1. Introduction

Recently, white light-emitting-diodes (WLEDs) have attracted growing amounts of attention, owing to their advantages of being environmentally friendly and having high brightness, eco-friendliness, long-life time, small size, low power consumption, and fast response time, as compared to conventional incandescent and fluorescent lamps [1–3]. White light can currently be generated by blue LED chips and the yellow phosphor YAG: Ce³⁺ (YAG), which is the method most frequently used in practice. Nevertheless, this combination only obtains a cool white light and low color rendering, due to the deficient red emission in the visible spectrum, which restricts their applications in the areas of residential lighting and commercial lighting. Of course, this is not the case and those LEDs are commercially available, as there are several Eu²⁺ or Mn⁴⁺ red phosphors available [4,5]. Moreover, an alternative approach to obtaining white light is the combination of UV or near-UV (n-UV) LED chips coupled with tricolor (red, green, and blue) phosphors, which have been increasing studied and have developed rapidly over the past few years. WLEDs fabricated in this way can overcome the above problems and produce excellent color rendering indexes as well as easily controlled emission color properties. Hence, there is an urgent need to develop new n-UV excitable phosphors with high thermal stabilities and high color rendering properties [6].

Recently, rare-earth doped oxonitridosilicate phosphors have attracted more and more attention based on their high luminescence efficiency as well as their good thermal and chemical stabilities for application in WLEDs, as compared to oxosilicates [7,8]. This is because of the high covalency and strong crystal fields in the strong covalent Si[O/N]₄ network of the lattice. The Si[O/N]₄ units are stacked together by sharing their corners and edges so as to form reactively condensed Si[O/N]₄ frameworks, making oxonitridosilicates excellent hosts for rare earth ions. Therefore, oxonitridosilicates have been used as excellent potential host materials for phosphors, such as: Ba₃Si₆O₁₂N₂ [9,10], (Ba, Ca)Si₂O₂N₂ [11], La_4-xCa_xSi₁₂O_3+xN_18-x [12], (Sr, Ca)₃Si₂O₄N₂ [13,14], Ca₁₅Si₂₀O₁₀N₃₀ [15], Ca_m/2Si_12-m-nAl_m+nO_nN_16-n [16,17], and Ca_1.5Ba_0.5Si₅O₃N₆ [18]. The preparation of most nitridosilicates requires critical synthesis conditions, such as high temperature and high pressure. Recently, a new Eu²⁺ doped green phosphor BaSi₃O₄N₂, has attracted great attention due to its easy synthesis condition and high thermal quenching temperature. BaSi₃O₄N₂ crystalizes in a hexagonal system, and is composed of corrugated layers of the corner-sharing SiNO₃ tetrahedral stacked by Ba²⁺ spacers, as shown in Fig. 1(a). The luminescence properties of the Eu²⁺ and Eu²⁺/Mn²⁺ in BaSi₃O₄N₂ were first reported by D.C. Huang et al [19,20]. BaSi₃O₄N₂: Eu²⁺ shows a highly intense green broad emission. The green emission intensity of Eu²⁺ was greatly enhanced by adding Mn²⁺ due to the co-excitation and energy transfer between Mn²⁺ and Eu²⁺.

 

Fig. 1. The crystal structure of BaSi₃O₄N₂ (a), X-ray diffraction patterns of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/ Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) (b, c), the variations of unit cell parameters of Ba_1-xSr_xSi₃O₄N₂ with the increasing Sr²⁺ concentration x (d).

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The spectral properties of Eu²⁺ and Ce³⁺ ions with 5d - 4f transitions are strongly dependent on their local environment, due to the fact that the 5d excited state is not shielded from the crystal field by the 5s² and 5p⁶ electrons. As a result, the tuning of emission wavelength and other luminescence properties including quantum efficiency and quenching temperature of Eu²⁺ and Ce³⁺ -activated phosphors can be achieved, through the cationic substitution based on the local crystal structural change of phosphors [21–23]. Thus, the effects of Sr²⁺ substitution on the structure and luminescence properties of the Eu²⁺ and Ce³⁺ ion in the BaSi₃O₄N₂ lattice should be research target in the search for exploration of novel phosphors. In this work, a series of phosphors Ba_1-xSr_xSi₃O₄N₂:Eu²⁺ (0 ≤ x ≤ 0.5) and Ba_1-xSr_xSi₃O₄N₂: Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) have been synthesized through the solid-state reaction method. The crystal structure, photoluminescence properties and thermal quenching behavior of Ba_1-xSr_xSi₃O₄N₂:Eu²⁺ (0 ≤ x ≤ 0.5) and Ba_1-xSr_xSi₃O₄N₂: Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) are investigated in detail. In the present paper, Ba_1-xSr_xSi₃O₄N₂:Eu²⁺ (0 ≤ x ≤ 0.5) and Ba_1-xSr_xSi₃O₄N₂: Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) are presented as independent systems, and Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/ Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) is used for simplification.

2. Experimental section

Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) samples were synthesized through a high temperature solid-state reaction. The starting materials were BaCO₃ (A.R.), SrCO₃ (A.R.), SiO₂ (A.R.), Si₃N₄ (A.R.), Li₂CO₃ (A.R.), Eu₂O₃ (99.99%) and CeO₂ (99.99%). The nominal doped lanthanide (Eu²⁺ and Ce³⁺) concentrations were 2 mol% and 1 mol% relative to barium, respectively. The raw materials were weighed and mixed in the given stoichiometric proportion, then thoroughly ground in an agate mortar. Subsequently, the mixtures were placed into BN crucibles and then sintered between 1300 and 1350 °C for 4 h in a horizontal tube furnace under a reducing atmosphere of H₂/N₂ (5/95%). After sintering, these samples were gradually furnace-cooled to room temperature, then ground into powder for further analysis. The prepared samples had no apparent reaction with the BN crucibles.

The X-ray diffraction (XRD) patterns were collected in the 2θ range of 10 - 75 ° using a D/max-2550 X-ray diffractionmeter (18 kV, 10 mA). A step scan with a step size of 0.02 ° and a count time of 1 s per step were used to collect all XRD patterns. The XRD measurements were performed at room temperature in air. The unit cell parameters of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) were calculated from the XRD patterns using the Fullprof software package. The UV excitation and emission spectra, were measured with a Hitachi F-4600 spectrometer. The scan speed was fixed at 240 nm/min, the voltage was 400 V, and the slits were fixed at 2.5 nm. The temperature-variable luminescence properties were studied on the same device, which was coupled with a self-made heating attachment and a computer-controlled electric furnace. The luminescence decay profiles were measured on an Edinburgh FLS980 spectrometer equipped with a µF2 lamp as excitation source, and the external quantum efficiencies were measured using a barium sulfate coated integrating sphere (150 mm in diameter) attached to the FLS 980.

3. Results and discussions

3.1 Phase identification of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/Ce³⁺, Li⁺

The power XRD patterns of a series of the synthesized Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) phosphors with various concentrations of Sr²⁺ are shown in Fig. 1(b, c), and they are consistent with the reported data of D. C. Huang et al [20], as all of the diffraction peaks are well indexed to those of the reported BaSi₃O₄N₂ phase. The doping of Sr²⁺ ions does not induce any other distinct impurity phases when x reaches in the range of 0 ≤ x ≤ 0.5, indicating the successful incorporation of Sr²⁺ into the BaSi₃O₄N₂ host. When the value of x is up to 0.6, the impurity can be clearly observed. Notably, the unit cell parameters a, c and V decrease with increasing Sr²⁺ concentration (Fig. 1(d)), which can be ascribed to the ionic radii of Sr²⁺ (r = 1.18 Å, CN = 6) being smaller than that of Ba²⁺ (r = 1.35 Å, CN = 6) [24]. This indicates that Ba²⁺ is substituted by Sr²⁺ and that the intermediate solid-solutions Ba_1-xSr_xSi₃O₄N₂:Eu²⁺ are formed with the same hexagonal structure with increasing Sr²⁺ concentrations in an approximate range.

3.2 Luminescence properties of Ba_1-xSr_xSi₃O₄N₂:Eu²⁺

The room temperature excitation and emission spectra of BaSi₃O₄N₂:Eu²⁺ (i.e. x = 0) are illustrated in Fig. 2(a). The excitation spectrum of BaSi₃O₄N₂:Eu²⁺ shows a broad band covering the wavelength range of 250 to 450 nm when monitored at 515 nm. The peaks of the excitation band at about 274, 308 and 361 nm are attributed to the 4f-5d transition of Eu²⁺ [25]. Upon excitation at 361 nm, the phosphor exhibits a green emission band in the wavelength range of 450-650 nm with a peak located around 516 nm and full width at a half-maximum (FWHM) of about 62 nm. The Eu ion is present as a divalent ion in all Eu-doped samples, which has been demonstrated by the absence of sharp f-f line emission and excitation transitions for typical Eu³⁺ characteristics. In BaSi₃O₄N₂ host lattice, there are two different kinds of Ba²⁺ coordination environment: the trigonal antiprism environment, namely Ba1, surrounded by six oxygen atoms and one nitrogen atom; and the distorted octahedron environment, namely Ba2 surrounded by six oxygen atoms [20], as shown in Fig. 2(b). Considering the two different crystallographic cation sites of Ba²⁺ in the BaSi₃O₄N₂:Eu²⁺ lattice and the fact that 5d - 4f emission band is not symmetrical, the broad emission band could be well decomposed into two sub-bands centered at 19650 cm^-1 (i.e. 509 nm) and 18850 cm^-1 (i.e. 531 nm). The high-energy emission band at 19650 cm^-1 assigned to the Eu²⁺ occupied the Ba1 site (CN = 7) with a stronger crystal field strength, while the low-energy emission band at 18850 cm^-1 results from Eu²⁺ on the Ba2 site (CN = 6) with a weaker crystal field strength [26]. It is clear that the overall emission intensity contributed by Eu1 is higher than that of Eu2, and this leads to a green emission color of the sample. For the sake of comparison, some characteristics of the BaSi₃O₄N₂: Eu²⁺ and other typical Eu²⁺-activated oxynitride phosphors are presented in Table 1.

 

Fig. 2. Excitation (black) and emission (red) spectra of the BaSi₃O₄N₂:Eu²⁺, the inset shows the Gaussian fitting of the emission band (a), coordination polyhedral of (Ba1)(O, N)₇ and (Ba2)(O, N)₆ (b), excitation and emission spectra of the Ba_1-xSr_xSi₃O₄N₂:Eu²⁺ (0 ≤ x ≤ 0.5) phosphors (c); the variation of the emission band, FWHM, Stokes shift and external quantum efficiency dependent on Sr content (d).

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Table 1. Photoluminescence properties, crystal structure of BaSi₃O₄N₂:Eu²⁺ and other typical Eu²⁺-doped oxynitride phosphors.

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Figure 2(c) shows the excitation and emission spectra of Ba_1-xSr_xSi₃O₄N₂:Eu²⁺ (0 ≤ x ≤ 0.5). It is clear that all of the excitation and emission spectra profile of the as-synthesized samples are similar. There is no significant change in the shape of the excitation band with increasing Sr concentration, but a small red shift of the excitation band at the lowest energy (i.e. longest wavelength) can be observed, indicating that the crystal field strength around the Eu²⁺ ions is little larger when Ba is substituted by Sr, as demonstrated by the shrinkage of the unit cell volume (Fig. 1(d)). The observed broad excitation is indicative of the fact that the phosphor can be well excited using the emission of near-UV LEDs and blue In/GaN chips. When the samples were excited at 361 nm, the emission bands are clearly attributable to the allowed transitions of Eu²⁺ between the excited 5d and the ground 4f levels. The emission band of Ba_1-xSr_xSi₃O₄N₂:Eu²⁺ changed from 516 nm (i.e. 2.41 eV, blue-green) to 546 nm (i.e. 2.28 eV, green-yellow) with a 30 nm redshift, increasing the x value of Sr from 0 to 0.5. Figure 2(d) displays the emission band, full-width at half maximum (FWHM), Stokes shift and external quantum efficiency of Ba_1-xSr_xSi₃O₄N₂:Eu²⁺ (0 ≤ x ≤ 0.5). With increasing Sr²⁺ concentration from 0 to 0.5, there is clearly a red-shift of the emission peak with 30 nm and the increment of FWHM and Stokes shift with 16 nm and 745 cm^-1, respectively. The red-shift behavior of the emission band can be explained by increment of the crystal field strength and Stokes shift, as shown in Fig. 3. BaSi₃O₄N₂:Eu²⁺ (i.e. x = 0) exhibits the highest external quantum efficiency, and it decreases gradually from 51% to 13% as the x value varies from 0 to 0.5.

 

Fig. 3. The mechanism of red shift of the emission spectra for the substitution of Ba by Sr in BaSi₃O₄N₂:Eu²⁺.

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The correlation between the Stokes shift (ΔS) and the FWHM of the Eu²⁺ 5d-4f emission band can be estimated approximately using the Huang-Rhys parameter S (S =$0.5\left( {\frac{{\Delta {\mathrm{S}}}}{{\mathrm{\hbar} \omega }} + 1} \right)$) and the phonon energy of the lattice ħω, which are related to the Frank-Condon offset and the curvature of the parabola in the configurational diagram, according to the following formula [31]:

3.3 Luminescence properties of Ba_1-xSr_xSi₃O₄N₂:Ce³⁺, Li⁺

 Figure 4(a) shows the excitation and emission spectra of the BaSi₃O₄N₂:Ce³⁺, Li⁺ sample. When monitored at 400 nm, there are two obvious bands in the wavelength range of 250-380 nm in the excitation spectra, corresponding to the transition from the Ce³⁺ 4f¹ ground state to the split 5d levels. Upon excitation at 336 nm, the phosphor displays a blue broad emission band in the wavelength range of 350-600 nm with a peak centered at about 400 nm and a FWHM of about 54 nm. Similar to the Eu²⁺-doped sample, considering the two different crystallographic cation sites of Ba²⁺ in the BaSi₃O₄N₂:Ce³⁺, Li⁺ lattice and the fact that 5d -4f emission band is also not symmetrical, the broad emission band could be well decomposed into four sub-bands centered at 22600 (i.e. 442 nm), 24600 (i.e. 406 nm), 24150 (i.e. 414 nm) and 26150 cm^-1 (i.e. 382 nm) by using Gaussian functions in the inset of Fig. 4(a). It is accepted that the emission of Ce³⁺ is attributed to transitions from the lowest 5d excited state to the ²F_5/2 state and the ²F_7/2 ground state, so that two distinct emission spectra with the same theoretical energy value of 2000 cm^-1 could be decomposed [33–35]. The above values can be set into two groups according to the calculated Gaussian energy values, which confirms that Ce³⁺ can occupy different Ba²⁺ centers in the BaSi₃O₄N₂ host. In addition, the Stokes shift of Ce³⁺ in BaSi₃O₄N₂ lattice is estimated to be about 4750 cm^-1.

 

Fig. 4. The excitation and emission spectra of the BaSi₃O₄N₂:Ce³⁺, Li⁺, the inset is the Gaussian fitting of the emission band (a), Excitation and emission spectra of the Ba_1-xSr_xSi₃O₄N₂:Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) phosphors (b), the inset is enlarged PL spectra, showing the red shift; the variation of the emission band, FWHM, Stokes shift, and external quantum efficiency depend on Sr content (c). CIE chromaticity coordinates for Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/Ce³⁺, Li⁺ phosphors (d).

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Figure 4(b) shows the normalized excitation and emission spectra of Ba_1-xSr_xSi₃O₄N₂:Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5). Similar to that observed in the Eu²⁺-activated samples, there was no significant change in the shape of the excitation band when Sr replaced the Ba in BaSi₃O₄N₂, but a small red shift of the excitation band at the lowest energy (i.e. longest wavelength) can be observed, indicating that the crystal field strength around the Ce³⁺ ions is little larger when Ba is substituted by Sr, as demonstrated by the shrinkage of the unit cell volume (Fig. 1(d)). In Fig. 4(c), the emission band, FWHM, Stokes shift and external quantum efficiency of Ba_1-xSr_xSi₃O₄N₂:Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) are displayed for the sake of comparison. When the samples were excited at 288 nm, the red-shifts of the Ce-doped samples have also been observed such that the emission peak of Ba_1-xSr_xSi₃O₄N₂:Ce³⁺, Li⁺ changed from 400 to 407 nm with the increase of the x value of Sr from 0 to 0.5. The Stokes shifts of Ce³⁺-doped samples increased from 4750 to 5180 cm^-1 with a 500 cm^-1 increment. Therefore, it can be understood the red-shift of Ce³⁺ resulted from the increasing of crystal field strength and Stokes shift. The external quantum efficiency of the Ce-doped sample also decreased compared to that of BaSi₃O₄N₂:Ce³⁺, Li⁺.

The Commission International de l'Eclairage (CIE) chromaticity coordinates of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/ Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) phosphors with different Sr dopant contents are shown in Fig. 4(d). The emission of the Ba_1-xSr_xSi₃O₄N₂: Eu phosphors changed from blue-green at 516 nm to green-yellow at 546 nm with increasing Sr content, and the CIE coordinates of Eu²⁺-doped samples varied systematically from (0.209, 0.619) for the composition with 0 mol% Sr²⁺ concentration to (0.350, 0.573) for the composition with 50 mol% Sr²⁺ content. This indicates that cationic substitution by Sr²⁺ can effectively change the emitted color of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ phosphors. However, the CIE coordinates of Ce³⁺-doped samples varied in a small region.

3.4 Decay curves of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/Ce³⁺, Li⁺

In order to investigate the presence of two Ba²⁺ sites in Ba_1-xSr_xSi₃O₄N₂, decay curves of the Eu-sample were monitored at 509 and 531 nm, respectively, under the excitation of 361 nm, as shown in Fig. 5(a).

 

Fig. 5. Luminescence decay curves of BaSi₃O₄N₂: Eu²⁺ (a), Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ (x = 0, 0.1, 0.3 and 0.5) (b) and Ba_1-xSr_xSi₃O₄N₂:Ce³⁺, Li⁺ (x = 0, 0.1, 0.3 and 0.5) (c) monitored at 509 and 531 nm, the excitation wavelength is 361 nm.

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All of the luminescence decay curves could be fitted well with a double-exponential function using the following equation:

3.5 Thermal quenching behavior and CIE chromaticity coordinates of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/Ce³⁺, Li⁺

The thermal quenching behaviors of the specific phosphor should be evaluated when the selected phosphor was applied in WLEDs. As is well known, thermal quenching stability would play an important role for phosphors used in WLEDs, since it has a considerable influence on the light output and CRI [11]. Thus, the thermal quenching properties of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ samples were investigated. Figures 6(a) and (b) show the temperature-dependent photoluminescence spectra of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ (x = 0, 0.4) under excitation at 361 nm. A blue-shift was clearly observed in the spectra when the temperature increased from 50 to 230 °C for the samples. This phenomenon can be attributed to the thermally active phonon-assisted tunneling from the excited states of the lower-energy emission band to those of the higher-energy emission band in the configuration coordinate diagram. The inset of Fig. 6(b) displays the temperature dependence of the integrated emission intensity of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ (x = 0, 0.4). By increasing the temperature to 150 °C, the normal working temperature for WLEDs, the emission intensity of the Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ (x = 0, 0.4) phosphors remains at 96 and 92% of that measured at room temperature, respectively. The emission intensity of Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ (x = 0.4) decreases faster with increasing temperature than that of BaSi₃O₄N₂: Eu²⁺, at 230 °C, the emission intensity of the Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ (x = 0.4) phosphor only remains at 68%, which is lower than that (80%) of the BaSi₃O₄N₂: Eu²⁺, implying that the doping of Sr²⁺ may disadvantage thermal stability at high temperatures. As shown in Fig. 6(c), the curve g is the ground state of Eu²⁺. The curves e₁ and e₂ are the excited states of Eu²⁺ of the [EuO₆]^10- octahedron neighboring with the [BaO₆]^10- and [SrO₆]^10- octahedra, respectively [9]. The ΔR is the departure from the ground state to the excited states along the R axis [22]. A and B are the crossing points of g and e₁ and e₂, respectively. The Δ E₁ and Δ E₂ are the energy differences of A and B to the lowest positions of the e₁ and e₂ curves, respectively. In our work, the Stokes shift (Δ R) becomes larger with Sr substitutions. As a consequence, the energy between the exited state and intersection, which is the activation energy for thermal quenching, decreases, which is the Δ E₁ >Δ E₂ in the configurational coordinate diagram. This means that electrons need less energy (lower temperature) to reach the intersection in order to achieve quenching. Therefore, the thermal stability becomes worse with the replacement with Sr²⁺ ions [22]. For Ba_1-xSr_xSi₃O₄N₂:Ce³⁺, Li⁺ (x = 0.4) phosphor, the emission intensity of the Ce-doped phosphor remains at 66% of that measured at room temperature, and it decreases faster with increasing temperature such that at 230 °C, the emission intensity of the Ce³⁺-doped phosphor only remains at 42% in Fig. 6(d). The inset of Fig. 6(d) shows the temperature dependence of the integrated emission intensity of Ba_1-xSr_xSi₃O₄N₂: Ce³⁺ (x = 0, 0.4, 0.5). The integral PL intensity of BaSi₃O₄N₂: Ce³⁺ remains at 92% of its value when the temperature is raised from 50 °C to 150 °C. However, when x = 0.4 and 0.5, the integral PL intensity of Ce³⁺ drastically decreases with about 45% when the temperature is raised from 50 °C to 150 °C. This result indicates that the incorporation of Sr²⁺ reduces thermal stability at high temperatures, similar to that of Eu²⁺-doped Ba_1-xSr_xSi₃O₄N₂.

 

Fig. 6. The PL spectra of BaSi₃O₄N₂: Eu²⁺ (a) and Ba_1-xSr_xSi₃O₄N₂: Eu² (x = 0.4) (b) phosphors under various temperatures; The inset of (b) shows the dependence of normalized PL intensities on temperature for Eu-doped phosphors excited at 361 nm; The configurational coordinate diagram of Ba_1-xSr_xSi₃O₄N₂: Eu² (x = 0, 0.4) showing the thermal quenching process (c). The PL spectra of Ba_1-xSr_xSi₃O₄N₂:Ce³⁺, Li⁺ (x = 0.4) phosphor under various temperatures (d); The inset of (d) shows the dependence of normalized PL intensities on temperature for Ce-doped phosphors excited at 336 nm.

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4. Conclusion

In summary, a series of oxynitride phosphors, Ba_1-xSr_xSi₃O₄N₂: Eu²⁺/Ce³⁺, Li⁺ (0 ≤ x ≤ 0.5) have been investigated in this work. BaSi₃O₄N₂: Eu²⁺ showed intense absorption in the near ultraviolet region and exhibited bright green emission in the wavelength range of 450-650 nm. BaSi₃O₄N₂: Ce³⁺, Li⁺ can be excited efficiently over a broad spectral range between 250-350 nm, and exhibits a single intense purple-blue emission at 400 nm. Through the Sr substitution at Ba sites, a red-shift of about 30 and 7 nm in the emission spectra for Eu²⁺ and Ce³⁺-activated samples has been observed, respectively. This red-shift originates from the increase of crystal field strength and Stokes shift. Aside from that, the thermal quenching stabilities of both Eu²⁺ and Ce³⁺ Ba_1-xSr_xSi₃O₄N₂ worsen with the replacement with Sr²⁺ ions. Their fluorescence decay behavior and CIE values were discussed. Based on these results, the Ba_1-xSr_xSi₃O₄N₂: Eu²⁺ can be a good candidate phosphor applicable to n-UV light-emitting diodes for solid-state lighting.