Single-Composition White Light Emission from Dy³⁺ Doped Sr₂CaWO₆

Abstract

A series of Dy³⁺ ion doped Sr₂CaWO₆ phosphors with double perovskite structure were synthesized by traditional high temperature solid-state method. It was found that there is significant energy transfer between Dy³⁺ and the host lattice, and the intensities of emission peaks at 449 nm (blue), 499 nm (cyan), 599 nm (orange), 670 nm (red), and 766 nm (infra-red) can be changed by adjusting the concentration of dopant amount of Dy³⁺ ion in Sr₂CaWO₆. The correlated color temperature of Dy³⁺ ion doped Sr₂CaWO₆ phosphors can be tuned by adjusting the concentration of Dy³⁺ ion. Upon optimal doping at 1.00 mol% Dy³⁺, white light with chromaticity coordinate (0.34, 0.33) was emitted under excitation at 310 nm. Thus, single composition white emission is realized in Dy³⁺ doped Sr₂CaWO₆.

1. Introduction

In recent years, phosphor-covered white light-emitting diodes (pc-WLEDs) have become more and more prevalent due to their superior performance, such as energy saving, long life time, small volume, and high brightness, compared to the conventional white light sources [1]. The commercial approach for obtaining white light-emitting diode (WLED) typically involves covering blue chips with yellow light-emitting phosphors such as Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺). Nevertheless, the lack of strong visible red-light emission makes it difficult to fabricate WLEDs with a high color-rendering index (CRI) and low correlated color temperature (CCT) [2,3,4,5,6], which limit their applications. In order to overcome these difficulties, red, green, and blue (RGB) tricolor phosphors pumped by ultra-violet (UV) LED chips (300–400 nm) are used to fabricate WLEDs with high CRI. However, it is difficult to fabricate WLED with high conversion efficiency as there is strong reabsorption of the blue-light by the green and red phosphors [7,8]. Thus, single-composition white light-emitting phosphors pumped by near ultra-violet (n-UV) light-emitting diodes are desired to fabricate WLEDs [9,10,11].

Double perovskite has the formula of A₂BB’O₆ [12,13]. The tungstate with double perovskite structure has important physical properties, such as magnetic [14], electrical [15], photocatalytic [16], and optical properties [17]. The luminescence properties of [WO₆]⁶⁻ group in A₂BWO₆ has been reported [18]. As previously reported, the emission colors of [WO₆]⁶⁻ group in A₂BWO₆ could range from blue to yellow (e.g., Ba₂CaWO₆, blue; Sr₃WO₆, green, and Ba₂MgWO₆, yellow), which strongly depends on the choice of A and B ions [18]. Rare earth (RE) ion doped double perovskite tungstate has been shown to have promising photoluminescence properties [19], such as Ba₂CaWO₆:Eu³⁺ [20], Ba₂ZnWO₆:Eu³⁺, Li⁺ [17], Sr₂CaWO₆:Eu³⁺, Na⁺ [21], and Sr₂CaWO₆:Sm³⁺, Na⁺ [22]. The emission peak at yellow region of Dy³⁺ ion which corresponding to the transition of ⁴F_9/2 → ⁶H_13/2 is greatly influenced by the coordination environment [23]. Furthermore, the ratio of emission peak intensities in blue (⁴F_9/2 → ⁶H_15/2) and yellow (⁴F_9/2 → ⁶H_13/2) region for Dy³⁺ ion is influenced by the coordination environment, and when there is an inversion center the emission in blue region is of significant intensity [24].

In the present work, Dy³⁺ was used to replace the Ca²⁺ ion in Sr₂CaWO₆, which is with an inversion center. Single-composition white light emitting phosphors with tunable correlated color temperature were successfully synthesized by adjusting the concentration of Dy³⁺ ions. There are some single-composition white light emission phosphors materials can be obtained at near ultra-violet(n-UV), such as Ca₉Gd(PO₄)₇:Eu²⁺, Mn²⁺ [25], NaBaBO₃:Dy³⁺, K⁺ [26], and Sr₃Y(PO₄)₃:Dy³⁺ [27]. However, many materials need UV excitation under 300 nm to realize white light emission, such as Y(P,V)O₄:Dy³⁺ [28], β-GdB₃O₆:Bi³⁺, Tb³⁺, Eu³⁺ [29], LaNbO₄:Dy³⁺ [30], and LuNbO₄:Dy³⁺ [31]. Taking LuNbO₄:Dy³⁺ as example, the excitation peak of LuNbO₄:Dy³⁺ phosphors centered in 261 nm. In comparison, Sr₂CaWO₆:Dy³⁺ phosphors can be excited under a relatively longer wavelength of 310 nm. In the present work, the photoluminescence properties of Dy³⁺ ions doped Sr₂CaWO₆ with different concentrations of Dy³⁺ ions and white light emission upon optimal doping are reported.

2. Material and Methods

The powder samples of Sr₂Ca_(1−1.5x%)WO₆: x mol% Dy³⁺ (x = 0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0) and Sr₂Ca_0.99WO₆: 0.5 mol% Dy³⁺, 0.5 mol% M⁺ (M⁺ = Li⁺, Na⁺, K⁺) were synthesized by high-temperature solid-state method. The starting materials SrCO₃ (99.9%, Sigma-Aldrich), CaCO₃ (99.95–100.05%, Alfa), WO₃ (99.90%, Adamas), Li₂CO₃ (99%, Alfa), Na₂CO₃ (99.8%, General-Reagent), K₂CO₃ (99%, Sigma-Aldrich), and Dy₂O₃ (99.99%, Adamas) were weighed by stoichiometric ratio. Then the starting materials were mixed and ground in an agate mortar. The mixture was transferred to a corundum crucible and preheated under 850 °C for 5 h in a box furnace. After the sample was cooled down to room temperature, the mixture was thoroughly ground, calcined at 1200 °C for 12 h, cooled to room temperature, and reground to obtain the final powder sample.

The powder XRD date of Sr₂Ca_(1−1.5×%)WO₆: x mol% Dy³⁺ were collected in the range of 10° ≤ 2θ ≤ 70° using a Rigaku D/MAX 2550 diffract meter (Cu-Kα, λ = 1.54059 Å), operated at 40 kV and 40 mA. The UV–Vis absorption spectra of Sr₂CaWO₆ and Sr₂CaWO₆: doped with 1 mol% Dy³⁺ were collected with a UV–Vis spectrometer (Lambda 950, Perkin-Elmer, Waltham, MA, USA) with a polytetrafluoroethylene plate as a reference. The excitation spectra and emission spectra were collected with a photoluminescence spectrometer (FS5, Edinburgh Instruments, Livingston, UK) equipped with a 150 W xenon lamp as the excitation source. The lifetime of Sr₂Ca_0.985WO₆: 1.00 mol% Dy³⁺ was measured with an FLS-980 fluorometer (Edinburgh Instruments, Livingston, UK).

3. Results and Discussion

3.1. Structural characterization

XRD patterns were collected to determine the phases of Sr₂Ca_(1−1.5x%)WO₆: x mol% Dy³⁺ (x = 0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0). Meanwhile, we also characterized the structure of Sr₂Ca_0.99WO₆: 0.5% Dy³⁺, 0.5 mol% M⁺ (M⁺ = Li⁺, Na⁺, K⁺), for which Li⁺, Na⁺, and K⁺ ions were introduced as charge compensators. The ionic radius of Dy³⁺ ion in six-fold coordination is 0.912 Å, close to Ca²⁺ ion, which has an ionic radius of 1.0 Å for six-fold coordination, while the ionic radius of Sr²⁺ ion is 1.44 Å [32]. As shown in Figure 1a, the XRD patterns of Sr₂Ca_(1−1.5x%)WO₆: x mol% Dy³⁺ (x = 0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0) matched well with JPCDS card of Sr₂CaWO₆ (JPCDS #76-1983). As shown in Figure 1b, compared with JPCDS cards of Sr₂CaWO₆, the XRD patterns of Sr₂Ca_0.99WO₆: 0.5 mol% Dy³⁺, 0.5 mol% M⁺ (M⁺ = Li⁺, Na⁺, or K⁺) have no obvious change. Thus, the introduction of the charge compensator has little influence on the crystal structure of Sr₂CaWO₆. These results indicated that the host structure of double perovskite was well preserved for both the samples with and without charge compensators.

The host compound Sr₂CaWO₆ is in orthorhombic system with Pmm2 space group (a = 8.1918 Å, b = 5.7653 Å, c = 5.8491 Å, V = 276.24 ų). In the host lattice of Sr₂CaWO₆, with the formula of A₂BB’O₆, Ca²⁺ ions and W⁶⁺ ions reside at B and B’ sites, respectively. Ca atoms and W atoms are coordinated by 6 O atoms (Figure 2). The cations and the coordinated oxygen ions form an octahedral structure with an inversion center. Each CaO₆ octahedron shared its O atoms with six adjacent WO₆ octahedron, and each WO₆ octahedron also shared its O atoms with six adjacent CaO₆ octahedron. Sr atoms are located at the interspace of CaO₆ octahedron and WO₆ octahedron and coordinated by 12 O atoms, without an inversion center [21,22].

3.2. UV–Vis Absorption Spectra

The UV–Vis absorption spectrum of Sr₂CaWO₆ is shown in Figure 3a. There is a broad absorption band in UV region. The calculated band structure and partial densities of Sr₂CaWO₆ and the atoms constituting Sr₂CaWO₆, such as strontium, calcium, tungsten, and oxygen have been reported before [22]. The strong absorption in the ultraviolet region 270–330 nm is attributed to the charge transfer from O atom to W atom. With the UV–Vis absorption spectra, the optical band gap (E_g) of Sr₂CaWO₆ can be calculated with the following equation [33]:

αhν = k(hν − E_g) ⁿ

(1)

where α is the absorbance, h is the Planck’s constant, ν is the frequency, k is a constant, n is equal to 1/2, 2, 3/2, or 3, which is dependent on whether the transition is direct allowed, indirect allowed, direct forbidden of indirect forbidden, respectively. Wang et al reported the calculated band structure of Sr₂CaWO₆ and the result indicated that the Sr₂CaWO₆ is an indirect band gap insulator [22]. Considering the transition is indirect allowed, here n = 2. The optical band gap of Sr₂CaWO₆ is calculated to be 3.79 eV, while the optical band gap of Sr₂CaWO₆: 1.0 mol% Dy³⁺ is 3.81 eV (Figure 3b). The optical band gap of Sr₂CaWO₆ synthesized by sol-gel method was calculated to be 3.51 eV, which is 0.30 eV smaller than the value obtained from the present sample [22]. This suggests that the preparation conditions have appreciable influence on the optical band gap.

3.3. Luminescence Properties

The photoluminescence excitation spectra of Sr₂Ca _(1−1.5x%) WO₆: x mol% Dy³⁺ (x = 0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0), which were measured at emission wavelength of 499 nm, are shown in Figure 4a. As shown in Figure 4a, there is a broad excitation band in the region of 270–330 nm. The calculated band structure and total densities of states of Sr₂CaWO₆ near the Fermi energy level have been reported [22]. The broad excitation band centered at 310 nm was attributed to the charge transfer from O²⁻ to W⁶⁺ ions. The doping concentrations of Dy³⁺ ion have significant influence on the excitation band of the host lattice. With the increase of doping concentration, the excitation intensity at 310 nm decreased. The concentration dependent excitation intensity of phosphors at 310 nm is shown in Figure 4b.

The photoluminescence emission spectra of Dy³⁺ doped Sr₂CaWO₆ phosphors were collected under excitation at 310 nm. Figure 5a shows the emission spectra of Sr₂Ca_0.99WO₆: 0.5 mol% Dy³⁺, 0.5 mol% M⁺ (M⁺ = Li⁺, Na⁺ or K⁺). The introduction of charge compensation ions had no significant influence on the emission of Dy³⁺ ion doped Sr₂CaWO₆ phosphors. A broad emission band centered at 449 nm, which is due to self-trapped luminescent recombination in [WO₆]⁶⁻ octahedral [19,34]. The emission peaks at 499 nm (⁴F_9/2 → ⁶H_15/2), 599 nm (⁴F_9/2 → ⁶H_13/2), 670 nm (⁴F_9/2 → ⁶H_11/2) and 766 nm (⁴F_9/2 → ⁶H_9/2) are attributed to f–f transitions of Dy³⁺ ions. As shown in Figure 5a, the emission intensity of Sr₂Ca_0.99WO₆: 0.5 mol% Dy³⁺, 0.5 mol% M⁺ decreased in different degrees with introducing charge compensation ions, which indicated that the defects caused by the introduction of charge compensation ions cause more energy loss. The emission spectra of Sr₂Ca_(1−1.5x%)WO₆: x mol% Dy³⁺ (x = 0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0) is shown in Figure 5b. The concentration-dependent emission intensity variations at 449 nm, 499 nm, 599 nm and 670 nm are shown in Figure 5c.

The electric dipole transition of ⁴F_9/2 → ⁶H_13/2 emission of Dy³⁺ ion is sensitive to surrounding environment [24]. If there is no inversion center, the ⁴F_9/2 → ⁶H_13/2 emission of Dy³⁺ ions will be strong. Otherwise, the ⁴F_9/2 → ⁶H_13/2 emission of Dy³⁺ ion will be weak. However, the transition of ⁴F_9/2 → ⁶H_15/2 is not as sensitive to coordinate surroundings [24,35]. Therefore, the symmetry of the environment in which Dy³⁺ ions are located can be judged by comparing the relative intensity of ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2 transition [36]. In most fluorescent materials, with Dy³⁺ ions in the asymmetric position the transition intensity of ⁴F_9/2 → ⁶H_13/2 is much stronger than that of ⁴F_9/2 → ⁶H_15/2 (such as SrMoO₄:Dy³⁺ [37], LuNbO₄:Dy³⁺ [31], and Sr₂ZnWO₆:Dy³⁺ [19]). With Dy³⁺ ion located at a position with high symmetry, the transition intensity of ⁴F_9/2 → ⁶H_13/2 is almost the same as that of ⁴F_9/2 → ⁶H_15/2, or even weaker than that of ⁴F_9/2 → ⁶H_15/2 (such as Ba₃La_2−x(BO₃)₄:xDy³⁺ [38], Ba₂Ca_(1−x)WO₆:xDy³⁺ [20], Sr₃Sc_1−x(PO₄)₃:xDy³⁺ [39]). As can be seen from the emission spectra of Dy³⁺ ion doped Sr₂CaWO₆, the emission intensity of ⁴F_9/2 → ⁶H_15/2 at 499 nm is similar to ⁴F_9/2 → ⁶H_13/2 centered at 599 nm, which indicates that Dy³⁺ replaces the position with high symmetry.

As shown in Figure 6, the excitation peaks located at 352 nm, 366 nm, and 455 nm are attributed to the f-f transition absorptions of Dy³⁺ ions. The excitation spectra of Dy³⁺ ions have significant overlap with the emission spectra of Sr₂CaWO₆, and there is energy radiation transfer from Sr₂CaWO₆ host lattice (donors) to Dy³⁺ ions (acceptors) [40]. Hence, the emission peak intensity at 449 nm decreased with the increase of the concentration of Dy³⁺ ion, while the emission peak intensities at 499 nm, 599 nm, 670 nm, and 766 nm increased with higher doping concentration of Dy³⁺ ion when x ≤ 1.0. However, when x >1.0, the emission peak intensities at 499 nm, 599 nm, 670 nm, and 766 nm decreased with the increase of the concentration of Dy³⁺ ion. The emission peak-intensities at 449 nm, 499 nm, 599 nm, and 670 nm changed with concentration of Dy³⁺ ion, and single-composition WLED phosphors with tunable correlated color temperature were successfully generated through adjusting the concentration of Dy³⁺ ion.

The critical distance (R_c) between Dy³⁺ ions were calculated by the concentration quenching method. The critical transfer distance (R_c) was calculated with the following formula (Equation (2)) which was proposed by Blasse [41]:

$$R\mathrm{c}\approx 2{\left[\frac{3V}{4\pi x_{c}N}\right]}^{\frac{1}{3}}$$

(2)

In this equation, V is the volume of crystallographic unit cell, x_c is the critical concentration, and N is the lattice site number in a unit cell which can be replaced by sensitizers. In Sr₂CaWO₆, V = 276.24 ų, N = 2, and x_c = 0.01, R_c of Dy³⁺ ion is calculated to be 29.8 Å. In general, for the energy transfer process, the exchange interaction and multipole interaction are the two mechanisms that can play important roles [40]. Exchange interactions take place over a distance shorter than 5 Å, while multipole interaction can occur at a distance as large as 30 Å [42,43]. The critical distance of Dy³⁺ ion in Sr₂CaWO₆ equal to 29.8 Å, which is much larger than 5 Å. Therefore, the energy transfer process belongs to multipole interaction instead of exchange interaction.

Van Uitert [40] has pointed out that when non-radiative losses are attributed to multipolar transfer, the strength of multipolar interaction can be determined from the change in the emission intensity of activator. The relation between the emission intensity of each activator and the concentration of each activator can be expressed by Equation (3) [44,45]:

$$\lg \frac{I}{x}=-\frac{s}{3}\lg x+A$$

(3)

where I is the integral emission intensity of ⁶F_5/2 → ⁴H_13/2, x is the corresponding doping concentration, A is a constant which independent on the dopant concentration, and s is dependent on the interaction process. The value of s can be 6, 8, and 10, corresponding to electric dipole–dipole, electric dipole–quadrupole or electric quadrupole–quadrupole interaction, respectively. When s equal to 3, the energy transfer among nearest-neighbor ions plays a major role in quenching. As shown in Figure 7, the slope was calculated to be −0.87, thus s is most approximate 3. Therefore, the energy transfer process is most likely caused by energy transfer among nearest-neighbor ions.

Figure 8 shows the decay curve of Sr₂CaWO₆: 1.00 mol% Dy³⁺ phosphors excited at 310 nm and monitored at 499 nm. The decay curve fits well with the following single-exponential Equation (4):

I(t) = A exp (−t/τ)

(4)

where I(t) is the emission intensity at time t and A is a constant. Thus, the lifetime value of τ is calculated to be 0.48 ns. The reported lifetime value of Sr_1.99CaWO₆:0.01Dy³⁺ is 127 μs [34]. Therefore, synthesis methods and doping sites appear to have significant influence on fluorescence lifetime.

The proposed energy transfer mechanism of Sr₂CaWO₆:Dy³⁺ phosphors are shown in Figure 9. In Sr₂CaWO₆, electrons at valance band top were excited under 310 nm ultraviolet irradiation and transferred to conduction band, which is mainly attributed to the charge transfer from O atoms to W atoms. Electrons at conduction band returned to conduction band bottom through non-radiative transition. When electrons at conduction band bottom transfer to the top of valance band, energy is released by radiative transition. Thus, there is a broad blue light emission band in Sr₂CaWO₆. In Sr₂Ca_(1−1.5x%)WO₆: x mol% Dy³⁺ phosphors, besides the excitation of charge transfer from O atoms to W atoms, Dy³⁺ ions are also excited by the ultraviolet under 310 nm. Dy³⁺ ions can be excited to energy levels higher than ⁴F_9/2 by visible light from 350 nm until 450 nm. Electrons at high energy levels return to ⁴F_9/2 configuration through non-radiative transition, then release to ⁶H_J (J = 11/2, 13/2, 15/2) configuration through radiative transition. It is well known that Dy³⁺ ions have matched energy level pairs which produce strong cross relaxation and lead to concentration quenching. Therefore, it can be inferred that besides the energy transfer among nearest-neighbor ion, the concentration quenching is also related to cross relaxation of Dy³⁺ ions (⁴F_9/2:⁶H_15/2 → ⁶H_9/2 + ⁶F_11/2:⁶F_3/2 and ⁴F_9/2:⁶H_15/2 → ⁶F_5/2:⁶H_7/2 + ⁶F_9/2, where ⁶H_9/2 + ⁶F_11/2 and ⁶H_7/2 + ⁶F_9/2 mean the energy level of ⁶H_9/2 and ⁶H_7/2 are very close to those of ⁶F_11/2, and ⁶F_9/2, respectively) [45].

3.4. Commission International de I’Eclairage (CIE) chromaticity diagram

As shown in

Table 1. Commission International de I’Eclairage (CIE) coordinates of different Dy³⁺ ion concentration of Sr₂CaWO₆.

Number	Phosphors	CIE Coordinates (x,y)
1	Sr2CaWO6	(0.18, 0.16)
2	Sr2Ca0.9985WO6:0.001Dy3+	(0.23, 0.21)
3	Sr2Ca0.9955WO6:0.003Dy3+	(0.27, 0.26)
4	Sr2Ca0.9925WO6:0.005Dy3+	(0.30, 0.29)
5	Sr2Ca0.99WO6:0.005Dy3+,0.005Li+	(0.30, 0.29)
6	Sr2Ca0.99WO6:0.005Dy3+,0.005Na+	(0.29, 0.28)
7	Sr2Ca0.99WO6:0.005Dy3+,0.005K+	(0.29, 0.28)
8	Sr2Ca0.985WO6:0.010Dy3+	(0.34, 0.33)
9	Sr2Ca0.9775WO6:0.015Dy3+	(0.35, 0.34)
10	Sr2Ca0.97WO6:0.020Dy3+	(0.37, 0.36)
11	Sr2Ca0.955WO6:0.030Dy3+	(0.37, 0.36)

, with the increase of doping concentration, the CIE coordinates of Sr₂Ca_(1-1.5x%)WO₆: x mol% Dy³⁺ phosphors can be adjusted from the blue region (0.18, 0.16) to white (0.34, 0.33) (x = 1), which is very close to the coordinate of standard white light (0.33, 0.33). As the doping concentration continues to increase, the CIE coordinates gradually shift to the yellow region (0.37, 0.36). By adjusting the concentration of Dy³⁺ ion, a series of phosphors with different CIE coordinates were successfully obtained and white light from a single host was successfully obtained (Figure 10).

Chromaticity coordinates of Sr₂Ca_(1−1.5x%)WO₆: x mol% Dy³⁺ (x = 0, 0.1, 0.3, 0.5, 1.0, 2.0, 3.0) and Sr₂Ca_0.99WO₆: 0.5 mol% Dy³⁺, 0.5 mol% M⁺ (M⁺ = Li⁺, Na⁺ or K⁺) were compared at various Dy³⁺ ion concentration. It can be seen that the CIE coordinates of the samples with and without charge compensation ions are very close (Figure 10). Therefore, the introduction of charge compensation ion does not significantly affect the chromaticity coordinates.

4. Conclusions

In summary, phosphors with Dy³⁺ doped on the Ca site of Sr₂CaWO₆ were prepared by high temperature solid state method, and they can be excited under 310 nm ultraviolet. The host compound, Sr₂CaWO₆ emits blue light centered at 449 nm with the color coordinate of (0.18, 0.16) under ultraviolet excitation at 310 nm. The intensity of emission peaks under 310 nm excitation can be tuned by adjusting the concentration of Dy³⁺ ion. White light emission with CIE coordinate (0.34, 0.33) was successfully generated in Sr₂CaWO₆:Dy³⁺ phosphors at the doping level of 1 mol% on the Ca site. Considering the overlap between the emission spectra of host lattice and the excitation spectra of Dy³⁺ ions, it is expected that there is efficient energy transfer from host lattice to Dy³⁺ ions.