ABSTRACT

Synthesis of oxynitride solid solutions CaAl_4−xSi_xO_7−xN_x:Eu²⁺ (x = 0 - 4) was attempted by the solid state reaction (SSR) methods using Si₃N₄ and AlN as nitrogen sources. The Ca₃Al₈Si₄O₁₇N₄ (x = 4/3) sample with the high phase purity was obtained when AlN was used as a nitrogen source whereas the sample synthesized using Si₃N₄ as another nitrogen source contained a Ca₂Al₂SiO₇ impurity. Thus, it was revealed that AlN was a preferable nitrogen source for the synthesis of Ca₃Al₈Si₄O₁₇N₄ by the SSR method. The solid solutions around x = 4/3 activated with Eu²⁺ exhibited bluish-green luminescence with emission maxima at 480 nm by the excitation at 250 - 450 nm. Thus, the CaAl_4-xSi_xO_7-xN_x: Eu²⁺ solid solutions especially for Ca₃Al₈Si₄O₁₇N₄:Eu²⁺ (x = 4/3) were developed as novel Eu²⁺-activated oxynitride phosphors capable of the excitation by the near ultraviolet (NUV) LEDs.

1. Introduction

White light-emitting diodes (LEDs) have been developing rapidly over the past decade, since its advanced properties such as long life time, high efficiency, and environmentally friendliness without use of mercury. The application of white LEDs is expanding into extensive fields such as residential lighting, medical lighting, mobile, back lights, traffic lights, emotional lighting and so on. General white LEDs are composed of a blue LED chip and yellow phosphor such as Y₃Al₅O₁₂:Ce³⁺ [1]. Such combination certainly achieves generation of artificial white light; however, it is not preferable in respect of the high color temperature and the low color rendering index value (CRI < 75) owing to the lack of red and green components. In case of residential lighting, CRI value should satisfy over 80 [2]. In order to realize such a high CRI value for the white LED, highly efficient bluegreen (470 - 510 nm) and red (650 nm) emitting phosphors capable of excitation by blue or NUV LEDs are demanded.

Eu²⁺ ions are widely used as activators in phosphors since the emission from Eu²⁺ attributed to the electron transition 4f⁶5d¹ → 4f⁷ is strongly affected by its surrounding environment, i.e. symmetry, covalence, bond length, crystal-field strength. In other words, the emission wavelengths from Eu²⁺ ions are able to be tuned from blue to red region with the selection of suitable materials as the hosts. Oxynitrides are regarded as suitable hosts since excitation and emission bands at longer wavelength are expected from the nephelauxetic effect owing to the larger covalence nature for M-N (M: metal) bonds than that for M-O [3,4]. It results in the extensive research for the Eu²⁺-doped oxynitrides particularly silicon-contained oxynitrides such as β-SiAlON:Eu²⁺ [5], MSi₂O₂N₂:Eu²⁺ (M = Ca, Sr, and Ba) [6-8] and Ba₃Si₆O₁₂N₂:Eu²⁺ [9]. Development of new oxynitride phosphors is an important research topic to enrich the phosphor library with various excitation and emission properties. Sun et al. have reported the synthesis of solid solutions between CaAl₄O₇ and Ca₃Al₈Si₄O₁₇N₄ [10]. Their research has attracted the authors’ interest in investigation of photoluminescence properties of Eu²⁺-activated CaAl_4−xSi_xO_7−xN_x solid solutions.

On the other hand, homogeneous distribution of the Eu²⁺ activators in given host materials is one of the important factors in order to achieve high luminescence efficiency. Solution-based processes are potential methods to achieve homogeneous distribution of the activators [11-18]. Our research group recently has succeeded in improvements of emission intensities for oxynitride phosphors, Na_1–xM_xAlSiO_4–xN_x:Eu²⁺ (M = Mg²⁺, Ca²⁺, Sr²⁺) [13] and Ba₃Si₆O₁₂N₂:Eu²⁺ [14], by the combined synthesis methods composed of the preparation of oxide precursors by solution-based method and its subsequent nitridation under ammonium atmosphere.

Based on the background described above, the synthesis of oxynitride solid solutions CaAl_4−xSi_xO_7−xN_x (x = 0 - 4) by the SSR method and investigation of their photoluminescence properties with the Eu²⁺ activation were examined in the present study. In addition, the combined method involving the solution-based process and the subsequent nitridation of oxide precursor was applied for the synthesis of Ca₃Al₈Si₄O₁₇N₄:Eu²⁺.

2. Experimental

Two series of synthesis were examined for the CaAl_4−xSi_xO_7−xN_x:Eu²⁺ solid solutions by a conventional solid state reaction (SSR) method as summarized in Table 1. Raw materials of CaCO₃, Eu₂O₃, α-Al₂O₃, SiO₂, AlN and Si₃N₄ were weighed and mixed thoroughly in an agate mortal with a pestle according to the ratios listed in Table 1. 2 mol% of europium was substituted for calcium. The mixed powder was heat-treated at 1673 K for 4 h in a H₂(4%)-Ar stream. For the x = 4/3 sample corresponding to Ca₃Al₈Si₄O₁₇N₄, the combined synthesis method involving the preparation of an oxide precursor by an amorphous metal complex (AMC) method, one of solution-based methods, and its subsequent nitridation under ammonia atmosphere was also examined [13,14]. The oxide precursor having a composition of (Ca, Eu):Al:Si = 3:8:4 was prepared by the AMC method. After dissolving CaCO₃ in an aqueous citric acid solution, aqueous solutions of 1 M Eu(NO₃)₃ and 1 M Al(NO₃)₃∙9H₂O were added. Then, an aqueous solution of propylene glycol-modified silane (PGMS) was added.

PGMS was obtained by an alkoxy group exchange reaction for tetraethoxysilane with propylene glycol at 353 K in the presence of hydrochloric acid as a catalyst [11,15]. The mixed solutions were heated on a heating plate operated at 393 K with stirring to promote polymerization. The obtained polymer gel was pyrolyzed at 723 K for 3 h, and subsequently at 823 K for 10 h to remove organic compounds gradually. The obtained oxide precursor, finally, was heat-treated at 1673 K for 4 h under an ammonia stream (50 ml/min).

The X-ray diffraction (XRD; Bruker AXS: D2 Phaser) was used for the phase identification. Photoluminescence spectra were measured using a fluorescence spectrometer (Hitachi: F-4500) at room temperature. Internal quantum efficiency was measured using another fluorescence spectrometer (Jasco: FP-6500) equipped with an integrating sphere.

3. Results and Discussion

Figure 1 shows XRD patterns of the samples in the sersies A using Si₃N₄ as a nitrogen source as listed in Table 1. CaAl₄O₇ (x = 0) was successfully obtained without any impurities. The x = 0.5 sample contained CaAl₄O₇ and Ca₂Al₂SiO₇ whereas the x = 1 sample was the mixture of Ca₂Al₂SiO₇, Ca₃Al₈Si₄O₁₇N₄, Al₂O₃, and CaAl₄O₇. The x = 4/3 sample whose nominal composition was equal to Ca₃Al₈Si₄O₁₇N₄ of the known Ca-Al-Si-O-N compound was also crystallized in the multiphase of Ca₃Al₈Si₄O₁₇N₄, Ca₂Al₂SiO₇, and Al₂O₃ although there is a report on the synthesis of Ca₃Al₈Si₄O₁₇N₄ in a single phase [10]. The x = 2 sample showed a diffraction pattern similar to that for the x =3/4 sample. The x = 3 sample was composed of Ca₂Al₂SiO₇ and unknown phase while the x = 4 sample was mixture of Ca₃Si₃O₉, Si₂N₂O, and Si₃N₄. Thus, no known Ca-Al-Si-O-N phases were formed in the samples of x = 3 and 4. The results in the synthesis of the samples in the series A implied the difficulties in the synthesis of the CaAl_4–xSi_xO_7–xN_x samples using Si₃N₄ as the nitrogen source even for the known