mater., 21(2009) 316.
[4] Bong-Goo Yun et al., J. Electrochem. Soc., 154(2007) 320.
[5] Markus Seibald et al., Solid State Sciences, 13 (2011) 1769.