Crystallographic and Luminescent Characteristics of New Blue Phosphor (Sr,Ba)Al2Si3O4N4:Eu2+

 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) ų [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 (Sr_1-xBa_x)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 Sr_0.5Ba_0.5Si₂O₂N₂: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.