The Preparation and Optical Properties of Novel LiLa(MoO4)2:Sm3+,Eu3+ Red Phosphor

Novel LiLa1−x−y(MoO4)2:xSm3+,yEu3+ (in short: LL1−x−yM:xSm3+,yEu3+) double molybdate red phosphors were synthesized by a solid-state reaction at as low temperature as 610 °C. The optimal doping concentration of Sm3+ in LiLa1−x(MoO4)2:xSm3+ (LL1−xM:xSm3+) phosphor is x = 0.05 and higher concentrations lead to emission quenching by the electric dipole—electric dipole mechanism. In the samples co-doped with Eu3+ ions, the absorption spectrum in the near ultraviolet and blue regions became broader and stronger than these of the Sm3+ single-doped samples. The efficient energy transfer from Sm3+ to Eu3+ was found and the energy transfer efficiency was calculated. Under the excitation at 403 nm, the chromaticity coordinates of LL0.95−yM:0.05Sm3+,yEu3+ approach to the NTSC standard values (0.670, 0.330) continuously with increasing Eu3+ doping concentration. The phosphor exhibits high luminous efficiency under near UV or blue light excitation and remarkable thermal stability. At 150 °C, the integrated emission intensity of the Eu3+ remained 85% of the initial intensity at room temperature and the activation energy is calculated to be 0.254 eV. The addition of the LL0.83M:0.05Sm3+,0.12Eu3+ red phosphors can improve the color purity and reduce the correlated color temperature of WLED lamps. Hence, LL1−x−yM:xSm3+,yEu3+ is a promising WLED red phosphor.


Introduction
With the problem of environmental pollution and energy crisis becoming increasingly prominent in recent years, the white light emitting diode (WLED) has attracted wide attention due to the advantages of energy saving, long lifetime, high efficiency and environmental friendliness [1,2]. These attributes have made WLEDs to be the perfect candidate for the next generation of light sources. WLEDs can be used widely in lighting, backlighting, display, indicating areas, urban lighting, etc. [3]. White light can be produced by exciting multi-phosphors by a UV-LED, or by the combination of a blue LED chip with a yellow phosphor, or by blending multi-LEDs [4]. The combination of the commercial YAG:Ce 3+ yellow phosphor pumped by a blue InGaN chip has some drawbacks which are difficult to overcome: a high correlated color temperature (CCT) and low color rendering index (CRI) due to lack of red component. Hence this color cannot meet indoor light requirements which require warm white light and an excellent CRI [5]. In addition, the blue chip InGaN and yellow phosphor possess a different failure rate and in the case of long working hours this may cause chromatic aberration and reduced white performance [6][7][8][9][10]. To solve this problem, researchers have developed red, green

Characterization of Samples
The phase purity of the prepared phosphors was analyzed by X-ray powder diffraction using Cu Kα radiation (wavelength = 0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The cell parameters were calculated by MDI Jade 6.5 software. The diffuse reflectance spectrum of sample was measured on a Cary 5000 UV-Vis-NIR spectrophotometer attached to an integrating sphere, using polytetrafluoroethene as a standard for measurement. The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra of the obtained powders were recorded by a Hitachi F-7000 Fluorescence Spectrophotometer (Hitachi, Tokyo, Japan) equipped with a 450 W xenon lamp as excitation source. Lifetime measurements were carried out with an Edinburgh Instruments spectrometer (model FLS 980). Temperature-dependent PL properties were investigated by a Renishaw Spectrometer (Renishaw, Hong Kong, China) equipped with a 325 nm laser and temperature control system.
An WLED lamp was fabricated using a 455 nm blue LED chip pre-coated with LL 0.83 M:0.05Sm 3+ , 0.12Eu 3+ red phosphor and commercial yellow phosphor encapsulated in a transparent epoxy resin based on the standard LED technology in Guangzhou LEDteen Optoelectronics Co., Ltd. (Guangzhou, China).The pumping LED source is an 455 nm InGaN LED COB (chip on board) in which 12 chips are connected in series; the driving voltage of single chip is 2.9-3.3 V. The commercial yellow phosphor is from Jiangsu Bree Optronics Co., Ltd. (Nanjing, China). The material is rare earth aluminates, the excitation wavelength is 440-455 nm, the peak wavelength is 537 nm, CIE is x = 0.380 ± 0.005, y = 0.571 ± 0.005, peak half-width bandwidth is 105.7 nm, color purity is 88%, and particle size is 12.5 ± 0.5 µm. The dispensing type silicon encapsulant for LED is ENA6550 A/B (Tianbao Science&Technology Co., Ltd., Taiwan).

X-Ray Diffraction Patterns
X-ray powder diffraction is an important method to estimate the phase purity. The X-ray diffractograms of four kinds of samples are shown in Figure 1a. The positions of all diffraction peaks are very close to that of LiLa(MoO 4 ) 2 (LLM) standard card (JCPDS 18-0734) [24]. Thus, the doping with rare earth ions did not change the crystal structure and the samples prepared by this method are of pure LLM phase. According to Bragg's law, 2d sin θ = nλ, the unit cell group parameters for LLM are calculated as a = b = 5.293 Å, c = 11.598 Å, Z = 2, V = 324.97 Å 3 (space group I4 1 /a).  Figure 1c displays the diffuse reflectance spectrum of the phosphor. The sample has a broad absorption band in the range of 200-380 nm due to host absorption and obvious absorptions in the longer wavelength region due to the 4f N -4f N transitions of Sm 3+ and Eu 3+ ions, as indicated.   Figure 1c displays the diffuse reflectance spectrum of the phosphor. The sample has a broad absorption band in the range of 200-380 nm due to host absorption and obvious absorptions in the longer wavelength region due to the 4f N -4f N transitions of Sm 3+ and Eu 3+ ions, as indicated.

The Excitation and Emission Spectra of LiLa 1−x (MoO 4 ) 2 Doped with Sm 3+
The electronic ground state of Sm 3+ is 6 H 5/2 . The excitation spectrum of LL 0.95 M:0.05Sm 3+ recorded by monitoring the 4 G 5/2 → 6 H 5/2 emission of Sm 3+ at 598 nm is shown in Figure 2. The excitation spectra are composed of a broad band in the ultraviolet region and a number of narrow-band peaks in the near-ultraviolet to visible region. There is a wide absorption band from 230 to 320 nm, attributed to the charge transfer transition O 2− → Mo 6+ due to the transition of a 2p electron of O 2− to the d orbital of Mo 6+ [48]. The narrow bands detected over 340-550 nm are the typical weak, sharp 4f-4f transitions of rare earth ions and the terminal states of Sm 3+ are labeled. The appearance of the charge transfer band is beneficial to the absorption of UV photons. The energy absorbed by the host is then transferred to the rare earth ion. Also, the absorption band of 6 H 5/2 → 4 K 11/2 at 403 nm is the strongest of the Sm 3+ 4f-4f transitions and its wavelength is well matched with that of near UV LED chips.

The Excitation and Emission Spectra of LiLa1−x(MoO4)2 Doped with Sm 3+
The electronic ground state of Sm 3+ is 6 H5/2. The excitation spectrum of LL0.95M:0.05Sm 3+ recorded by monitoring the 4 G5/2 → 6 H5/2 emission of Sm 3+ at 598 nm is shown in Figure 2. The excitation spectra are composed of a broad band in the ultraviolet region and a number of narrow-band peaks in the near-ultraviolet to visible region. There is a wide absorption band from 230 to 320 nm, attributed to the charge transfer transition O 2− → Mo 6+ due to the transition of a 2p electron of O 2− to the d orbital of Mo 6+ [48]. The narrow bands detected over 340-550 nm are the typical weak, sharp 4f-4f transitions of rare earth ions and the terminal states of Sm 3+ are labeled. The appearance of the charge transfer band is beneficial to the absorption of UV photons. The energy absorbed by the host is then transferred to the rare earth ion. Also, the absorption band of 6 H5/2 → 4 K11/2 at 403 nm is the strongest of the Sm 3+ 4f-4f transitions and its wavelength is well matched with that of near UV LED chips. A series of LL1−xM:xSm 3+ phosphors with different x was prepared to study the relationship between luminescence intensity and the doping concentration of Sm 3+ . Figure 3 shows the relevant emission spectra under 403 nm excitation. The LL1−xM:xSm 3+ samples have three emission peaks at 562 nm ( 4 G5/2 → 6 H5/2), 598 nm ( 4 G5/2 → 6 H7/2) and 644 nm ( 4 G5/2 → 6 H9/2) [49,50]. The band at 598 nm corresponds to yellow-orange light and that at 644 nm to deep red, so LL1−xM:xSm 3+ samples emit red-orange light, having potential applications in the LCD back light and white LED. According to magnetic dipole and electric dipole transition selection rules, when Sm 3+ is at the inversion center of the host crystal, the emission of Sm 3+ is predominantly from the 4 G5/2 → 6 H7/2 (598 nm) magnetic dipole allowed transition. When Sm 3+ ion is not at a site with inversion symmetry, the emission of Sm 3+ is predominantly from the 4 G5/2 → 6 H9/2 (644 nm) forced electric dipole transition, but the 4 G5/2 → 6 H7/2 transition also gains forced electric dipole character. In the present case, the emission intensity at 598 nm is similar to that at 644 nm.   A series of LL 1−x M:xSm 3+ phosphors with different x was prepared to study the relationship between luminescence intensity and the doping concentration of Sm 3+ . Figure 3 shows the relevant emission spectra under 403 nm excitation. The LL 1−x M:xSm 3+ samples have three emission peaks at 562 nm ( 4 G 5/2 → 6 H 5/2 ), 598 nm ( 4 G 5/2 → 6 H 7/2 ) and 644 nm ( 4 G 5/2 → 6 H 9/2 ) [49,50]. The band at 598 nm corresponds to yellow-orange light and that at 644 nm to deep red, so LL 1−x M:xSm 3+ samples emit red-orange light, having potential applications in the LCD back light and white LED. According to magnetic dipole and electric dipole transition selection rules, when Sm 3+ is at the inversion center of the host crystal, the emission of Sm 3+ is predominantly from the 4 G 5/2 → 6 H 7/2 (598 nm) magnetic dipole allowed transition. When Sm 3+ ion is not at a site with inversion symmetry, the emission of Sm 3+ is predominantly from the 4 G 5/2 → 6 H 9/2 (644 nm) forced electric dipole transition, but the 4 G 5/2 → 6 H 7/2 transition also gains forced electric dipole character. In the present case, the emission intensity at 598 nm is similar to that at 644 nm.
According to the theory of Dexter, the mechanism of photoluminescence concentration quenching in inorganic non-conductive materials is the electric multipole interaction between the ions [51]. When the activator concentration, X, is large enough, there is relationship between the emission intensity I and the activator concentration x: [52], where β is constant, and θ = 6, 8 or 10 corresponds to electric dipole-electric dipole, electric dipoleelectric quadrupole or electric quadrupole-electric quadrupole interaction, respectively [52]. The emission spectra of LL1−xM:xSm 3+ were measured under 403 nm excitation and the curve vs.
According to the theory of Dexter, the mechanism of photoluminescence concentration quenching in inorganic non-conductive materials is the electric multipole interaction between the ions [51]. When the activator concentration, X, is large enough, there is relationship between the emission intensity I and the activator concentration x: [52], where β is constant, and θ = 6, 8 or 10 corresponds to electric dipole-electric dipole, electric dipole-electric quadrupole or electric quadrupole-electric quadrupole interaction, respectively [52]. The emission spectra of LL 1−x M:xSm 3+ were measured under 403 nm excitation and the curve lg(I/x) vs. lgx was fitted, as it is shown in Figure 3b. The slope of the straight line is −θ/3 = −1.9, so that θ = 5.7~6. Hence, the concentration quenching mechanism in LL 1−x M:xSm 3+ is by electric dipole-electric dipole interaction. This concentration quenching mechanism is consistent with that in NaGd(MoO 4 ) 2 :Sm 3+ ,Eu 3+ [48]. 381 nm ( 7 F 0 → 5 L 7 ), 393 nm ( 7 F 0 → 5 L 6 ), 415 nm ( 7 F 0 → 5 D 3 ), 463 nm ( 7 F 0 → 5 D 2 ) and 534 nm ( 7 F 1 → 5 D 1 ). This indicates that Eu 3+ in the LLM matrix has a wide absorption range, beneficial to improving the efficiency of the sample in the application of LED. Also, in Figure 4, the excitation spectrum of the co-doped sample LLM:0.05Eu 3+ ,0.05Sm 3+ are shown, as monitored by the 5 D 0 → 7 F 2 emission of Eu 3+ at 614 nm. Two new absorption peaks appear after co-doping with Sm 3+ , located at 403 nm ( 6 H 5/2 → 4 K 11/2 ) and 460 nm ( 6 H 5/2 → 4 G 7/2 ), thereby broadening the excitation spectrum and indicating energy transfer from Sm 3+ to Eu 3+ ion in the LLM crystal. As seen in Figure 4, the absorption intensity at 403 nm is larger than that at 393 nm, so we can use 403 nm as the excitation wavelength to study the optical properties of the co-doped sample. In addition, all of the excitation peaks are enhanced in the co-doped sample, compared with the Eu 3+ -doped sample. Hence, Sm 3+ also acts as a sensitizer, which can effectively absorb the excitation energy and transfer it to the activator, so that the luminous efficiency of the material can be enhanced. The excitation spectrum of the co-doped powder matches more closely with the emission wavelength of the LED chip. transfer from Sm 3+ to Eu 3+ ion in the LLM crystal. As seen in Figure 4, the absorption intensity at 403 nm is larger than that at 393 nm, so we can use 403 nm as the excitation wavelength to study the optical properties of the co-doped sample. In addition, all of the excitation peaks are enhanced in the co-doped sample, compared with the Eu 3+ -doped sample. Hence, Sm 3+ also acts as a sensitizer, which can effectively absorb the excitation energy and transfer it to the activator, so that the luminous efficiency of the material can be enhanced. The excitation spectrum of the co-doped powder matches more closely with the emission wavelength of the LED chip. LL0.95M:0.05Eu 3+ has three emission bands at ~590, 614 and 701 nm under the excitation of 393 nm, corresponding to the 5 D0 → 7 F1, 7 F2 and 7 F4 transitions, as shown in Figure 5. The emission intensity at 614 nm is much larger than at 590 nm, which indicates that Eu 3+ was located in an asymmetric crystal field [53,54]. As it can be reasonably assumed, the Eu 3+ ion (r = 0.107 nm) replaces La 3+ (r = 0.106 nm) instead of Li + (r = 0.076 nm) in the crystal because of the same valence and similar radius. When the excitation wavelength was changed to 403 nm, the emission spectrum of LL0.95M:0.05Eu 3+ showed a very weak peak at 614 nm. This indicates that LL0.95M:0.05Eu 3+ cannot be efficiently excited by the 403 nm radiation. As shown in Figure 5, the emission spectrum of LL0.95M:0.05Sm 3+ under 403 nm excitation and it demonstrates that the emission spectrum of LL0.95M:0.05Sm 3+ ,0.05Eu 3+ is a superposition of emission spectra of the Sm 3+ and Eu 3+ ions. After codoping Sm 3+ and Eu 3+ ions, the characteristic peaks of Sm 3+ are reduced in intensity from the singly doped LL0.95M:0.05Sm 3+ , whereas the characteristic peaks of Eu 3+ are stronger than in LL0.95M:0.05Eu 3+ excited by 393 nm radiation. This also confirms the existence of the energy transfer from Sm 3+ to Eu 3+ . The LL0.95M:0.05Sm 3+ phosphor has the advantage of low synthesis temperature, simple preparation method and high luminous efficiency but it needs for some improvement to be used in practical applications: such as its color purity, which is far from the standard red region and its near UV/blue light absorption is not strong enough. These problems can be solved by the incorporation of Eu 3+ . The PLE (photoluminescence excitation) spectrum of LL 0.95 M:0.05Eu 3+ and LL 0.90 M:0.05Sm 3+ ,0.05Eu 3+ phosphors by monitoring the 5 D 0 → 7 F 2 emission of Eu 3+ at 614 nm. LL 0.95 M:0.05Eu 3+ has three emission bands at~590, 614 and 701 nm under the excitation of 393 nm, corresponding to the 5 D 0 → 7 F 1 , 7 F 2 and 7 F 4 transitions, as shown in Figure 5. The emission intensity at 614 nm is much larger than at 590 nm, which indicates that Eu 3+ was located in an asymmetric crystal field [53,54]. As it can be reasonably assumed, the Eu 3+ ion (r = 0.107 nm) replaces La 3+ (r = 0.106 nm) instead of Li + (r = 0.076 nm) in the crystal because of the same valence and similar radius. When the excitation wavelength was changed to 403 nm, the emission spectrum of LL 0.95 M:0.05Eu 3+ showed a very weak peak at 614 nm. This indicates that LL 0.95 M:0.05Eu 3+ cannot be efficiently excited by the 403 nm radiation. As shown in Figure 5, the emission spectrum of LL 0.95 M:0.05Sm 3+ under 403 nm excitation and it demonstrates that the emission spectrum of LL 0.95 M:0.05Sm 3+ ,0.05Eu 3+ is a superposition of emission spectra of the Sm 3+ and Eu 3+ ions. After co-doping Sm 3+ and Eu 3+ ions, the characteristic peaks of Sm 3+ are reduced in intensity from the singly doped LL 0.95 M:0.05Sm 3+ , whereas the characteristic peaks of Eu 3+ are stronger than in LL 0.95 M:0.05Eu 3+ excited by 393 nm radiation. This also confirms the existence of the energy transfer from Sm 3+ to Eu 3+ . The LL 0.95 M:0.05Sm 3+ phosphor has the advantage of low synthesis temperature, simple preparation method and high luminous efficiency but it needs for some improvement to be used in practical applications: such as its color purity, which is far from the standard red region and its near UV/blue light absorption is not strong enough. These problems can be solved by the incorporation of Eu 3+ . The emission spectra of LL0.95−yM:0.05Sm 3+ ,yEu 3+ (y = 0, 0.01, 0.03, 0.05, 0.08, 0.12) under 403 nm excitation are shown in Figure 6. With the Eu 3+ concentration increase, the luminescence intensity of Sm 3+ gradually decreased while the 5 D0 → 7 F2 (614 nm) emission of Eu 3+ is continuously enhanced.  The emission spectra of LL 0.95−y M:0.05Sm 3+ ,yEu 3+ (y = 0, 0.01, 0.03, 0.05, 0.08, 0.12) under 403 nm excitation are shown in Figure 6. With the Eu 3+ concentration increase, the luminescence intensity of Sm 3+ gradually decreased while the 5 D 0 → 7 F 2 (614 nm) emission of Eu 3+ is continuously enhanced.  The schematic energy level diagram depicted in Figure 7a indicates that the energy transfer pathway from Sm 3+ → Eu 3+ is probably from 4 G 5/2 level of Sm 3+ to 5 D 0 level of Eu 3+ , which is confirmed by the spectrum overlap between the emission of Sm 3+ and the excitation of Eu 3+ , as shown in Figure 7b. The schematic energy level diagram depicted in Figure 7a indicates that the energy transfer pathway from Sm 3+ → Eu 3+ is probably from 4 G5/2 level of Sm 3+ to 5 D0 level of Eu 3+ , which is confirmed by the spectrum overlap between the emission of Sm 3+ and the excitation of Eu 3+ , as shown in Figure  7b. In order to better understand the energy transfer processes in the LLM:Sm 3+ ,Eu 3+ phosphors, the luminescent dynamics for the 4 G5/2 → 6 H7/2 transition of Sm 3+ ions at 598 nm was studied. The PL decay curves of Sm 3+ emission from LLM:Sm 3+ ,Eu 3+ phosphors excited at 403 nm and monitored at 598 nm are shown in Figure 8 and they are fitted by the following double exponential function: where I(t) and I0 are the luminescence intensities at time t and , respectively. A1 and A2 are fitting parameters. τ1, τ2 are the fast decay and slow decay lifetimes, respectively. The average decay time was calculated using the following formula When co-doping with Eu 3+ , the Sm 3+ 4 G5/2 lifetime decreases due to energy transfer from Sm 3+ to Eu 3+ . The energy transfer efficiency (ηET) from the sensitizer Sm 3+ to the activator Eu 3+ can be calculated by the following formula [55]: where τ0 is the average decay lifetime of the sensitizer (Sm 3+ ) and τ is the average decay lifetime of the sensitizer in the presence of the activator (Eu 3+ ). The energy transfer efficiency from Sm 3+ to Eu 3+ is calculated to be 39.0%. In order to better understand the energy transfer processes in the LLM:Sm 3+ ,Eu 3+ phosphors, the luminescent dynamics for the 4 G 5/2 → 6 H 7/2 transition of Sm 3+ ions at 598 nm was studied. The PL decay curves of Sm 3+ emission from LLM:Sm 3+ ,Eu 3+ phosphors excited at 403 nm and monitored at 598 nm are shown in Figure 8 and they are fitted by the following double exponential function: where I(t) and I 0 are the luminescence intensities at time t and ∞, respectively. A 1 and A 2 are fitting parameters. τ 1 , τ 2 are the fast decay and slow decay lifetimes, respectively. The average decay time was calculated using the following formula When co-doping with Eu 3+ , the Sm 3+ 4 G 5/2 lifetime decreases due to energy transfer from Sm 3+ to Eu 3+ . The energy transfer efficiency (η ET ) from the sensitizer Sm 3+ to the activator Eu 3+ can be calculated by the following formula [55]: where τ 0 is the average decay lifetime of the sensitizer (Sm 3+ ) and τ is the average decay lifetime of the sensitizer in the presence of the activator (Eu 3+ ). The energy transfer efficiency from Sm 3+ to Eu 3+ is calculated to be 39.0%.

Color Coordinates of LL0.95−yM:0.05Sm 3+ ,yEu 3+ Phosphors
LLM phosphors doped with Sm 3+ ions emit orange-red light but the color coordinates are far from the standard red color coordinates of (0.670, 0.330), which limits applications in display products. In this work, the co-doping of Eu 3+ has improved the color purity, so that the emission color is closer to that of standard red light. The color coordinates of the samples are shown in Figure 9 and, with the increase of the Eu 3+ concentration, the sample points gradually approach to standard red light. The chromaticity coordinates of the LL0.83M:0.05Sm 3+ ,0.12Eu 3+ sample are (0.629, 0.370), which are close to the color coordinates (0.631, 0.350) of the commercial red phosphor Y2O2S:Eu 3+ . Hence, the luminous color of LL0.95−yM:0.05Sm 3+ ,yEu 3+ can be adjusted so that it has application as a LED red phosphor.

Color
Coordinates of LL 0.95−y M:0.05Sm 3+ ,yEu 3+ Phosphors LLM phosphors doped with Sm 3+ ions emit orange-red light but the color coordinates are far from the standard red color coordinates of (0.670, 0.330), which limits applications in display products. In this work, the co-doping of Eu 3+ has improved the color purity, so that the emission color is closer to that of standard red light. The color coordinates of the samples are shown in Figure 9 and, with the increase of the Eu 3+ concentration, the sample points gradually approach to standard red light. The chromaticity coordinates of the LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ sample are (0.629, 0.370), which are close to the color coordinates (0.631, 0.350) of the commercial red phosphor Y 2 O 2 S:Eu 3+ . Hence, the luminous color of LL 0.95−y M:0.05Sm 3+ ,yEu 3+ can be adjusted so that it has application as a LED red phosphor.

Color Coordinates of LL0.95−yM:0.05Sm 3+ ,yEu 3+ Phosphors
LLM phosphors doped with Sm 3+ ions emit orange-red light but the color coordinates are far from the standard red color coordinates of (0.670, 0.330), which limits applications in display products. In this work, the co-doping of Eu 3+ has improved the color purity, so that the emission color is closer to that of standard red light. The color coordinates of the samples are shown in Figure 9 and, with the increase of the Eu 3+ concentration, the sample points gradually approach to standard red light. The chromaticity coordinates of the LL0.83M:0.05Sm 3+ ,0.12Eu 3+ sample are (0.629, 0.370), which are close to the color coordinates (0.631, 0.350) of the commercial red phosphor Y2O2S:Eu 3+ . Hence, the luminous color of LL0.95−yM:0.05Sm 3+ ,yEu 3+ can be adjusted so that it has application as a LED red phosphor.

Thermal Stability Analysis
The thermal quenching property of phosphor is an important parameter to be considered for application in high power LEDs because the junction temperature of typical LEDs can be above 100 • C. The thermal quenching of LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ phosphors was evaluated by measuring the temperature-dependent emission intensity from 25 to 250 • C. As shown in Figure 10, the temperature dependence of PL intensity indicated that the integrated emission intensity of the Eu 3+ at 150 • C remained more than 85% of this measured at room temperature. In the inset (Figure 10) is shown the fitting of the emission intensity according to the following Arrhenius Equation (4) [30]: where I 0 and I T are the emission intensities at room and testing temperatures, respectively, c is a constant, and k is the Boltzmann constant (k = 8.617 × 10 −5 eVK −1

Thermal Stability Analysis
The thermal quenching property of phosphor is an important parameter to be considered for application in high power LEDs because the junction temperature of typical LEDs can be above 100 °C. The thermal quenching of LL0.83M:0.05Sm 3+ ,0.12Eu 3+ phosphors was evaluated by measuring the temperature-dependent emission intensity from 25 to 250 °C. As shown in Figure 10, the temperature dependence of PL intensity indicated that the integrated emission intensity of the Eu 3+ at 150 °C remained more than 85% of this measured at room temperature. In the inset (Figure 10) is shown the fitting of the emission intensity according to the following Arrhenius Equation (4)

The Luminous Efficiency and the Performance for LED Applications
The luminous efficiency of phosphor is an important parameter for practical application. The comparative emission spectra and excitation spectra between the LL0.83M:0.05Sm 3+ ,0.12Eu 3+ phosphor and commercial Y2O3:0.06Eu 3+ red phosphor measured under the same experiment conditions are shown in Figure 11. Under 403 or 460 nm excitation, the LL0.83M:0.05Sm 3+ ,0.12Eu 3+ phosphor exhibits stronger emission than the commercial red Y2O3:Eu 3+ phosphor.

The Luminous Efficiency and the Performance for LED Applications
The luminous efficiency of phosphor is an important parameter for practical application. The comparative emission spectra and excitation spectra between the LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ phosphor and commercial Y 2 O 3 :0.06Eu 3+ red phosphor measured under the same experiment conditions are shown in Figure 11. Under 403 or 460 nm excitation, the LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ phosphor exhibits stronger emission than the commercial red Y 2 O 3 :Eu 3+ phosphor. To estimate the potential of the Sm 3+ /Eu 3+ co-doped LiLa(MoO4)2 phosphor for application, WLED lamps were fabricated by coating 455 nm blue LED chips with LL0.83M:0.05Sm 3+ ,0.12Eu 3+ red phosphor and commercial yellow phosphor of different mass ratios. The emission spectra and the CIE diagrams of the obtained WLED lamps are shown in Figure 12, while the color and electrical parameters are summarized in Table 1. The emission spectra of the WLED lamps exhibited a blue band at 455 nm which emits from the blue chip, a yellow band at 537 nm which emits from the yellow phosphors and a red peak at 615.7 nm which comes from our samples.
In Figure 12a-c, with the mass ratio of red phosphors increasing from 0.2 to 0.8, while that of yellow phosphor fixed at 0.2, the CCT is decreased from 4811 K to 4553 K and the color purity is increased from 73.6% to 81.0%, which indicate that the addition of the LL0.83M:0.05Sm 3+ ,0.12Eu 3+ red phosphors can improve the color purity and reduce the CCT of WLED lamps. However, for the cases, the CRI is low, which can be improved by decreasing the mass of yellow phosphors. The emission spectrum and the CIE diagram of the WLED lamp with red phosphor mass ratio of 0.52 and yellow phosphor mass ratio decreasing to 0.1, are given in Figure 12d. As can be found from Table 1, the CRI, chromaticity coordinates, CCT, luminous efficacy and color purity are 62.6, (0.3363, 0.4498), 5426 K and 107.08 lm W −1 , 36.2% respectively, and the CRI is indeed improved. From Figure 12d, it can be noticed that the blue band is comparatively weak, which should be improved by decreasing the absorption of blue light, so as to obtain white light emission. For the purpose, the phosphor glue coating on the chip was thinned, and the emission spectrum and the CIE diagram of the WLED lamp are shown in Figure 12e. Now, the CIE indeed approaches the white light region, and the CRI, chromaticity coordinates, CCT, luminous efficacy and color purity are 65.0, (0.3055, 0.3849), 6482 K and 121.36 lm W −1 , 10.4% respectively. The performance of the WLED lamps can be further optimized To estimate the potential of the Sm 3+ /Eu 3+ co-doped LiLa(MoO 4 ) 2 phosphor for application, WLED lamps were fabricated by coating 455 nm blue LED chips with LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ red phosphor and commercial yellow phosphor of different mass ratios. The emission spectra and the CIE diagrams of the obtained WLED lamps are shown in Figure 12, while the color and electrical parameters are summarized in Table 1. The emission spectra of the WLED lamps exhibited a blue band at 455 nm which emits from the blue chip, a yellow band at 537 nm which emits from the yellow phosphors and a red peak at 615.7 nm which comes from our samples.
In Figure 12a-c, with the mass ratio of red phosphors increasing from 0.2 to 0.8, while that of yellow phosphor fixed at 0.2, the CCT is decreased from 4811 K to 4553 K and the color purity is increased from 73.6% to 81.0%, which indicate that the addition of the LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ red phosphors can improve the color purity and reduce the CCT of WLED lamps. However, for the cases, the CRI is low, which can be improved by decreasing the mass of yellow phosphors. The emission spectrum and the CIE diagram of the WLED lamp with red phosphor mass ratio of 0.52 and yellow phosphor mass ratio decreasing to 0.1, are given in Figure 12d. As can be found from Table 1, the CRI, chromaticity coordinates, CCT, luminous efficacy and color purity are 62.6, (0.3363, 0.4498), 5426 K and 107.08 lm W −1 , 36.2% respectively, and the CRI is indeed improved. From Figure 12d, it can be noticed that the blue band is comparatively weak, which should be improved by decreasing the absorption of blue light, so as to obtain white light emission. For the purpose, the phosphor glue coating on the chip was thinned, and the emission spectrum and the CIE diagram of the WLED lamp are shown in Figure 12e. Now, the CIE indeed approaches the white light region, and the CRI, chromaticity coordinates, CCT, luminous efficacy and color purity are 65.0, (0.3055, 0.3849), 6482 K and 121.36 lm W −1 , 10.4% respectively. The performance of the WLED lamps can be further optimized by adjusting the particle size and shape, the particle size distribution of the phosphor and the ratio of yellow and red phosphor. The results indicate that the phosphor-coated white LED had a high luminous efficacy and low CCT, demonstrating that the LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ is a very promising WLED red phosphor.
Materials 2018, 11,297 13 of 18 by adjusting the particle size and shape, the particle size distribution of the phosphor and the ratio of yellow and red phosphor. The results indicate that the phosphor-coated white LED had a high luminous efficacy and low CCT, demonstrating that the LL0.83M:0.05Sm 3+ ,0.12Eu 3+ is a very promising WLED red phosphor.   Table 1.

Figure 12.
The emission spectra and CIE diagrams of the WLED lamps coated with LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ red phosphors and commercial yellow phosphors of different mass ratios, the mass ratios of phosphor glues coating on the chip of WLED lamps as shown in (a-f) are written in Table 1. Table 1. The color and electrical parameters of WLEDs coated with phosphor glues of different mass ratios.

Conclusions
The Sm 3+ /Eu 3+ co-doped LiLa(MoO 4 ) 2 phosphor was prepared by the traditional solid state method of sintering the reactants for 6 h in an ambient air atmosphere at 610 • C. The excitation spectra show that LL 1−x M:xSm 3+ can effectively absorb photons of 403 and 460 nm wavelengths and this absorption matches well with the near UV LED and blue chip emission. The optimal dopant concentration of Sm 3+ is x = 0.05, as evident from the emission spectra of LL 1−x M:xSm 3+ , and higher concentrations exhibit concentration quenching by the electric dipole-electric dipole mechanism.
With Sm 3+ co-doping, the absorption spectrum of LLM:Eu 3+ in the near ultraviolet and blue region becomes wider and stronger. Due to the energy transfer from Sm 3+ to Eu 3+ , the Eu 3+ emission in the co-doped phosphor can be effectively excited by 403 nm and 460 nm photons. The Sm 3+ ions play the significant role in regulating luminous efficiency and color coordinates of the co-doped phosphor. The color coordinates of LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ are (0.629, 0.370) under 403 nm excitation and they are close to these of the standard red light point O (0.670, 0.330).
The LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ phosphor exhibits stronger emission than the commercial red Y 2 O 3 :Eu 3+ phosphor under both 403 and 460 nm excitations, and it has superior thermal stability in comparison with that of the commercial red Y 2 O 3 :Eu 3+ phosphor. The red LL 0.83 M:0.05Sm 3+ ,0.12Eu 3+ phosphor-precoated WLED lamp has a high luminous efficacy and a low CCT. In summary, LL 1−x−y M:xSm 3+ ,yEu 3+ is a very promising WLED red phosphor.