Single-Composition White Light Emission from Dy3+ Doped Sr2CaWO6

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.


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 Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce 3+ ). 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].

Structural characterization
XRD patterns were collected to determine the phases of Sr 2 Ca (1−1.5x%) WO 6 : x mol% Dy 3+ (x = 0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0). Meanwhile, we also characterized the structure of Sr 2 Ca 0.99 WO 6 : 0.5% Dy 3+ , 0.5 mol% M + (M + = Li + , Na + , K + ), for which Li + , Na + , and K + ions were introduced as charge compensators. The ionic radius of Dy 3+ ion in six-fold coordination is 0.912 Å, close to Ca 2+ ion, which has an ionic radius of 1.0 Å for six-fold coordination, while the ionic radius of Sr 2+ ion is 1.44 Å [32]. As shown in Figure 1a, the XRD patterns of Sr 2 Ca (1−1.5x%) WO 6 : x mol% Dy 3+ (x = 0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.0, 3.0) matched well with JPCDS card of Sr 2 CaWO 6 (JPCDS #76-1983). As shown in Figure 1b, compared with JPCDS cards of Sr 2 CaWO 6 , the XRD patterns of Sr 2 Ca 0.99 WO 6 : 0.5 mol% Dy 3+ , 0.5 mol% M + (M + = Li + , Na + , or K + ) have no obvious change. Thus, the introduction of the 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 Å 3 ). In the host lattice of Sr2CaWO6, with the formula of A2BB'O6, Ca 2+ ions and W 6+ 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 [22,33]. The host compound Sr 2 CaWO 6 is in orthorhombic system with Pmm2 space group (a = 8.1918 Å, b = 5.7653 Å, c = 5.8491 Å, V = 276.24 Å 3 ). In the host lattice of Sr 2 CaWO 6 , with the formula of A 2 BB'O 6 , Ca 2+ ions and W 6+ 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 CaO 6 octahedron shared its O atoms with six adjacent WO 6 octahedron, and each WO 6 octahedron also shared its O atoms with six adjacent CaO 6 octahedron. Sr atoms are located at the interspace of CaO 6 octahedron and WO 6 octahedron and coordinated by 12 O atoms, without an inversion center [21,22].

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 (Eg) of Sr2CaWO6 can be calculated with the following equation [34]: 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 Sr2CaWO6:1.0mol% Dy 3+ 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.

UV-Vis Absorption Spectra
The UV-Vis absorption spectrum of Sr 2 CaWO 6 is shown in Figure 3a. There is a broad absorption band in UV region. The calculated band structure and partial densities of Sr 2 CaWO 6 and the atoms constituting Sr 2 CaWO 6 , 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 Sr 2 CaWO 6 can be calculated with the following equation [33]: 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 Sr 2 CaWO 6 and the result indicated that the Sr 2 CaWO 6 is an indirect band gap insulator [22]. Considering the transition is indirect allowed, here n = 2. The optical band gap of Sr 2 CaWO 6 is calculated to be 3.79 eV, while the optical band gap of Sr 2 CaWO 6 : 1.0 mol% Dy 3+ is 3.81 eV ( Figure 3b). The optical band gap of Sr 2 CaWO 6 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. Materials 2018, 11, x FOR PEER REVIEW 5 of 15 Figure 3. The UV-Vis absorption spectra of Sr2CaWO6 (a) and Sr2CaWO6:1.0mol% Dy 3+ (b), the insert shows variation of (αhν) 1/2 under different photon energy.

Luminescence properties
The photoluminescence excitation spectra of Sr2Ca (1−1.5x%) WO6: x mol% Dy 3+ (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 O 2− to W 6+ ions. The doping concentrations of Dy 3+ 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 electric dipole transition of 4 F 9/2 → 6 H 13/2 emission of Dy 3+ ion is sensitive to surrounding environment [24]. If there is no inversion center, the 4 F 9/2 → 6 H 13/2 emission of Dy 3+ ions will be strong. Otherwise, the 4 F 9/2 → 6 H 13/2 emission of Dy 3+ ion will be weak. However, the transition of 4 F 9/2 → 6 H 15/2 is not as sensitive to coordinate surroundings [24,35]. Therefore, the symmetry of the environment in which Dy 3+ ions are located can be judged by comparing the relative intensity  [36]. In most fluorescent materials, with Dy 3+ ions in the asymmetric position the transition intensity of 4 F 9/2 → 6 H 13/2 is much stronger than that of 4 F 9/2 → 6 H 15/2 (such as SrMoO 4 :Dy 3+ [37], LuNbO 4 :Dy 3+ [31], and Sr 2 ZnWO 6 :Dy 3+ [19]). With Dy 3+ ion located at a position with high symmetry, the transition intensity of 4 F 9/2 → 6 H 13/2 is almost the same as that of 4 F 9/2 → 6 H 15/2 , or even weaker than that of 4 F 9/2 → 6 H 15/2 (such as Ba 3 La 2−x (BO 3 ) 4 :xDy 3+ [38], Ba 2 Ca (1−x) WO 6 :xDy 3+ [20], Sr 3 Sc 1−x (PO 4 ) 3 :xDy 3+ [39]). As can be seen from the emission spectra of Dy 3+ ion doped Sr 2 CaWO 6 , the emission intensity of 4 F 9/2 → 6 H 15/2 at 499 nm is similar to 4 F 9/2 → 6 H 13/2 centered at 599 nm, which indicates that Dy 3+ replaces the position with high symmetry.  [38]. In most fluorescent materials, with Dy 3+ ions in the asymmetric position the transition intensity of 4 F9/2→ 6 H13/2 is much stronger than that of 4 F9/2→ 6 H15/2 (such as SrMoO4: Dy 3+ [39], LuNbO4: Dy 3+ [40], and Sr2ZnWO6: Dy 3+ [19]). With Dy 3+ ion located at a position with high symmetry, the transition intensity of 4 F9/2→ 6 H13/2 is almost the same as that of 4 F9/2→ 6 H15/2, or even weaker than that of 4 F9/2→ 6 H15/2 (such as Ba3La2−x(BO3)4: xDy 3+ [41], Ba2Ca(1−x)WO6: xDy 3+ [20], Sr3Sc1−x(PO4)3: xDy 3+ [42]). As can be seen from the emission spectra of Dy 3+ ion doped Sr2CaWO6, the emission intensity of 4 F9/2→ 6 H15/2 at 499 nm is similar to 4 F9/2→ 6 H13/2 centered at 599 nm, which indicates that Dy 3+ 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 Dy 3+ ions. The excitation spectra of Dy 3+ ions have significant overlap with the emission spectra of Sr2CaWO6, and there is energy radiation transfer from Sr2CaWO6 host lattice (donors) to Dy 3+ ions (acceptors) [43]. Hence, the emission peak intensity at 449 nm decreased with the increase of the concentration of Dy 3+ ion, while the emission peak intensities at 499 nm, 599 nm, 670 nm, and 766 nm increased with higher doping concentration of Dy 3+ 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 Dy 3+ ion. The emission peak-intensities at 449 nm, 499 nm, 599 nm, and 670 nm changed with concentration of Dy 3+ ion, and single-composition WLED phosphors with tunable correlated color temperature were successfully generated through adjusting the concentration of Dy 3+ ion.  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 Dy 3+ ions. The excitation spectra of Dy 3+ ions have significant overlap with the emission spectra of Sr 2 CaWO 6 , and there is energy radiation transfer from Sr 2 CaWO 6 host lattice (donors) to Dy 3+ ions (acceptors) [40]. Hence, the emission peak intensity at 449 nm decreased with the increase of the concentration of Dy 3+ ion, while the emission peak intensities at 499 nm, 599 nm, 670 nm, and 766 nm increased with higher doping concentration of Dy 3+ 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 Dy 3+ ion. The emission peak-intensities at 449 nm, 499 nm, 599 nm, and 670 nm changed with concentration of Dy 3+ ion, and single-composition WLED phosphors with tunable correlated color temperature were successfully generated through adjusting the concentration of Dy 3+ ion. 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 Dy 3+ ions. The excitation spectra of Dy 3+ ions have significant overlap with the emission spectra of Sr2CaWO6, and there is energy radiation transfer from Sr2CaWO6 host lattice (donors) to Dy 3+ ions (acceptors) [43]. Hence, the emission peak intensity at 449 nm decreased with the increase of the concentration of Dy 3+ ion, while the emission peak intensities at 499 nm, 599 nm, 670 nm, and 766 nm increased with higher doping concentration of Dy 3+ 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 Dy 3+ ion. The emission peak-intensities at 449 nm, 499 nm, 599 nm, and 670 nm changed with concentration of Dy 3+ ion, and single-composition WLED phosphors with tunable correlated color temperature were successfully generated through adjusting the concentration of Dy 3+ ion.  The critical distance (R c ) between Dy 3+ 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]: 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 Sr 2 CaWO 6 , V = 276.24 Å 3 , N = 2, and x c = 0.01, R c of Dy 3+ 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 Dy 3+ ion in Sr 2 CaWO 6 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]: where I is the integral emission intensity of 6 F 5/2 → 4 H 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. The critical distance (Rc) between Dy 3+ ions were calculated by the concentration quenching method. The critical transfer distance (Rc) was calculated with the following formula (Equation (2)) which was proposed by Blasse [44]: c ≈ 2 3 4 (2) In this equation, V is the volume of crystallographic unit cell, xc 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 Å 3 , N = 2, and xc = 0.01, Rc of Dy 3+ 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 [43]. Exchange interactions take place over a distance shorter than 5 Å, while multipole interaction can occur at a distance as large as 30 Å [45,46]. The critical distance of Dy 3+ 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 [43] 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) where I is the integral emission intensity of 6 F5/2→ 4 H13/2, 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 dipolequadrupole 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% Dy 3+ phosphors excited at 310 nm and monitored at 499 nm. The decay curve fits well with the following single-exponential Equation (4):  Figure 8 shows the decay curve of Sr 2 CaWO 6 : 1.00 mol% Dy 3+ phosphors excited at 310 nm and monitored at 499 nm. The decay curve fits well with the following single-exponential Equation (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 1.99 CaWO 6 :0.01Dy 3+ is 127 µs [34]. Therefore, synthesis methods and doping sites appear to have significant influence on fluorescence lifetime. 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 Sr1.99CaWO6: 0.01Dy 3+ is 127 μs [35]. Therefore, synthesis methods and doping sites appear to have significant influence on fluorescence lifetime. The proposed energy transfer mechanism of Sr2CaWO6:Dy 3+ 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% Dy 3+ phosphors, besides the excitation of charge transfer from O atoms to W atoms, Dy 3+ ions are also excited by the ultraviolet under 310 nm. Dy 3+ ions can be excited to energy levels higher than 4 F9/2 by visible light from 350 nm until 450 nm. Electrons at high energy levels return to 4 F9/2 configuration through non-radiative transition, then release to 6 HJ (J = 11/2, 13/2, 15/2) configuration through radiative transition. It is well known that Dy 3+ 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 Dy 3+ ions ( 4 F9/2: 6 H15/2→ 6 H9/2+ 6 F11/2: 6 F3/2 and 4 F9/2: 6 H15/2 → 6 F5/2: 6 H7/2 + 6 F9/2, where 6 H9/2+ 6 F11/2 and 6 H7/2 + 6 F9/2 mean the energy level of 6 H9/2 and 6 H7/2 are very close to those of 6 F11/2, and 6 F9/2, respectively) [48]. The proposed energy transfer mechanism of Sr 2 CaWO 6 :Dy 3+ phosphors are shown in Figure 9. In Sr 2 CaWO 6 , 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 Sr 2 CaWO 6 . In Sr 2 Ca (1−1.5x%) WO 6 : x mol% Dy 3+ phosphors, besides the excitation of charge transfer from O atoms to W atoms, Dy 3+ ions are also excited by the ultraviolet under 310 nm. Dy 3+ ions can be excited to energy levels higher than 4 F 9/2 by visible light from 350 nm until 450 nm. Electrons at high energy levels return to 4 F 9/2 configuration through non-radiative transition, then release to 6 H J (J = 11/2, 13/2, 15/2) configuration through radiative transition. It is well known that Dy 3+ 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 Dy 3+ ions ( 4 F 9/2 : 6 H 15/2 → 6 H 9/2 + 6 F 11/2 : 6 F 3/2 and 4 F 9/2 : 6 H 15/2 → 6 F 5/2 : 6 H 7/2 + 6 F 9/2 , where 6 H 9/2 + 6 F 11/2 and 6 H 7/2 + 6 F 9/2 mean the energy level of 6 H 9/2 and 6 H 7/2 are very close to those of 6 F 11/2 , and 6 F 9/2 , respectively) [45]. Figure 9. Schematic illustration of the energy transfer mechanism for Sr2CaWO6:Dy 3+ . The red solid curve with an arrow means the vibrational relaxation of excited Dy 3+ ion in Sr2CaWO6. The green solid line with an arrow and the yellow one means the cross relaxation 4 F9/2: 6 H15/2→ 6 H9/2+ 6 F11/2: 6 F3/2 and 4 F9/2: 6 H15/2 → 6 F5/2: 6 H7/2 + 6 F9/2 of Dy 3+ ions, respectively.

Commission International de I'Eclairage (CIE) chromaticity diagram
As shown in Table 1, with the increase of doping concentration, the CIE coordinates of Sr2Ca(1-1.5x%)WO6: xmol%Dy 3+ 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 Dy 3+ ion, a series of phosphors with different CIE coordinates were successfully obtained and white light from a single host was successfully obtained (Fig.10).

Commission International de I'Eclairage (CIE) chromaticity diagram
As shown in Table 1, with the increase of doping concentration, the CIE coordinates of Sr 2 Ca (1-1.5x%) WO 6 : x mol% Dy 3+ 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 Dy 3+ ion, a series of phosphors with different CIE coordinates were successfully obtained and white light from a single host was successfully obtained ( Figure 10).

Conclusion
In summary, phosphors with Dy 3+ 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 Dy 3+ ion. White light emission with CIE coordinate (0.34, 0.33) was successfully generated in Sr2CaWO6:Dy 3+ phosphors at the doping level of 1mol% on the Ca site. Considering the overlap between the emission spectra of host lattice and the excitation spectra of Dy 3+ ions, it is expected that there is efficient energy transfer from host lattice to Dy 3+ ions.   Chromaticity coordinates of Sr 2 Ca (1−1.5x%) WO 6 : x mol% Dy 3+ (x = 0, 0.1, 0.3, 0.5, 1.0, 2.0, 3.0) and Sr 2 Ca 0.99 WO 6 : 0.5 mol% Dy 3+ , 0.5 mol% M + (M + = Li + , Na + or K + ) were compared at various Dy 3+ 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.

Conclusions
In summary, phosphors with Dy 3+ doped on the Ca site of Sr 2 CaWO 6 were prepared by high temperature solid state method, and they can be excited under 310 nm ultraviolet. The host compound, Sr 2 CaWO 6 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 Dy 3+ ion. White light emission with CIE coordinate (0.34, 0.33) was successfully generated in Sr 2 CaWO 6 :Dy 3+ 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 Dy 3+ ions, it is expected that there is efficient energy transfer from host lattice to Dy 3+ ions.