Optical properties of yellow-emitting Sr₃LaNb₃O₁₂:Dy³⁺ phosphors with an abnormal thermal quenching for white light-emitting diode applications

Abstract

In this study, a series of Sr₃LaNb₃O₁₂ phosphors doped with Dy³⁺ ions was prepared by a high-temperature solid-state reaction. The crystal structure, phase purity, particle morphology, photoluminescence (PL) properties, and thermal stability of the phosphors were described. And the Sr₃LaNb₃O₁₂ crystal symmetry is hexagonal. Under the optimal excitation wavelength of 352 nm, the three prominent emission peaks of the Sr₃LaNb₃O₁₂:Dy³⁺ focused on 483 nm (⁴F_9/2-⁶H_15/2), 575 nm (⁴F_9/2-⁶H_13/2), and 668 nm (⁴F_9/2-⁶H_11/2). The optimal Dy³⁺-doped concentration in the Sr₃LaNb₃O₁₂ phosphor was 0.05 mol. The phosphor had an abnormal thermal quenching phenomenon and activation energy (E_a = 0.29 eV). The prepared white light-emitting diode (w-LED) had a low correlation color temperature of 4162 K, chromaticity coordinates (0.365, 0.368), and the color rendering index 91. The feasibility of the Sr₃LaNb₃O₁₂:Dy³⁺ luminescent material for w-LED was verified by its performance.

1 Introduction

White light-emitting diodes (W-LEDs) have the advantages of low energy consumption, environmental protection, and higher efficiency, which remedies some disadvantages of incandescent and traditional fluorescent lamps [1,2,3,4,5,6,7,8]. Nowadays, commercial w-LEDs are based on blue light-emitting diode (LED) chips using the YAG:Ce³⁺. The blue chips may take a toll on health [9, 10]. For example, it leads to the inhibition of melatonin secretion, myopia, and yellow spots [11, 12]. The phosphor conversion to n-UV chips stimulating w-LEDs can solve these problems. Therefore, it is of great significance to develop a new yellow phosphor for w-LEDs based on n-UV chips.

In recent years, Dy³⁺ ions doped phosphors have been studied for w-LED in many rare-earth ions. And Dy³⁺ ions doped phosphors have good optical properties in many reports [13,14,15]. The rare-earth Dy³⁺ ions are highly efficient activators and usually have a blue peak, prominent yellow emission peak, and low-intensity red emission peak, corresponding to ⁴F_9/2-⁶H_J/2 (J = 15, 13, 11) transitions [16,17,18]. White light is obtained by taking the host materials and doped Dy³⁺ ions in combination. Emission peaks of rare-earth Dy³⁺ ions doped materials can show white light and the CIE coordinates can be reflected in the white light area. Therefore, many Dy³⁺-doped materials are ideally combined with an n-UV chip and successfully applied in the w-LED [19,20,21].

An excellent host material favors the luminescence properties of doped ions. The hexagonal perovskite Sr₃LaNb₃O₁₂ is a good microwave dielectric material with low loss, whose space group is R_3m. At present, many hexagonal perovskite phosphors are applied in w-LEDs. Peng et al. compounded Sr₃La(VO₄)₃:Sm³⁺ phosphors which have good thermal stability [22]. Priyanka et al. researched the luminescence properties of Dy³⁺-activated BaYAlZn₃O₇ phosphors, a fine cold-white-emitting material [23]. Zhang et al. studied luminescence properties and the thermal stability of Sr₂CaLa(VO₄)₃:Sm³⁺ phosphors with higher activation energy [24]. These show that the phosphors with hexagonal perovskite structures have good optical properties. However, the luminescence properties of Sr₃LaNb₃O₁₂:Dy³⁺ phosphors haven’t been studied.

Here, novel Dy³⁺-doped Sr₃LaNb₃O₁₂ phosphors were compounded by a solid-state reaction. The structural and optical properties of phosphors were researched, including phase purity, crystal structure, particle morphology, element composition, CIE chromaticity coordinates, photoluminescence properties, and the abnormal thermal stability [25,26,27]. Finally, w-LEDs were prepared using multicolor phosphors based on an n-UV chip.

2 Experimental section

Dy³⁺-doped Sr₃La_1-xDy_xNb₃O₁₂ (x = 0.01, 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 mol) phosphors were prepared by the traditional high-temperature solid-state technique. High purity raw materials were SrCO₃ (99%), La₂O₃ (99.99%), Dy₂O₃ (99.99%), and Nb₂O₅ (99.9%). These raw materials were weighed by following the equilibrium equation and ground for around 16 min in the agate mortar. Then the samples were put in the small crucibles. The samples firstly were heated at 1200 °C for four hours in the muffle furnace. When the preparation firing reaction was completed, they were cooled to room temperature. Afterward, the prepared samples were continued to be heated at 1320 °C for twelve hours. After a series of reactions were completed, the samples were fetched and completely smashed. Then color and hardness of products were recorded and used for the next luminescence characterization. The related experimental equation is given as follows:

$$\begin{aligned} 3{\text{SrCO}}_{3} + & 0.5(1 - x){\text{La}}_{2} {\text{O}}_{3} + 0.5x{\text{Dy}}_{2} {\text{O}}_{3} \\ + & 1.5{\text{Nb}}_{2} {\text{O}}_{3} \mathop{\longrightarrow}\limits^{(1)1200^{\circ } {\text{C}} \times 4{\text{h}}}_{(2)1320 \times 12h}{\text{Sr}}_{3} {\text{La}}_{{(1 - x)}} {\text{Dy}}_{x} {\text{Nb}}_{3} {\text{O}}_{{12}} \\ \end{aligned}$$

The phase purity analysis of Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) phosphors were revealed on the X-ray diffractor (XRD, Bruker D2 PHASER) with Cu Ka source (λ = 0.154184 nm) by scanning from 10° to 70° range. The crystal structure was refined the Maud software. The sample particle size was obtained passing by a laser particle size analyzer (Malvern MS2000). The appearance and composition of the phosphor element were determined with scanning electron microscopy (Nova Nano SEM-450) equipped with energy-dispersive X-ray spectrum (EDS). The photoluminescence (PL) spectra and temperature-dependent spectra were measured via Edinburgh FLS 980 spectrometer, which used a 450 W Xenon lamp as the excitation source.

The prepared yellowing Sr₃LaNb₃O₁₂:0.05Dy³⁺ phosphor was mixed with a bit of commercial red phosphor ((Sr, Ca)AlSiN₃:Eu²⁺) and blue phosphor (BaMgAl₁₀O₁₇:Eu²⁺ (BAM)) to obtain the required mixture, whose ratio is 98%, 0.5%, and 1.5%, respectively. The mixture was added with a moderate amount of shadowless adhesive and combined with a 388 nm n-UV InGaN chip. The obtained product was cured under a UV lamp for 5 min, and finally, the w-LED can be fabricated. Then CIE chromaticity coordinates, electroluminescence (EL) spectrum, R_a, and correlation color temperature (CCT) can be achieved using the USB 4000 fiber optic spectrometer (Ocean Optics).

3 Results and discussion

The crystal structure of the Sr₃LaNb₃O₁₂ phosphor is shown in Fig. 1. It indicates that M (M = Nb/Sr/La) ions occupy a 3a lattice site in the proportion of 14% La, 28% Sr, and 58% Nb. M coordinating with six O ions at the position 3a lattice site forms the group, which is a regular octahedral biconical whose M–O distance is 1.91 Å. In the standard data, the cell parameters of the Sr₃LaNb₃O₁₂ are a = b = 5.6590 Å, c = 27.1780 Å, a/c = 0.2082 Å, V = 753.8 ų, and Z = 3.

Fig. 1

a, b The crystal structure of the Sr₃LaNb₃O₁₂, c the coordination environment of M (Nb, Sr, and La)

The standard (PDF#00–041-0148) card of the Sr₃LaNb₃O₁₂ and eight XRD patterns of the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) samples are represented in Fig. 2a. The peaks of XRD have very narrow diffraction and show no evident mismatched peaks, which proves that the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) samples were successfully synthesized, and these phosphors are high phase purity. Using the substitution formula of the Dy³⁺ can calculate the substitution of the host lattices [28, 29]:

$$D_{{\text{r}}} = \frac{{R_{{\text{S}}} \left( {{\text{CN}}} \right) - R_{{\text{d}}} \left( {{\text{CN}}} \right)}}{{R_{{\text{S}}} \left( {{\text{CN}}} \right)}} \times 100\%,$$

(1)

where R_S is represented for the radius of the substituted cation, CN is on behalf of the coordination number, and D_r stands for the difference between the radius percentage, R_d represents the radius of the doped ions. The radius of Sr²⁺ ions is 1.180 Å and the radius of Nb³⁺ is 0.720 Å, whose D_r is 22.71% and -26.67%. But the radius of Dy³⁺ ions is 0.912 Å and its D_r is 11.62% which is the smallest and far less than 30%. Dy³⁺ ions and La³⁺ ions stay in the uniform valence state. The results indicate that the Dy³⁺ ion prefers to replace the La³⁺ ion. So The Dy³⁺ can quickly get into the host lattice, and it has no evident effect on the Sr₃LaNb₃O₁₂ crystal system.

Fig. 2

a The XRD patterns of the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) phosphors, b Rietveld refinement results of the Sr₃LaNb₃O₁₂:0.01Dy³⁺

In the Fig. 2b, the Rietveld refinement of the Sr₃LaNb₃O₁₂:0.01Dy³⁺ phosphor was researched by using Maud software [30]. In Tables 1 and 2, the crystal structure parameters and Rietveld refinement data are shown. The lattice parameters of the Sr₃LaNb₃O₁₂:0.01Dy³⁺ sample is a = b = 5.65 Å, c = 27.19 Å, a/c = 0.2078, V = 753.28 ų, and Z = 3. The obtained fitting parameters of Sr₃LaNb₃O₁₂:0.01Dy³⁺ sample are R_wp = 14.95% and χ² = 7.57, which indicates that the Dy³⁺ ions well replace La³⁺ ions in the Sr₃LaNb₃O₁₂ phosphors.

Table 1 The cell parameters of the Sr₃LaNb₃O₁₂:0.01Dy³⁺ phosphor

Table 2 The Rietveld refinement data of the Sr₃LaNb₃O₁₂:0.01Dy³⁺ phosphor

The surface morphology and size of phosphor is also necessary to study the use of w-LEDs. Figure 3a indicates SEM image of the Sr₃LaNb₃O₁₂:0.01Dy³⁺ phosphor at 2400 magnification. The products are large conglomerations due to a high-temperature process. Figure 3b shows the particle size distribution of phosphor. D(50) is 24.418 μm, which represents that 50% of the particles were less than 24.418 μm. D(90) is 68.662 μm, which represents that 90% of the particles were less than 68.662 μm. The particle size distribution of 22.556 μm is the largest cell volume percentage, which describes that they are suitable for use in solid-state lighting.

Fig. 3

a The SEM image of the Sr₃LaNb₃O₁₂:0.01Dy³⁺, b the particle distribution of the Sr₃LaNb₃O₁₂:0.01Dy³⁺

To confirm the elemental composition of samples, the type of the element can be analyzed using the EDS spectrum. By the EDS spectrum of the Sr₃LaNb₃O₁₂:Dy³⁺ phosphor, all the elemental peaks can be observed containing Sr, La, Nb, O, and Dy. Figure 4b–g shows all the elements were even distribution. The weight percentages of the elements Sr, La, Nb, O and Dy are 29.27%, 16.74%, 31.34%, 22.29%, and 0.36%. The atomic percentages are 15.27%, 5.51%, 15.42%, 63.70%, and 0.10%. The results were consistent with the outcome of Sr₃LaNb₃O₁₂:0.01Dy³⁺ phosphors. These proved that we successfully made the Sr₃LaNb₃O₁₂:Dy³⁺ phosphors.

Fig. 4

a EDS spectrum and b–g elemental mapping of Sr, Nb, O, La, and Dy of the Sr₃LaNb₃O₁₂:0.01Dy³⁺ phosphor

The excitation spectrum of the Sr₃LaNb₃O₁₂:0.05Dy³⁺ is shown in Fig. 5a (λ_em = 575 nm). The O²⁻—Dy³⁺ charge transfer (CT) causes a weak broad band ranging from 240 to 310 nm [31]. In the 300–500 nm excitation spectrum, the seven remarkable peaks lie in 326, 352, 366, 388, 426, 454, and 475 nm, corresponding to the transition of the ⁶H_15/2 level to the ⁶P_3/2, ⁶P_7/2, ⁶P_5/2, ⁴I_13/2, ⁴G_11/2, ⁴I_15/2, and ⁴F_9/2 level [32, 33]. Among these excitation peaks, the 352 nm excitation is the most favorable wavelength because of the Dy³⁺ transition (⁶H_15/2-⁶P_7/2), which will be suitable for the development of the n-UV-based w-LEDs.

Fig. 5

a The excitation spectrum of the Sr₃LaNb₃O₁₂:0.05Dy³⁺ (λ_em = 575 nm), b The emission spectrum of the Sr₃LaNb₃O₁₂:0.05Dy³⁺ (λ_ex = 352 nm)

Figure 5b describes the emission spectrum of Sr₃LaNb₃O₁₂:0.05Dy³⁺ under the 352 nm excitation, delegated the three prominent emission peaks at 483 nm (⁴F_9/2-⁶H_15/2), 575 nm (⁴F_9/2-⁶H_13/2), and 668 nm (⁴F_9/2-⁶H_11/2). The most substantial peak was the yellow emission (⁴F_9/2-⁶H_13/2), but it was easily impacted by the external coordination environment, which is ascribed to the pure electric dipole transition. In contrast, the lower peak was the blue emission (⁴F_9/2-⁶H_13/2), which was not affected by the crystal field. The blue area (483 nm) of Dy³⁺ ions was lower than the yellow emission (575 nm) [34]. This phenomenon was identified with the fact that the Dy³⁺ ions occupied a lower symmetrical position.

To study the relationship between the doped Dy³⁺ concentration and the emission intensities, the emission spectra (λ_ex = 352 nm) of the Sr₃LaNb₃O₁₂:xDy³⁺ phosphors (x = 0.01–0.30 mol) were shown in Fig. 6a. Figure 6b clearly describes the relationship between the doping concentration of Dy³⁺ and the peak intensity of Sr₃LaNb₃O₁₂:xDy³⁺ by normalizing the maximum emission intensity. At the low doping concentration of Dy³⁺ ions, the luminous intensities gradually rises till the distance of centers reaches the critical distance where the optimal doping concentration was 0.05 mol in the Sr₃LaNb₃O₁₂:Dy³⁺. After this point, the luminescence intensity presents the trend of falling, and the energy transfer ability has weakened. Figure 6c indicates the corresponding energy level schemes of the Dy³⁺. Under the excitation of 352 nm, electrons are firstly transmitted from the ground state ⁶H_15/2 level to the higher excited state ⁴I_15/2 level. Then the excited electrons go through the non-radiative (NR) transition to ⁴F_9/2 level. In the end, the typical radiative transitions ⁴F_9/2 to ⁶H_M/2 (M = 15, 13, and 11) of Dy³⁺ ions by can be observed, corresponding to the blue peak at 483 nm, the yellow main peak at 575 nm, and the red peak at 668 nm. Meanwhile, this process quickly releases the energy of phosphors through the non-radioactive transition.

Fig. 6

a The PL emission of the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) at 352 nm, b The relationship between the doping concentrations of Dy³⁺ and the peak intensities of Sr₃LaNb₃O₁₂:xDy³⁺, c The energy electron transition processes of Dy³⁺ levels diagram, d The relationship between lg (I/x) and lg(x) in the Sr₃LaNb₃O₁₂:Dy³⁺ samples

To study the interaction mechanism between Dy³⁺ ions, the critical distance (R_c) is calculated. The value of the R_c is calculated by the subsequent formula [35, 36]:

$$R_{{\text{c}}} \approx 2\left[\frac{3V}{{4\pi X_{{\text{c}}} N}}\right]^{\frac{1}{3}},$$

(2)

where N is the number of cations, X_c is the optimal Dy³⁺-doped concentration of the Sr₃LaNb₃O₁₂, and V is the volume of the cell. By substituting the value of X_c (0.05 mol), V (752.11 ų), and N (3), the energy transfer distance can be calculated (21.25 Å), which is far larger than the critical distance (5 Å) of exchange. Therefore, the concentration quenching (CQ) is caused by the multipole-multipole interaction.

To further explore the connection between the Dy³⁺-doping concentration and the luminous intensity, Dexter’s model can be used. Dexter’s model established the energy transfer between the same kinds of activators. On the basis of Dexter’s theory, the specific role of the multipole energy transfer mechanism can be explored [37, 38]:

$$\frac{I}{x} = K\left[ {1 + \beta \left( x \right)^{{\frac{\alpha }{3}}} } \right]^{ - 1},$$

(3)

where I stands for emission intensity of different Dy³⁺ concentrations; x is the Dy³⁺ concentration; β and K are constants at a certain excitation wavelength in the same system. α represents a multipole interaction constant including three types of mechanism interaction: dipole–dipole, dipole-quadrupole, and quadrupole–quadrupole, corresponding to the α = 6, 8, and 10. Figure 6d shows that the connection with x and I/x has a negative correlation relation, and the slope is determinately -1.81. So the value of α is 5.43, which is close to 6. Therefore, the combined action of dipole–dipole causes the CQ of the Dy³⁺-doped Sr₃LaNb₃O₁₂ phosphors.

The intensity of ⁴F_9/2-⁶H_15/2 and ⁴F_9/2-⁶H_13/2 transition at different concentrations is shown in Fig. 7. ⁴F_9/2-⁶H_15/2 represents the magnetic dipole (MD) transition, and ⁴F_9/2-⁶H_13/2 transition denotes the electric dipole (ED) transition. ED intensity is affected by the ligand ions in the crystal field [39]. The intensity of MD transition is weaker than the ED, which states that Dy³⁺ occupies the low symmetry site in the Sr₃LaNb₃O₁₂ matrix. Here, the asymmetric ratio ED/MD (R) play an important role in analyzing the symmetry of the lattice environment around Dy³⁺. The R by calculating is 3.27, 3.00, 2.93, 2.77, 2.60, 2.42, 2.11, and 3.19, corresponding to x = 0.01, 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30, respectively. The R of Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) phosphors is greater than 1, which explains that Dy³⁺ ion occupied inversion symmetric lattice sites in the Sr₃LaNb₃O₁₂ crystal structure [40].

Fig. 7

The intensity of ⁴F_9/2-⁶H_15/2 and ⁴F_9/2-⁶H_13/2 transition at different concentrations

Thermal stability plays an important role that is analyzed to decide whether the phosphors are suitable for the w-LED application. When the LED chip temperature can reach about 420 K, excellent phosphors can sustain high temperatures and against a color shift. As follows, the effect of changing temperature in the range of 300–480 K on the luminescent intensity of the Sr₃LaNb₃O₁₂:0.02Dy³⁺ and Sr₃LaNb₃O₁₂:0.05Dy³⁺ were also studied in Fig. 8a, b. The emission intensity rise and then fall when the temperature increase but the emission peaks of both phosphors hardly shift. It can also be observed through the normalized integral emission intensity. Figure 8c, d clearly plots the normalized integrated PL intensities versus temperature for representative samples. It engenders thermal quenching, and compared with the luminous intensity of the initial temperature, the intensity of 480 K has not obviously fallen. For the Sr₃LaNb₃O₁₂:0.02Dy³⁺ phosphor, the PL intensity increased abnormally by approximately 6% from 300 to 340 K. For the Sr₃LaNb₃O₁₂:0.05Dy³⁺ phosphor, the PL intensity increased abnormally by approximately 13% from 300 to 360 K. Then the intensities of the both phosphors decreased with the further increase in temperature.

Fig. 8

(a, b) and (c, d) of the temperature-dependent emission spectra of the Sr₃LaNb₃O₁₂:0.02Dy³⁺ and Sr₃LaNb₃O₁₂:0.05Dy³⁺, and the emission intensity of the Sr₃LaNb₃O₁₂:0.02Dy³⁺ and Sr₃LaNb₃O₁₂:0.05Dy³⁺

The phosphor exhibited abnormal thermal quenching, first discovered in Na₃Sc₂(PO₄)₃:Eu²⁺ blue phosphors in 2017 [41]. It was later found in several phosphors [42,43,44,45,46]. In the Sr₃LaNb₃O₁₂ crystal, Sr, La, and Nb ions occupy the same crystal sites. Moreover, the radius of Sr²⁺ (r = 1.18 Å, CN = 6) is very close to that of Dy³⁺ (r = 0.912 Å, CN = 6). Dy³⁺ may enter into the Sr²⁺ lattice of Sr₃LaNb₃O₁₂, an unbalanced charge substitution causing lattice defects [47]. Therefore, this abnormal thermal quenching phenomenon can ascribe to the lattice defects, which function like electron capture centers. In the end, the remaining energy of the excited electron was transferred and stored in Dy³⁺, which was higher than the loss caused by NR transition. Therefore, the emission intensity became stronger as temperature increased. It showed good thermal stability and potential in high-power photoelectric devices, especially w-LEDs.

Besides the variation of emission spectral intensity, the change of CIE chromaticity coordinates can also be used to evaluate the stability of phosphors at different temperatures. The CIE chromaticity coordinates of the Sr₃LaNb₃O₁₂:0.02Dy³⁺ and Sr₃LaNb₃O₁₂:0.05Dy³⁺ vary only slightly from 300 to 480 K in Fig. 9a, b. Therefore, the phosphors are almost the same color. The changes in CIE chromaticity coordinates of the Sr₃LaNb₃O₁₂:0.02Dy³⁺ and Sr₃LaNb₃O₁₂:0.05Dy³⁺, respectively are (∆x = 3.89 × 10^–5, ∆y = 6.94 × 10^–5) and (∆x = 3.33 × 10^–6, ∆y = 0.89 × 10^–5) by calculating. So the phosphors manifest good coloring stability at high temperatures.

Fig. 9

a, b Chromaticity coordinates at different temperatures of the Sr₃LaNb₃O₁₂:0.02Dy³⁺ and Sr₃LaNb₃O₁₂:0.05Dy³⁺

For the sake of learning the mechanism of thermal quenching, the research knows more about the activation energy (E_a) of thermal quenching. By the Arrhenius model combined with experimental data, the relational graph of ln[(I₀/I) − 1] ~ 1/T was used to achieve the activation energy in Fig. 10. The following is the Arrhenius equation [48,49,50]:

$$I\left( T \right) = \frac{{I_{0} }}{{1 + c {\text{exp}}\left( { - E_{{\text{a}}} /kT} \right)}},$$

(4)

where I₀is the initial luminous intensity; k is a Boltzmann constant, which is 8.62 × 10^–5 eV/K; I(T) is the luminous intensity in different temperatures; c is a constant; E_a is the constant activation energy. The calculated E_a of the Sr₃LaNb₃O₁₂:Dy³⁺ was 0.29 eV, which further indicated that the Sr₃LaNb₃O₁₂:xDy³⁺ have good thermal stability.

Fig. 10

The relevance of ln [(I₀/I) − 1] versus 1/T

CIE chromaticity coordinates play an important role in evaluating the luminescence properties of the phosphors. The CIE chromaticity coordinates of the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01, 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 mol) phosphor samples namely are (0.426, 0.452), (0.420, 0.448), (0.425, 0.450), (0.421, 0.449), (0.424, 0.446), (0.422, 0.442), (0.416, 0.434), and (0.435, 0.455), respectively, as listed in Table 3. The CIE chromaticity coordinates of the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) samples are shown in Fig. 11. They all drop into the yellow-light region of the CIE 1931 chromaticity coordinates, and all points are distributed in one piece. In addition, with increasing doping concentration, the CIE chromaticity coordinates (x, y) show a little change, and the chromaticity coordinate displacement (∆x, ∆y) is (0.019, 0.022). The small change indicate the different Dy³⁺-doped concentrations do not make a difference in color. The data indicated that the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) phosphors have good color stability to apply in w-LEDs.

Table 3 The CIE color coordinates and CCT of the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) phosphors

Fig. 11

CIE chromaticity coordinates for the prepared Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) samples

CCT is also important for the luminescence properties of phosphors. The CCT value affects whether the lamp emits warm or cold light. The light shows a cold lamp when CCT is more significant than 4000 K, the light emits a warm lamp while CCT is less than 3200 K. By the McCamy formula, CCT can be calculated [51, 52]:

$${\text{CCT}} = - 449n^{3} + 3525n^{3} - 6823n + 5520.33,$$

(5)

where inverse slope line, n = (x-x_e)/(y-y_e), (x, y) are the chromaticity coordinates of the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) samples, and (x_e, y_e) are the coordinates of the epicenter, which are (0.332, 0.186). The CCT value corresponding to the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) phosphors can be calculated in Table 2. These CCTs correspond to 3245, 3375, 3252, 3346, 3258, 3271, 3275, and 3242 K, respectively, which prove that these phosphors could be applied in w-LEDs.

The QY play an important role in w-LED application, which is calculated by the following equation [53]:

$$\eta_{{{\text{QY}}}} { = }\frac{{\smallint {\text{ L}}_{{\text{s}}} }}{{\smallint {\text{ E}}_{{\text{R}}} { - }\smallint {\text{ E}}_{{\text{s}}} { }}},$$

(6)

where L_S is the emission spectrum of the Sr₃LaNb₃O₁₂:0.05Dy³⁺, E_S and E_R represent the excitation light with and without the sample in the integrating sphere, respectively. The calculating QY value of the Sr₃LaNb₃O₁₂:0.05Dy³⁺ phosphor is 18.24% under 352 nm in Fig. 12, which shows that the Sr₃LaNb₃O₁₂:0.05Dy³⁺ phosphor is suitable for application for w-LEDs.

Fig. 12

QY for the Sr₃LaNb₃O₁₂:0.05Dy³⁺

The synthesized Sr₃LaNb₃O₁₂:0.05Dy³⁺ yellow phosphor, BAM blue phosphor, and (Sr, Ca)AlSiN₃:Eu²⁺ red phosphor were combined with the n-UV InGaN chip (388 nm) to make up a w-LED. The prepared w-LED has high R_a (91), low CCT (4162 K), and CIE coordinates (0.365, 0.368). Figure 13a reveals that the manufactured w-LED compares with the commercial w-LED in terms of CRI values. R_a plays a vital role in describing a parameter of the w-LED luminescence properties. The histogram comparison of the particular rendering index (R₁–R₁₄) shows the rendering performance of different colors. It is significantly higher than the commercial w-LED values (R_a = 80, R₉ = 15) [54,55,56]. Thus, a new component phosphor is used to produce w-LED to make up for the traditional limitations and has excellent potential in the future. Figure 13b shows the CIE coordinates for preparing the w-LED, which is lying white area and approaching the equal-energy point coordinates (0.333, 0.333). Therefore, this w-LED can become an excellent alternative to commercial w-LEDs.

Fig. 13

a A comparison of the prepared w-LED and the commercial w-LED in color rendering index (CRI), b The CIE coordinates of w-LED at a forward-bias current of 300 mA

4 Conclusions

In summary, the Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) of space group R_3m perovskite was successfully synthesized by high-temperature solid-phase method. The XRD patterns proved that the prepared Sr₃LaNb₃O₁₂:xDy³⁺ (x = 0.01–0.30 mol) phosphors were a pure phase, and the Dy³⁺ ions can quickly replace the La³⁺ ions which has no effect on the crystal structure. The element distribution maps of the prepared samples are distributed. Sr₃LaNb₃O₁₂:0.05Dy³⁺ had the most optimal luminescence intensity among a series of Sr₃LaNb₃O₁₂ doped with different concentrations of Dy³⁺. Under the excitation of 352 nm, Sr₃LaNb₃O₁₂:0.05Dy³⁺ had a prominent yellow peak of 575 nm, which was ⁴F_9/2-⁶H_13/2. The observed CQ phenomenon was caused by the interaction of dipole–dipole. The abnormal thermal phenomenon was due to lattice defects where Dy³⁺ replaces Sr²⁺. The activation energy of phosphor was calculated to be 0.29 eV. The CIE coordinates were located in the yellow region. The changes in temperature and doping concentration had only a slight effect on the color stability. White light with low CCT (4162 K) and R_a (91) was successfully prepared by an n-UV InGaN chip (388 nm), whose CIE chromaticity coordinates were (0.365, 0.368). Sr₃LaNb₃O₁₂:Dy³⁺ was expected to replace commercial yellow phosphor in w-LED applications due to its high thermal stability and excellent optical performance.