1. Introduction

Due to the benefits of long lifetime, low energy consumption, small size and eco-friendliness, the white light-emitting diodes (wLEDs) are widely used and have gained enormous commercial interest as solid state lighting sources [1,2]. At present, the most used way to generate white light is combining a blue InGaN chip with Y₃Al₅O₁₂:Ce³⁺ (YAG: Ce) yellow phosphor [3]. However, this kind of white emission suffers a poor color rendering index (CRI≈70-80) and a high correlated color temperature (CCT≈7500K) because of the lack of red-light component [4,5]. In recent years, many researchers have devoted to the study of red phosphors which can be excited by blue light. Such phosphors are Na₅Ln(MoO₄)₄: Eu³⁺ (Ln = La, Gd, Y) [6], CaS: Eu²⁺ [7], CaAlSiN₃:Eu²⁺ [8], and K₂SiF₆:Mn⁴⁺ [9,10], etc. With these phosphors, the phosphor converted wLEDs can present low CCT and high CRI values. Unfortunately, poor blue emission efficiency due to the strong re-absorption among green and red phosphors and high manufacturing cost are other challenging problems. Therefore, numerous studies have been focused on developing efficient, durable and single-phase white light emitting phosphors with the red, green and blue (RGB) components based on the mechanism of the energy transfer (ET) from sensitizes to activators [11,12].

Among various kinds of wLED phosphors, co-doping Ce³⁺, Tb³⁺ or Mn²⁺ ions is the common strategy for realizing white light emission through ET in the single-phase phosphors [11–13]. Ce³⁺ has a 4f¹5d⁰ ground state and a 4f⁰5d¹ excited state and therefore shows typical parity allowed 5d-4f electronic transitions. The distribution of Ce³⁺ energy level is deeply dependent on the host lattice, so the emission due to 5d-4f transition can appear in a large wavelength range. Tb³⁺ and Mn²⁺ ions have been frequently used as green and red emission components, respectively. However, the excitation bands of Tb³⁺ and Mn²⁺ ions from UV to visible region are very weak due to the forbidden 4f-4f transition of Tb³⁺ and ⁴T₁-⁶A₁ transition of Mn²⁺ [14,15]. In order to enhance the absorption of Tb³⁺ and Mn²⁺ in the UV region, a frequently used method is to introduce an efficient sensitizer such as Ce³⁺, and then the strongly absorbed excitation energy can be transferred from the 5d level of Ce³⁺ to the ⁵D_3,4 level of Tb³⁺ or the 4G level of Mn²⁺ [11,16,17]. In this way, the emission-tunable single-phase phosphors can be obtained, which has advantages such as lower manufacturing cost, good color reproducibility and high luminescence efficiency [18,19].

So far, the apatites have proven to be excellent host materials for phosphors due to their good physical and chemical stability [20–24]. Sr₄La(PO₄)₃O (SLPO) is isomorphic to the apatite compound and consists of two cationic sites, a nine-fold coordination 4f site with C₃ point symmetry and a seven-fold coordination 6h site with C_s point symmetry [21]. In our previous report, the SLPO: Eu³⁺/Tb³⁺/Ce³⁺ phosphors have been studied [22]. In this work, we introduced Tb³⁺ or Mn²⁺ into SLPO: Ce³⁺. Through ET, a green emission peak of Tb³⁺ at 539 nm and a red emission band of Mn²⁺ with a maximum at around 605 nm can be obtained. Their photoluminescence properties and ET behaviors from Ce³⁺ to Tb³⁺ and Mn²⁺ were systematically studied. By adjusting the dopant concentration of Tb³⁺ or Mn²⁺, tunable emission colors from blue to green and warm white have been realized, which implies its promising application in UV-pumped wLEDs.

2. Experimental

The Sr₄La_1-x-y(PO₄)₃O: xCe³⁺, yTb³⁺ and Sr_4-zLa_1-x(PO₄)₃O: xCe³⁺, zMn²⁺ phosphors with different compositions were prepared by conventional high temperature solid-state reactions. The starting materials included SrCO₃ (99%), (NH₄)₂HPO₄ (99%), La₂O₃ (99.99%), Tb₄O₇ (99.99%), MnCO₃ (99%) and CeO₂ (99.99%). 2wt% Li₂CO₃ (99%) was introduced as flux. The stoichiometric starting reagents were thoroughly mixed and ground with an agate mortar. The mixture was first pre-fired in air at 600°C for 3 hours, ground again, and calcined at 1200°C for 5 hours in a reducing atmosphere (N₂: H₂ = 95: 5).

The phase purity was determined using an ARL X'TRA powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5406 Å) operating at 40 kV and 35 mA. Diffuse reflection spectra (DRS) were obtained with a UV/visible spectrophotometer (UV-3600, SHIMADZU) using BaSO₄ as a reference in the 200-700 nm range. The morphologies of the as-prepared samples were examined by a field emission scanning electronmicroscope (FESEM, FEI, Quanta FEG). The photoluminescence excitation (PLE) spectra and photoluminescence (PL) spectra were recorded on an Edinburgh FS5 fluorescence spectrophotometer with a 150 W xenon lamp as the light source. The temperature dependent measurement was performed by the FS5 spectrofluorometer system, and the samples were mounted on a heating device that can be heated from 25 to 200 °C with the step of 1°C. The photoluminescence external quantum yields of the samples were obtained with an integrating sphere in the FS5 spectrophotometer. The fluorescence lifetime was recorded on an Edinburgh FLS 920 Fluorescence Spectrophotometer.

3. Results and discussion

3.1. Phase formation and morphology of SLPO: xCe³⁺, yTb³⁺/ xCe³⁺, zMn²⁺

The Rietveld structural refinement of the laboratory XRD data for the synthesized compound SLPO at room temperature is shown in Fig. 1(a), using the structural parameters of Sr₅La(PO₄)₃F as an initial model [23]. The final refinement structure parameters are listed in Table 1. The as-obtained residual factors and cell parameters are χ² = 4.328, R_wp = 5.51%, R_p = 4.39%, and a = b = 9.721 Å, c = 7.235 Å, V = 592.135 ų. Figure 1(b) presents the crystal structure of SLPO modeled with the acquired atomic coordinates. In the SLPO compound, there are two types of cationic sites (6h and 4f), the Sr(I) site is surrounded by nine O atoms in C₃ symmetry, while the Sr(II)/La(II) position is with seven-coordinated C_S symmetry [22]. The cations in the host are connected by the isolated PO₄ tetrahedrons. According to charge balance and the effective ionic radius(r_Sr²⁺, _{CN = 9} = 1.31 Å, r_Sr²⁺, _{CN = 7} = 1.21 Å, r_Mn²⁺, _{CN = 7} = 0.9 Å, r_La³⁺, _{CN = 7} = 1.1 Å, r_Tb³⁺, _{CN = 7} = 0.98 Å, r_Ce³⁺, _{CN = 7} = 1.07 Å) of positive ions in the host lattice [25], the Ce³⁺/Tb³⁺ ions are possible to enter into the La³⁺ sites and Mn²⁺ ions are expected to substitute the Sr²⁺ sites primarily in the SLPO lattice.

 

Fig. 1 (a) XRD refinement for SLPO host. (b) Crystal structure of SLPO host along the c-axis direction, and coordinated environments of Sr(I) and Sr/La(II) atoms. XRD patterns of the samples: (c) SLPO: 0.12Ce³⁺, yTb³⁺ (0.01≤y≤0.15);(d) Enlarged view of (c); (e) SLPO: 0.12Ce³⁺, zMn²⁺ (0.2≤z≤0.5); (f) Enlarged view of (e).

Download Full Size | PDF

Table 1. Final refined structure parameters of SLPO host.

View Table | View all tables in this article

According to the previous work, for SLPO: xCe³⁺ phosphors, the most intense emission intensity can be obtained when x = 0.12. Here, the value of x in each SLPO: xCe³⁺, yTb³⁺/ xCe³⁺, zMn²⁺ sample is set to be 0.12. Figure 1(c) and 1(d) show the XRD patterns of SLPO: 0.12Ce³⁺, yTb³⁺ and SLPO: 0.12Ce³⁺, zMn²⁺ samples together with the standard pattern of Sr₄Bi(PO₄)₃O (JCPDS 44-0180) as a reference. For observation, the enlarged views (2θ = 30-35 degree) of Fig. 1(c) and 1(d) are given in Fig. 1(d) and 1(f). Comparing with the standard data, it can be noticed that the diffraction peaks of SLPO: 0.12Ce³⁺, yTb³⁺ samples almost have no change when increasing the Tb³⁺ doping concentration y, indicating that the phase formation of SLPO is not influenced by little amount of Tb³⁺ or Ce³⁺. However, with the increase of z, an unknown impurity phase emerges when co-doping Mn²⁺ into SLPO: 0.12Ce³⁺.

Figure 2 shows the SEM morphologies of the SLPO: 0.12Ce³⁺, 0.1Tb³⁺ and SLPO: 0.12Ce³⁺, 0.3Mn²⁺ samples. It can be found that the particles exhibit relatively aggregated and irregular morphology due to the solid-state reaction preparation. In addition, the average particle sizes are about 10 μm, which can meet the requirements of w-LED phosphors.

 

Fig. 2 The SEM images of (a) SLPO: 0.12Ce³⁺, 0.1Tb³⁺ and (b) SLPO: 0.12Ce³⁺, 0.3Mn²⁺ phosphors.

Download Full Size | PDF

3.2. Luminescence properties and energy transfer in SLPO: Ce³⁺, Tb³⁺

Figure 3 presents the diffuse reflection spectrum of the SLPO from 200 to 700 nm, which shows high reflectance in the visible range (400-700 nm), consistent with the white daylight color of the SLPO matrix.

 

Fig. 3 Diffuse reflection spectrum of SLPO. Inset: Bandgap value of SLPO calculated by the K-M formula.

Download Full Size | PDF

To determine the optical bandgap value of the SLPO compound experimentally, the absorption spectrum of SLPO (see the inset of Fig. 3) was obtained from its reflection spectrum with the Kubelka-Munk (K-M) function [4].

Figure 4(a) presents the normalized PLE of SLPO: 0.1Tb³⁺ and PL spectra of SLPO: 0.12Ce³⁺ phosphors. Upon 310 nm excitation, the Ce³⁺ doped SLPO phosphor exhibits a broad emission band from 350 to 600 nm with the emission peak at around 469 nm, which is ascribed to the electric-dipole-allowed transition from the 5d excited state level to the 4f ground state of the Ce³⁺ ions [5]. By monitoring 539 nm, the PLE spectrum of SLPO: 0.1Tb³⁺ contains two parts: the broad excitation band from 240 to 300 nm attributed to the 4f-5d transition of Tb³⁺and the sharp lines in the region of 300-400 nm, corresponding to the Tb³⁺ 4f-4f transitions. It can be observed that there is a distinct spectral overlap between the Ce³⁺ PL and Tb³⁺ PLE spectrum, indicating the possibility of ET from Ce³⁺ to Tb³⁺, and the emission intensity of Tb³⁺ can be enhanced via co-doping Ce³⁺ in the SLPO host.

 

Fig. 4 (a) PLE (SLPO: 0.1Tb³⁺) and PL (SLPO: 0.12Ce³⁺) spectra; (b) PLE and PL spectra of SLPO: 0.12Ce³⁺, 0.1Tb³⁺, inset shows the schematic of ET from Ce³⁺ to Tb³⁺.

Download Full Size | PDF

The PLE and PL spectra of SLPO: 0.12Ce³⁺, 0.1Tb³⁺ are given in Fig. 4(b). It can be noticed that the shape of the PLE spectrum monitored at 539 nm is similar to that monitored at 469 nm. According to the previous research, the PLE spectra of the SLPO: Ce³⁺ phosphors vary with the monitoring emission wavelengths, probably because the Ce³⁺ ions will occupy more than one site in the SLPO host. The positions of the absorption bands in Fig. 4(b) are almost the same as the ones in the singly Ce³⁺ doped SLPO phosphors [22]. Upon excitation of 310 nm, the PL spectrum exhibits a broadband emission from Ce³⁺ and a group of sharp characteristic emission lines of Tb³⁺. When excited upon 335nm, the intensity of Ce³⁺ emission center gets higher, whereas the emission intensity at 539nm from Tb³⁺ ions becomes weaker. These appearances reveal the occurrence of the ET from Ce³⁺ to Tb³⁺. The inset of Fig. 4(b) illustrates the possible ET (Ce³⁺ to Tb³⁺) process in the SLPO host. Electrons of Ce³⁺ can be pumped to the 5d level by absorbing UV light and then the ET process may occur from the excited 5d state of Ce³⁺ to the ⁵D₃ level of Tb³⁺, which can give lower vibration frequency of phonon emission. Then the Tb³⁺ ions on ⁵D₃ level incline to transits to the ⁵D₄ level through non-radiative relaxation and shows strong emission.

To further investigate the ET process between the Ce³⁺ and Tb³⁺ ions in the host lattice, a series of SLPO: 0.12Ce³⁺, yTb³⁺ (y = 0-0.15) samples were prepared. Figure 5(a) depicts the PL spectra of the SLPO: 0.12Ce³⁺, yTb³⁺ under the excitation of 310 nm. It can be found that with the increase of Tb³⁺ concentration, the emission intensity of the green line at 539 nm of Tb³⁺ first increases till y = 0.1, and then decreases due to concentration quenching which is caused by the enhanced non-radiative ET among Tb³⁺ ions. Meanwhile, the intensity of the sensitizer Ce³⁺ declines gradually, which further supports the ET from Ce³⁺ to Tb³⁺. It can be found that when increasing the Tb³⁺ concentration, there is a decrease for the Ce³⁺ emission on the short wavelength side but an increase on the long wavelength side. This is mainly due to the overlapping of the two emission spectra from Ce³⁺ and Tb³⁺ ions in the long waveband. The emission intensities of Ce³⁺ ions in such side are relatively weak, and as the Tb³⁺ emissions strengthen, the total intensities increase.

 

Fig. 5 (a) PL spectra of SLPO: 0.12, yTb³⁺; (b) decay curves of Ce³⁺ and Tb³⁺ (inset) in SLPO: 0.12Ce³⁺, yTb³⁺ (y = 0, 0.01, 0.03, 0.07, 0.1 and 0.15).

Download Full Size | PDF

The ET efficiency (ƞ_ET) from a sensitizer to an activator can be calculated with the following equation [12]:

The fluorescence decay curves of Ce³⁺ monitored at 469 nm in the SLPO: 0.12Ce³⁺, yTb³⁺ phosphors under the excitation of 310 nm were measured and investigated as shown in Fig. 6(a), in order to further validate the ET behavior from Ce³⁺ to Tb³⁺ ions in the SLPO host. It can be found that the decay curves of Ce³⁺ deviate from the single exponential one and with the increase of Tb³⁺ concentration, the deviation becomes more evident. Since the decay curves are non-exponential, the decay process of these samples can be characterized by the average fluorescence lifetime calculated as follows [28,29]:

 

Fig. 6 Dependence of I_s0/I_s of $C_{Ce+Tb}^{n/3}$ on the exponent (a) n = 6, (b) n = 8 and (c) n = 10. The red lines indicate the fitting behaviors.

Download Full Size | PDF

To further study the ET mechanism from Ce³⁺ to Tb³⁺ in the SLPO host, the following relation can be used, based on Dexter’s ET formula for exchange and multi-polar interactions as well as Reisfeld’s approximation [32].

According to Dexter’s ET theory [34], the critical distance (R_c) from the sensitizer to the acceptor in the ET progress can be calculated with the spectral overlap method. For the dipole-dipole interaction, R_c can be expressed as following [35,36]:

3.3 Photoluminescence properties and ET in SLPO: Ce³⁺, Mn²⁺

As a transition metal ion with 3d⁵ electronic configurations, Mn²⁺ generally exhibits a broad emission band due to ⁴T₁-⁶A₁ transitions, and its position strongly depends on the host lattice, i.e. green emission in weak crystal field and red emission in strong crystal field [4, 41]. However, the intensities of Mn²⁺ absorption and emission bands are very weak because the d-d transitions are spin and parity forbidden. To enhance the absorption and emission intensities of Mn²⁺ in SLPO, Ce³⁺ ions were co-doped and tunable emission colors were obtained through ET processes. Figure 7(a) presents the normalized PLE and PL spectra of SLPO: 0.4Mn²⁺ and SLPO: 0.12Ce³⁺. The main excitation peaks of the SLPO: 0.4Mn²⁺ spectrum in the range of 250-490 nm could be ascribed to the transitions from the ⁶A₁(⁶S) level to the ⁴T₁(⁴P), ⁴E(⁴D), ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)], and ⁴T₂(⁴G) energy levels of Mn²⁺, respectively [18,33,38,42,43]. It can be noticed that the emission band of SLPO: 0.12Ce³⁺ is overlapped with the absorption band of SLPO: 0.4Mn²⁺, which implies that the ET is probably to exist in the Ce³⁺ and Mn²⁺ co-doped SLPO phosphor. From the view of Ce³⁺ to Mn²⁺ transition in the SLPO host, the ET behavior between Ce³⁺ and Mn²⁺ ions can be explained by the similar value of the excited 5d state energy level of Ce³⁺ and the 3d state energy levels of Mn²⁺ ions, as shown in the inset of Fig. 7(a). In order to further investigate the Ce³⁺ to Mn²⁺ ET in SLPO, a series of SLPO: 0.12Ce³⁺, zMn²⁺ (z = 0, 0.2, 0.3, 0.35, and 0.4) phosphors have been prepared and their PL spectra under the excitation of 310nm UV are displayed in Fig. 7(b). Two emission bands can be noticed in the spectra: a broad blue band due to Ce³⁺ luminescence centers and an orange-red emission band (550-700nm, ⁴T₁-⁶A₁ transition) of the Mn²⁺ ions. With the increase of Mn²⁺ content, the PL intensity of Ce³⁺ decreases gradually, while that of Mn²⁺ increases. This result indicates the presence of the ET from Ce³⁺ to Mn²⁺. According to Eq. (4), the ET efficiency gradually increases from 8.46% to 22.30% when z increases from 0.2 to 0.4.

 

Fig. 7 (a) PLE (SLPO: 0.4Mn²⁺) and PL (SLPO: 0.12Ce³⁺) spectra, inset shows the schematic of ET from Ce³⁺ to Mn²⁺; (b) PL spectra of SLPO: 0.12, zMn²⁺ (z = 0, 0.2, 0.3, 0.35, and 0.4).

Download Full Size | PDF

Furthermore, the decay curves and lifetimes of Ce³⁺ and Mn²⁺ ions in SLPO: 0.12Ce³⁺, zMn²⁺ phosphors under the excitation of 310 nm were investigated. Figure 8(a) presents the decay curves of SLPO: 0.12 Ce³⁺, zMn²⁺ phosphors with 310 nm excitation and 469 nm emissions. According to Eq. (5), the calculated lifetimes of Ce³⁺ for SLPO: 0.12Ce³⁺, zMn²⁺ (z = 0.2, 0.3, 0.35, and 0.4) samples are 46.3, 42.6, 40.6, and 37.5 ns, respectively. The inset of Fig. 8(a) shows the decay curves of Mn²⁺ (λ_em = 610nm, λ_ex = 310nm) in SLPO: 0.12Ce³⁺, zMn²⁺ samples. As Mn²⁺ content increases, the lifetime shows obvious increase, which may be mainly originating from the ET between Ce³⁺ and Mn²⁺ ions. the lifetimes of Mn²⁺ ions can be calculated as 29.1, 43.5, 46.8, and 47.8 ms for z = 0.2, 0.3, 0.35, and 0.4, respectively. The ƞ_ET values of ET behavior from Ce³⁺ to Mn²⁺ are also estimated using Eq. (6), with the results of 7.82%, 15.16%, 19.14%, and 25.34%, respectively. Like SLPO: Ce³⁺, Tb³⁺, the ET from Ce³⁺ to Mn²⁺ in the SLPO host is also inefficient. According to Eq. (7), when n = 6, the I₀/I and $C_{Ce+Mn}^{6/3}$ plot shows the optimal linear relationship, which indicates that the ET from Ce³⁺ to Mn²⁺ ions in SLPO can be determined to be the electric dipole-dipole interaction, as shown in Fig. 8(b), 8(c) and 8(d). The critical distance (R_c) calculated with Eq. (8) is 4.97 Å (f_d = 10⁻⁷ when Mn²⁺ is the acceptor ion [35]).

 

Fig. 8 (a) Decay curves of Ce³⁺ and Mn²⁺ (inset) in SLPO: 0.12Ce³⁺, zMn²⁺ (z = 0, 0.2, 0.3, and 0.4). (b)-(d) Dependence of I_s0/I_s of $C_{Ce+Mn}^{n/3}$on the exponent (b) n = 6, (c) n = 8 and (d) n = 10. The red lines indicate the fitting behaviors.

Download Full Size | PDF

3.4 CIE chromaticity diagram, external quantum yield, and thermal stability

The chromaticity coordinates of SLPO: 0.12Ce, yTb³⁺ (0≤y≤0.15) and SLPO: 0.12Ce, zMn²⁺ (0≤z≤0.4) phosphors were calculated based on the corresponding PL spectra with 310 nm excitation, which are represented in Table 2, and the corresponding CIE chromaticity diagram is presented in Fig. 9. With the increasing y or z value, the CIE values of SLPO: 0.12Ce³⁺, yTb³⁺ and SLPO: 0.12Ce³⁺, zMn²⁺ can be tuned from greenish blue (0.2310, 0.2742) to green (0.2906, 0.3705) region (arrow A, point 1 to 6) and from greenish blue to warm white (0.3488, 0.336) (arrow B, point 1, 7, 8, 9, 10) region, respectively. White light emission can be achieved by co-doping Ce³⁺ and Mn²⁺ ions in SLPO under the excitation of 310 nm UV-light, with the composition of SLPO: 0.12Ce³⁺, 0.3Mn²⁺ or 0.35Mn²⁺ and CIE value of (0.3326, 0.3298) or (0.3417, 0.3336) as shown in Fig. 9 (point 8, 9). The external quantum yields of the samples excited at 310nm were also measured and listed in Table 2. It can be noted that the performances on quantum efficiencies are not so good and further studies are necessary to improve such properties.

Table 2. External quantum yields and CIE chromaticity coordinates (x, y) of SLPO: 0.12Ce³⁺, yTb³⁺ and SLPO: 0.12Ce³⁺, zMn²⁺ phosphors excited at 310nm.

View Table | View all tables in this article

 

Fig. 9 CIE chromaticity diagram for SLPO: 0.12Ce³⁺, yTb³⁺ (0≤y≤0.15, Point 1-6) and SLPO: 0.12Ce³⁺, zMn²⁺ (0≤z≤0.4, Point 1, 7, 8, 9, 10).

Download Full Size | PDF

Thermal quenching performance of phosphors is an important index for LED application. Generally, the emission intensity gradually decreases as the probability of non-radioactive transition increases when the phosphor is heated. Figure 10(a) and 10(b) show the temperature-dependent emission spectra of the SLPO: 0.12Ce³⁺, 0.1Tb³⁺ and SLPO: 0.12Ce³⁺, 0.3Mn²⁺ samples, respectively. The emission intensity decreases as the temperature increases from 25 to 200 °C. It can be noticed that both the emission intensities of Tb³⁺ and Mn²⁺ decrease more slowly than those of Ce³⁺ when heated. Besides, the Ce³⁺ emission shows a blue-shift, like the thermal property of SLPO: 0.12Ce³⁺ phosphor, which should be ascribed to thermally-active phonon-assisted tunneling from the lower-level excited states to the higher-level excited states of Ce³⁺ [22, 43].

 

Fig. 10 Temperature-dependent emission spectra of SLPO: 0.12Ce³⁺, 0.1Tb³⁺ (a) and 0.12Ce³⁺, 0.3Mn²⁺ (b); Insets: Relative PL intensity of Ce³⁺, Tb³⁺, and Mn²⁺ emission via temperature.

Download Full Size | PDF

Setting the emission intensities at 25 °C as the normalized standard values, the relative PL emission intensities of Ce³⁺, Tb³⁺ and Mn²⁺ via temperature in SLPO: 0.12Ce³⁺, 0.1Tb³⁺ and SLPO: 0.12Ce³⁺, 0.3Mn²⁺ phosphors are presented in the insets of Fig. 10(a) and Fig. 10(b), respectively. The values are calculated according to the peak emission intensities of the Ce³⁺, Tb³⁺, and Mn²⁺ ions [29, 31, 44]. The PL intensities of the Ce³⁺ and Tb³⁺ emission centers at 150 °C of SLPO: 0.12Ce³⁺, 0.1Tb³⁺ decrease to only 40% and 49% of the initial intensities, respectively. For the SLPO: 0.12Ce³⁺, 0.3Mn²⁺ phosphor, the values become 39% and 79%. It has been reported that the Ce³⁺ doped SLPO phosphor had a poor thermal stability, but the Tb³⁺ singly doped sample had a relatively good performance [22]. The intensity of the Tb³⁺ emission center in the SLPO: 0.07Tb³⁺ phosphor at 150 °C decreased to 79% of the initial intensity, which was more stable than that in the SLPO: 0.12Ce³⁺, 0.1Tb³⁺ sample. This phenomenon is most probably caused by the wide range overlap between the PL spectra of Ce³⁺ and Tb³⁺ ions in SLPO, so the emission intensity at 539 nm depends on both Tb³⁺ and Ce³⁺ emission centers. It’s worth noting that different thermal quenching rates of Ce³⁺ and Mn²⁺ ions in SLPO might cause unstable color output. Under 310nm excitation, the color coordinates of the SLPO: 0.12Ce³⁺, 0.3Mn²⁺ phosphor shifts from (0.3326, 0.3298) in 25 °C to (0.4204, 0.3514) in 150 °C. Therefore, SLPO: 0.12Ce³⁺, 0.3Mn²⁺ phosphor is not applicable for some highly-stable color-output lighting fields. Fortunately, the white emission will gradually turn into a “warm white light” region as the thermal quenching rate of Mn²⁺ is lower than that of Ce³⁺ ions in SLPO. Thus the use in CRI-tunable wLED devices with temperatures might be explored [45].

4. Conclusion

A series of SLPO: 0.12Ce³⁺, yTb³⁺ and SLPO: 0.12Ce³⁺, zMn²⁺ phosphors have been synthesized by the traditional solid state reaction. The ETs from Ce³⁺ to Tb³⁺ and from Ce³⁺ to Mn²⁺ were studied in detail. The mechanisms of the two ETs were predominated by dipole-dipole interaction. Varied emitting colors from greenish blue to green or warm white can be achieved by proper tuning of the concentration of Tb³⁺ or Mn²⁺ ions. White light emission with CIE value of (0.3326, 0.3298) can be achieved in SLPO: 0.12Ce³⁺, 0.3Mn²⁺ phosphor under the excitation of 310nm UV-light. With the development of semiconductor technology, the chips for fabricating with phosphors in LED can be produced with different emitting wavelengths, which indicates that the SLPO: Ce³⁺/Tb³⁺/Mn²⁺ is a promising single-composition phosphor for application involving white LEDs.