Effect of Y³⁺ on the local structure and luminescent properties of La_3−xY_xSi₆N₁₁:Ce³⁺ phosphors for high power LED lighting

Abstract

A series of Ce³⁺-activated La_3−xY_xSi₆N₁₁ yellow phosphors were synthesized by high temperature solid-state reaction; their structures and luminescent properties were systematically investigated. The results of structural and spectral characterization showed that with the increase of Y³⁺ replacing La³⁺ in La_3−xY_xSi₆N₁₁:Ce³⁺ (0 < x ≤ 0.9), the space group remains P4bm but with Y³⁺ occupying two crystallographic sites in different ratio, while the emission spectra exhibit a red-shift from 535 nm to 552 nm. Moreover, the thermal quenching properties of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ exhibit somewhat of a decrease with the increase of x value, but still remains at 95% (x = 0.9) of the initial emission intensity measured at 200 °C. In addition, the light quality is significantly improved by direct packaging with a blue LED chip. The above results indicate that La_3−xY_xSi₆N₁₁:Ce³⁺ may be a promising phosphor for high power white-light LEDs.

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1. Introduction

During the past few decades, rapid progress has been made in the development of white light-emitting diodes (WLEDs) because of their superior characteristics, such as high efficiency, long lifetime and environmental friendliness.^1–5 However, rare earth ion activated white LED phosphors are mainly used in interior illumination or other medium–low power lighting. With the expansion of applications, white LEDs will gradually penetrate into the high-power lighting fields, such as automobile headlights, stadium, street lighting, industrialized illumination, and laser illumination. Therefore, developing phosphors for high power white-light LEDs may be a new trend in the future.^6–9 However, the white light obtained by the combination of current commercial YAG:Ce³⁺ phosphor with blue LED chips¹⁰ not only suffers from problems of spectrum defect in the red region,^11,12 which would result in high color-temperature (CCT) and low color-rendering index (CRI), but also unsatisfactory thermal quenching properties limiting their applications in high power w-LEDs.¹³ The shortage of spectrum deficiency can be improved by spectral tuning or adding red phosphors.^14–16 However, highly resisting to thermal quenching is mainly depended on phosphor matrix.

In recent years, growing interest has been focused on Ce³⁺-activated La₃Si₆N₁₁ yellow phosphor because it exhibits a broad emission band, high quantum efficiencies, especially, it shows superior thermal quenching property than YAG:Ce³⁺ phosphor. For these advantages, La₃Si₆N₁₁:Ce³⁺ becomes a commercially available yellow phosphor and already applied in high power lighting fields. La₃Si₆N₁₁ was first reported by Woike et al. in 1995, which was isostructural with Ce₃Si₆N₁₁ and Sm₃Si₆N₁₁.¹⁷ However, there were limited reports on Ce³⁺ doped La₃Si₆N₁₁ yellow phosphors because the pure phase can hardly be formed due to the harsh synthesis conditions. The yellow-emitting La₃Si₆N₁₁:Ce³⁺ phosphor was studied as host crystals of phosphor for the first time by Kijima in 2009.¹⁸ In 2013, George et al. further studied the relationship between local structural of Ce³⁺ and optical properties in detail and revealed two luminescent centers with different ratios of Ce³⁺ content.¹⁹ However, the emission peak of La₃Si₆N₁₁:Ce³⁺ at greenish yellow region (∼535 nm), which would result in low color rendering index and high correlated color temperature. In order to obtain a high quality w-LED, it is necessary to shift the emission spectrum of La₃Si₆N₁₁:Ce³⁺ to longer wavelength.

In this work, we chose Y³⁺ to substitute La³⁺ ion mainly due to the smaller ionic radius of Y³⁺ ion may large lattice shrink, leading to crystal field strength and finally resulting in red-shift of spectrum. However, the structure of Y₃Si₆N₁₁ ²⁰ has a completely different structure with La₃Si₆N₁₁, therefore, the solubility of Y³⁺ in La₃Si₅N₁₁ is limited. The structure and ionic occupation were studied by high-resolution synchrotron X-ray powder diffraction, the luminescent properties and thermal quenching properties of La_3−xY_xSi₆N₁₁:Ce³⁺ phosphors were also investigated in detail. The results demonstrated that La_3−xY_xSi₆N₁₁:Ce³⁺ phosphors exhibited tunable emission wavelength and excellent thermal stability.

2. Experimental

2.1 Synthesis

The La_3−zSi₆N₁₁:zCe³⁺ (0 ≤ z ≤ 0.2) and La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (0 < x ≤ 0.9) were synthesized by a high temperature solid state reaction. The stoichiometric amounts of LaN, α-Si₃N₄, YN, CeN powers were weighed out and ground in an agate mortar. All processes were performed in a dry glove box with purity nitrogen because the raw materials were sensitive to moisture and oxygen. The mixed powder were put into molybdenum crucibles and heated at 1750–1900 °C for 10 h with N₂ gas flow of 0.7 L min⁻¹ in the graphite resistance furnace.

2.2 Characterization

The powder X-ray diffraction (XRD) date of the phosphors were collected by an X-ray powder diffractometer (Rigaku, Japan) with Co-Kα radiation (λ = 0.178752 nm) operated at 40 kV and 100 mA with a scan speed of 6° min⁻¹ in the 2θ range of 10–80°. High-angular resolution synchrotron X-ray powder diffraction data were recorded at the BL44B2 beamline^21,22 of SPring-8 synchrotron radiation facility (Japan) using a constant wavelength of 0.50026 Å and a large Debye–Scherrer camera at room temperature and using a constant wavelength of 0.5 Å. The lattice parameters and cell volume were calculated by the Fullprof suite software. The morphology was investigated by using scanning electron microscopy (SEM; S4800). The crystal structure was visualized by using the software VESTA. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at room temperature by a spectrofluorometer (Fluoromax-4, Edison, USA) equipped with a 200 W xenon lamp. The external quantum efficiency (EQE) was recorded by an intensified multichannel spectrometer (QE-2100, Otsuka electronics, Japan).

3. Results and discussion

3.1 Phase and crystal structure of La_3−zSi₆N₁₁:zCe³⁺ (0 ≤ z ≤ 0.2)

Fig. 1 shows the XRD patterns of La_3−zSi₆N₁₁:zCe³⁺ (z = 0, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20) samples. Most of the diffraction peaks of all samples can be index to the standard card no. 48-1805 of La₃Si₆N₁₁, and show agreement with the pure phase of Ce₃Si₆N₁₁ (standard card no. 85-0113). As La₃Si₆N₁₁ and Ce₃Si₆N₁₁ have the same crystal structure and similar ionic radii (r[Ce³⁺] = 0.103 nm, r[La³⁺] = 0.106 nm), it is possible that the JCPDS standard pattern of La₃Si₆N₁₁ should be further confirm. It is worthy to note that we have successfully avoided the impurity phase LaSi₃N₅ ^18,19,23,24 using our sintering process. As seen in Fig. 1, the diffraction peaks shift to higher angle with the increase of Ce³⁺ concentration, ascribing to substitution of the smaller ionic radii of Ce³⁺ (0.103 nm) ion by the larger La³⁺ (0.106 nm) ion.

Fig. 1 XRD patterns of La_3−zSi₆N₁₁:zCe³⁺ samples (z = 0, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20).

As shown in Fig. 2, the basic structural of the La₃Si₆N₁₁ consists of SiN₄ tetrahedral and La³⁺ ions. The SiN₄ tetrahedra form two types of rings by linking at their corners. There are two La (La₍₁₎/La₍₂₎) sites with same coordination number in the host, which are surrounded by the Si₄N₄ and Si₈N₈ rings, respectively. To investigate the crystal field environment of surrounding Ce³⁺ ion, Rietveld refinement of high-angular resolution synchrotron X-ray diffraction analysis for La_2.86Ce_0.14Si₆N₁₁ is carried out and shown in Fig. 3. In our study, we use Ce₃Si₆N₁₁ as the initial model for structural refinement and the refined results are summarized in Table 1. Table 1 gives the basic crystallographic data of La_2.86Si₆N₁₁:0.14Ce³⁺. The parameters of the refinement are R_p = 3.45%, R_wp = 4.9%, and lattice parameters of a = 10.18852(1) Å, c = 4.84067(1) Å. Thus, the refinement parameters are reliable and illustrate that the pure phase La_2.86Si₆N₁₁:0.14Ce³⁺ is formed. As the Table 1 shows, the host material has two sites for activator Ce³⁺ ion. However, the vast majority of Ce³⁺ ions occupy the La₍₂₎ sites when the concentration of Ce³⁺ is 0.14 in spite of the ratio of total La₍₁₎ : La₍₂₎ was 2 : 1, which is in agreement with Nathan's report.¹⁴ As they explained, La₍₂₎ site has a smaller average La–N distance compared to the La₍₁₎ site [2.70 Å for La₍₁₎ compared to 2.62 Å for La₍₂₎]. The smaller ionic radii of Ce³⁺ prefers to substitution on the smaller La₍₂₎ site for low Ce concentrations (z ≤ 0.14).

Fig. 2 (a) Crystal structure of La₃Si₆N₁₁. (b) Coordination environment of La³⁺ ions.

Fig. 3 Rietveld refinement patterns against SPD data for La_2.86Si₆N₁₁:0.14Ce³⁺ (z = 0.14) using Ce₃Si₆N₁₁ as structure models.

Table 1 Selected crystallographic parameters from Rietveld refinement for the La_2.86Si₆N₁₁:0.14Ce³⁺ sample^a

Atom	x	y	z	Biso	Occu.
a Space group: P4bm (no. 100), a = b = 10.18852(1) Å, c = 4.84067(1) Å, α = β = γ = 90°, Z = 2, V = 502.480(1) Å^3, Rp = 3.45%, Rwp = 4.9%, χ^2 = 7.50.
La(1)	0.31922(2)	0.18078(2)	0.62714(6)	0.24054(2)	0.99(0)
Ce(1)	0.31922(2)	0.18078(2)	0.62714(6)	0.24054(2)	0.01(0)
La(2)	0	0	0.65326(6)	0.24054(3)	0.85(0)
Ce(2)	0	0	0.65326(6)	0.24054(3)	0.15(0)
Si(1)	0.07682(9)	0.21267(9)	0.12674(7)	0.90612(19)	1
Si(2)	0.61760(9)	0.11759(9)	0.61002(7)	0.90612(19)	1
N(1)	0.08392(9)	0.23264	0.43413	0.50000(4)	1
N(2)	0.65535(3)	0.15536(3)	0.93407(9)	0.50000(4)	1
N(3)	0	0.71831(3)	0.00024(7)	0.50000(4)	1
N(4)	0.50000(3)	0	0.58290(14)	0.50000(4)	1

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3.2 Photoluminescence properties of La_3−zSi₆N₁₁:zCe³⁺ (0 ≤ z ≤ 0.2)

Fig. 4 shows the excitation and emission spectra of La_2.86Si₆N₁₁:0.14Ce³⁺. The excitation spectrum shows two broad excitation bands with peaks at 360 nm and 455 nm, which are attributed to the transitions from the 4f to 5d states of Ce³⁺. Under the 460 nm excitation, a broad-band emission peaking at 535 nm with FWHM of 115 nm can be observed. The emission spectrum can be decomposed into two bands centered at 530 nm and 580 nm using Gaussian simulations. The energy gap between two bands is calculated to be 1817 cm⁻¹, which is in agreement with the theoretical difference between the ²F_5/2 and ²F_7/2 levels (∼2000 cm⁻¹).²⁵ The PL intensity (λ_ex = 460 nm) of La_3−zSi₆N₁₁:zCe³⁺ with different Ce³⁺ dopant concentrations are shown in Fig. 5. The optimal emission intensity is obtained when z reached 0.14.

Fig. 4 Photoluminescence emission (λ_ex = 460 nm) and excitation (λ_em = 535 nm) spectra of the La_2.86Si₆N₁₁:0.14Ce³⁺ phosphor.

Fig. 5 The emission intensity and peaking location of La_3−zSi₆N₁₁:zCe³⁺ with varying Ce³⁺ concentrations (z = 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20).

Additionally, the PL spectra (Fig. S1†) show that there is no obvious change in the location of the emission band with different z (0 ≤ z ≤ 0.2). According to the previous analysis, the activators Ce³⁺ ions mainly occupy La₍₂₎ site, and there is a La₍₁₎ atom between two adjacent La₍₂₎ atom (as shown in Fig. 2), indicating that the interatomic distance between two activators has changed slightly, and the interaction can be ignored. On the other hand, with similar ionic radius of Ce³⁺ ion and La³⁺ ion, the crystal field strength surrounding Ce³⁺ ion also remains little change when La³⁺ ion is substituted by Ce³⁺ ion. Hence, the emission peak of samples did not show significant shift toward longer wavelength. However, as a commercial phosphor, a tunable spectrum may be more significant for application. Based on the yellow phosphor Y₃Si₆N₁₁:Ce³⁺ as Liu reported,²⁰ we chose Y³⁺ to substitute part of La³⁺, and synthesised solid solution La_3−xY_xSi₆N₁₁:Ce³⁺ to make large red-shift of emission spectrum for the large difference of ionic radii between Y³⁺ and La³⁺.

3.3 Phase and crystal structure characterizations of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (0 < x ≤ 0.9)

The XRD patterns of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ with different doping Y³⁺ ion concentration are given in Fig. 6. The XRD patterns match well with the standard La₃Si₆N₁₁ pattern and a little shifts in the diffraction peaks are observed, which indicated that the solid solutions are formed in the range of 0 < x ≤ 0.9. When x = 1.1, a small amount of impurities such as YSi₃N₅, Y₂Si₃N₆ and LaSi₃N₅ are detected. The shifts of the diffraction peak positions are due to the change cell volume (Fig. S2†). The lattice parameter a decreases with the increase in Y³⁺ ion doping concentration and cell parameter c essentially keeps unchanged, which shows that the tetrahedral gets shrinkage at the a–b plane.

Fig. 6 XRD patterns of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1) samples.

The micro morphology of the La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (x = 0, 0.1, 0.5, 0.9) phosphor samples are studied by SEM (Fig. 7). It is observed that the particles with clear edges columnar crystal and the diameters are ranging from 20 to 30 μm (x = 0, 0.1). Irregular and smaller agglomerates particles were appeared with the increase of Y³⁺ concentration.

Fig. 7 SEM patterns obtained from La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (x = 0, 0.1, 0.5, 0.9) phosphors.

To further confirm the exactly ionic occupation information with different percentage of Y³⁺, high-resolution synchrotron powder diffraction experiments were performed. The Rietveld refinements of the crystal structures used the Fullprof software. The patterns were fitted with the pseudo-Voigt profile function. The phosphors La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (x = 0, 0.1, 0.5, 0.9) have been tested and the results of Rietveld refinements of the powder X-ray patterns are given in Fig. 8(a). The major parameters are provided in Table S1† (x = 0 can be read from Fig. 3 and Table 1). In order to fully understand the crystal field environment changes of surrounding Ce³⁺ ion, the lattice parameters change regularities for the samples with different x values are investigated. The distribution of Y³⁺ ion in the host lattice is investigated from the refinements result. As shown in Fig. 9(a), Y³⁺ ions mainly occupy the La₍₁₎ sites when doping with a small amount of Y³⁺ (x ≤ 0.1), which is due to the average La₍₁₎–N bond length is larger than the La₍₂₎–N and Y³⁺ ion is easier to enter the site of La₍₁₎ with less lattice distortion (x ≤ 0.1). With an increasing Y³⁺ ion doping concentration, the different La³⁺ sites are substituted by Y³⁺ with different proportion and Y³⁺ prefers to substitute on La₍₂₎ site (Fig. 8(b)). This phenomena is caused by the lattice shrinkage with the introduction of more Y³⁺ ions. In order to weaken the lattice distortion, more Y³⁺ ions occupy La₍₂₎ site preferentially because of the ionic radii of Y³⁺ is relatively closer to the La₍₂₎ site. As shown in Fig. 9(b), the average La₍₁₎/Y₍₁₎–N and La₍₂₎/Y₍₂₎–N bond length both are decreased gradually. Moreover, the average La₍₂₎/Y₍₂₎–N bond length reduces faster than La₍₁₎/Y₍₁₎–N bond length, which is consistent with the previous result. The cell parameters a, c and the volume of polyhedral V_La(1)/Y(1), V_La(2)/Y(2) also have been investigated (shown in Fig. 9(c)). Due to smaller ionic radius of Y³⁺, lattice constant a and b are decreased with increasing Y³⁺ concentration, while c is unchanged in all samples, which is similar with the reported in the literature.¹⁹ As shown in the inset of Fig. 3, all of La³⁺ are distributed in a–b plane which is separated by the other planes (consist of [Si₁N₄] tetrahedral). Thus, the doping Y³⁺ and Ce³⁺ ions are also diffused in a–b plane, which results in smaller change of the parameter c.

Fig. 8 (a) Rietveld refinements of SPD for x = 0.1, 0.5, 0.9. Observed intensities (black mark), calculated patterns (red line), Bragg positions (green line) and difference profile (blue line) are presented. (b) The average structure of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ obtained by refining against SPD date, the Y and La atoms are shown yellow, red balls, respectively.

Fig. 9 (a) Y occupancy of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ from the Rietveld refinement of synchrotron X-ray. (b) The average bond of La₍₁₎/Y₍₁₎–N, La₍₂₎/Y₍₂₎–N. (c) The crystal parameters of a, c and the volume of polyhedral V_La(1)/Y(1), V_La(2)/Y(2) from the Rietveld refinement of synchrotron X-ray.

3.4 Photoluminescence properties of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (0 < x ≤ 0.9)

Fig. 10 illustrates the room temperature emission spectra of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (0 < x ≤ 0.9) under 460 nm blue light excitation. The excitation spectra of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (0 < x ≤ 0.9) is given in Fig. S3.† As shown in Fig. 10(a), with the Y³⁺ doping concentration (x value) increased gradually, the emission intensity of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ remained unchanged, while the emission intensity decreased rapidly as x > 0.1. There are two possible reasons for the low emission of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺. Firstly, the crystallization characteristics of solid solution may be weakened and irregular and smaller agglomerates particles are appeared as the doping Y³⁺ concentration increased (Fig. 7), this may lower the luminescent properties greatly; secondly, as the doping ions Y³⁺, Ce³⁺ were diffused in the shrinkage a–b plan, the distance between Ce³⁺ ions became shorter, which enhance the nonradiative energy transfers. The normalized emission spectra (Fig. 10(b)) showed that the emission peaks significantly shifted from 535 nm (x = 0) to 552 nm (x = 0.9) with increasing Y³⁺ ion concentration. The red-shift ascribed to the larger splitting of the 5d energy level due to changing the local environment of Ce³⁺ ion.¹⁴ According to the above analysis, Ce³⁺ ion mainly substitute on the La₍₂₎ site when the Ce³⁺ ion doping content is 0.14, indicating that the crystal field splitting of the 5d energy level is mainly decided by the intensity of crystal field of polyhedral V_La(2)/Y(2). The volume of polyhedral V_La(2)/Y(2) gets shrinkage from 29.8939 ų (x = 0) to 27.6786 ų (x = 0.9) (Table S2†). The volume changes will lead to decrease in the length of the average of Ce₍₂₎–N bond and the variance of Ce₍₂₎–N bonds gets enough strength.²⁶ On the other hand, the structure symmetry of La₃Si₆N₁₁ host becomes lower with increasing Y³⁺ concentration, both of two factors are enhanced the crystal field around the Ce³⁺ ion. Based on the mechanism of electron transition, the emit energy (ΔE) which is the electron transition from the lowest 5d energy level to the 4f energy level declined with increasing x. Due to the value of ΔE₂ smaller than ΔE₁, the red shift of emission peak takes place with the wavelength from 535 nm to 552 nm, as depicted in Fig. 11. The results demonstrate that the luminescence properties of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ is tunable by changing the La/Y ratios, which could be more flexible for the generation of white light by combination of other phosphors and LED chips.

Fig. 10 (a) Emission spectra of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ phosphors under 460 nm Blue light excitation. (b) Normalized emission spectra of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ phosphors.

Fig. 11 The changes in 5d energy levels of the Ce³⁺ excited state with Y³⁺ ion introduce in LSN phosphor.

3.5 Temperature-dependent photoluminescence properties and application in white LEDs

The excellent thermal stability of phosphors is one of important indicator for fabricating a high efficiency white LEDs, especially in high-power lighting fields. Fig. 12(a) shows the temperature-dependent luminescent spectra of La_2.86Si₆N₁₁:0.14Ce³⁺ (LSN) under 460 nm excited. La_2.86Si₆N₁₁:0.14Ce³⁺ exhibits a small thermal quenching, which remains 97.6% of the emission intensity measured at 200 °C. It is also observed that the emission spectra gets a little red shift with the temperature increasing from 50 °C to 200 °C, which may be caused by the larger Stokes shift at high temperature. The structure gets relaxation or lattice expansion with increasing temperature, just like the effect of the incorporation of Ce³⁺ ion, leading to the emission bond gets red shift.¹⁵ The temperature-dependent of the relative emission intensities for La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) (LYSN) and YAG:Ce³⁺ under 460 nm excitation in the range of 25–200 °C are shown in Fig. 12(b). The results suggest that the thermal quenching properties have a trend of slight drop with increasing Y³⁺ content. In order to further explain the high thermal quenching, configurational coordinate diagram was carried out, shown in Fig. S4.†^27–30 As shown, the position of 5d energy level is controlled by the emit energy and the force constants of the chemical bond. With the above analysis, the emit energy (ΔE) of LYSN is smaller than LSN, indicating that the 5d energy level will decline and the crossing point A is dropped to A′, as shown by the red line. The force constants of Ce–N bonds can be considered essentially unchanged due to the small red-shift of spectral. Moreover, the crystal structure symmetry of LYSN will be lower because of the difference of structure between LSN (tetragonal) and YSN (orthorhombic). These two factors are responsible for the thermal quenching of LYSN. However, compared with the commercial YAG:Ce³⁺ phosphor, La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ still show a small thermal quenching temperature. The extraordinary thermal quenching property indicates that La_3−xY_xSi₆N₁₁:Ce³⁺ have a wide application in high-power lighting fields.

Fig. 12 (a) Temperature-dependent PL spectra of La_2.86Si₆N₁₁:0.14Ce³⁺; inset shows the normalized emission intensity as a function of temperature (λ_ex = 460 nm). (b) The dependence of normalized PL intensities on temperature for phosphors, excited at 460 nm.

Fig. 13 shows the corresponding CIE coordinates of the samples of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ with different doping Y³⁺ content. The chromaticity coordinates of LYSN phosphors can be tuned from greenish-yellow region (0.4245, 0.5528) to yellow region (0.4727, 0.5150). To evaluate the new solid solutions La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ phosphors for application in white-LEDs, the La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (x = 0, 0.5, 0.9) samples are packaged with 445 nm InGaN-based chip with the driven current at 60 mA. The corresponding CCT, CRI, CIE, and (EQE) are summarized in Table 2. The results indicate that the optical characteristics have been improved apart from the luminous efficiency, which is a great significance for warm white light output. However, a further study on how to improve the efficiency of LYSN is still necessary, such as controlling the particle size, distribution, morphology and crystalline.³¹ The outstanding thermal stability and tunable luminescence properties of the LYSN are presented here for evaluating their potential applications for high quality w-LEDs.

Fig. 13 Color coordinate of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) as-prepared under 445 nm. The inset shows the photograph of the La_2.76Y_0.1Si₆N₁₁:0.14Ce³⁺ phosphor under the nature light.

Table 2 The CCT, CRI, CIE of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ samples (x = 0, 0.5, 0.9) under a drive current of 60 mA and external quantum efficiency of the samples under 450 nm

x	CCT (K)	CRI	CIE (x, y)	EQE
0	5570	64.5	(0.33, 0.36)	0.76
0.5	5146	68.9	(0.35, 0.40)	0.34
0.9	4437	74.2	(0.37, 0.41)	0.18

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4. Conclusions

A series of nitride yellow phosphors La_3−xY_xSi₆N₁₁:Ce³⁺ were synthesized by solid-state reaction. The crystal structure, luminescence property and thermal quenching property had been investigated. Two different sites for Ce³⁺ ion in the host and Ce³⁺ were favoring substitution on the smaller La₍₂₎ site. By introducing Y³⁺ into the La₃Si₆N₁₁ host and the structure determination demonstrates that the different La³⁺ sites were substituted by Y³⁺ with different proportion and Y³⁺ prefers to substitute on La₍₂₎ site, and the crystal field around the Ce³⁺ ion is enhanced, resulting the emission spectra tuned from 535 nm to 552 nm. A warm white light with relative high CRI and lower CCT can be obtained under the blue light excitation. La_3−xY_xSi₆N₁₁:Ce³⁺ (0 ≤ x ≤ 0.9) exhibits a small thermal quenching, which remains 97.6% (x = 0) and 95% (x = 0.9) of the initial emission intensity measured at 200 °C, respectively. The tunable luminescence properties and outstanding thermal stability indicate that this new solid solutions can be potential phosphor for blue-chip LED applications, especially for the warm white light in high power field.

Acknowledgements

The work is financially supported from the National Key Basic Research Program of China (2014CB643801) and the National Natural Science Foundation of China (51102021, 51302016). The synchrotron radiation experiments were performed at the BL44B2 of Spring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2015B1127 and 2016A1060).

References

1. E. F. Schubert and J. K. Kim, Science, 2005, 308, 1274–1278.
2. M. S. Shur and R. Zukauskas, Proc. IEEE, 2005, 93, 1691–1703.
3. T. Justel, H. Nikol and C. Ronda, Angew. Chem., Int. Ed., 1998, 37, 3084–3103.
4. R. J. Xie and N. Hirosaki, Sci. Technol. Adv. Mater., 2007, 8, 588–600.
5. C. C. Lin and R. S. Liu, J. Phys. Chem. Lett., 2011, 2, 1268–1277.
6. X. J. Zhang, L. Huang, F. J. Pan, M. M. Wu, J. Wang, Y. Chen and Q. Su, ACS Appl. Mater. Interfaces, 2014, 4, 2709–2717.
7. X. J. Zhang, J. Wang, L. Huang, F. J. Pan, Y. Chen, B. F. Lei, M. Y. Peng and M. M. Wu, ACS Appl. Mater. Interfaces, 2015, 18, 10044–10054.
8. X. J. Zhang, J. B. Yu, J. Wang, C. B. Zhu, J. H. Zhang, R. Zou, B. F. Lei, Y. L. Liu and M. M. Wu, ACS Appl. Mater. Interfaces, 2015, 51, 28122–28127.
9. L. Huang, Y. W. Zhu, X. J. Zhang, R. Zou, F. J. Pan, J. Wang and M. M. Wu, Chem. Mater., 2016, 5, 1495–1502.
10. C. H. Huang, Y. C. Chiu, Y. T. Yeh, T. S. Chan and T. M. Chen, ACS Appl. Mater. Interfaces, 2012, 4, 6661–6668.
11. R. Kasuya, A. Kawano, T. Isobe, H. Kuma and J. Katano, Appl. Phys. Lett., 2007, 91, 111916-1–111916-3.
12. R. M. Mach, G. Mueller, M. R. Krames, H. A. Hoppe, F. Stadler, W. Schnick, T. Juestel and P. Schmidt, Phys. Status Solidi A, 2005, 202, 1727–1732.
13. J. Y. Zhong, W. D. Zhuang, X. R. Xing, L. G. Wang, Y. F. Li, Y. L. Zheng, R. H. Liu, Y. H. Liu and Y. S. Hu, J. Alloys Compd., 2016, 674, 93–97.
14. A. A. Setlur, W. J. Heward, M. E. Hannah and U. Happek, Chem. Mater., 2008, 20, 6277–6283.
15. Y. Q. Li, N. Hirosaki, R. J. Xie, T. Takeda and M. Mitomo, Chem. Mater., 2008, 20, 6704–6714.
16. S. P. Lee, C. H. Huang, T. S. Chan and T. M. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 7260–7267.
17. M. Woike and W. Jeitschko, Inorg. Chem., 1995, 34, 5105–5108.
18. T. Seto, N. Kijima and N. Hirosaki, ECS Trans., 2009, 25, 247–252.
19. N. C. George, A. Birkel, J. Brgoch, B. C. Hong, A. A. Mikhailovsky, K. Page, A. Llobet and R. Seshadri, Inorg. Chem., 2013, 52, 13730–13741.
20. L. H. Liu, R. J. Xie, W. Y. Li, N. Hirosaki, Y. Yamamoto and X. D. Sun, J. Am. Ceram. Soc., 2013, 96, 1688–1690.
21. K. Kato, R. Hirose, M. Takemoto, S. Ha, J. Kim, M. Higuchi, R. Matsuda, S. Kitagawa and M. Takata, The RIKEN Materials Science Beamline at Spring-8: Towards Visualization of Electrostatic Interaction, Australia, 2010, pp. 9–10.
22. K. Kato and H. Tanaka, Advances in Physics: X, 2016, 1, 55–80.
23. W. B. Prak, T. H. Kwon and K. S. Sohn, J. Am. Ceram. Soc., 2015, 98, 490–494.
24. T. Suehiro, H. Hirosaki and R. J. Xie, ACS Appl. Mater. Interfaces, 2011, 3, 811–816.
25. Z. G. Xia and R. S. Liu, J. Phys. Chem. C, 2012, 116, 15604–15609.
26. W. F. Huang, F. Yoshimura, K. Ueda, Y. Shimomura, H. S. Sheu, T. S. Chan, H. F. Greer, W. Z. Zhou, S. F. Hu, R. S. Liu and J. P. Attfield, Angew. Chem., 2013, 125, 8260–8264.
27. C. Wang, Z. Y. Zhao, Q. S. Wu, S. Y. Xin and Y. H. Wang, CrystEngComm, 2014, 16, 9651–9656.
28. L. Chen, R. H. Liu, W. D. Zhuang, Y. H. Liu, Y. S. Hu, X. F. Zhou, W. Gao and X. L. Ma, CrystEngComm, 2015, 17, 3687–3694.
29. Y. T. Tsai, C. Y. Chiang, W. Z. Zhou, J. F. Lee, H. S. Sheu and R. S. Liu, J. Am. Ceram. Soc., 2015, 137, 8936–8939.
30. F. J. Pan, M. Zhou, J. H. Zhang, X. J. Zhang, J. Wang, L. Huang, X. J. Kuang and M. M. Wu, J. Mater. Chem. C, 2016, 4, 5671–5678.
31. Z. G. Xia, S. H. Miao, M. S. Molokeev, M. Y. Chen and Q. L. Liu, J. Mater. Chem. C, 2016, 4, 1336–1344.

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Footnote

† Electronic supplementary information (ESI) available: Parameters information of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ (x = 0.1, 0.5, 0.9) and crystallographic data after refinements of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺. Excitation spectra of La_2.86−xY_xSi₆N₁₁:0.14Ce³⁺ and emission spectra of La_3−zSi₆N₁₁:zCe³⁺. See DOI: 10.1039/c6ra18460j

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