A single-phase white light emitting phosphor Ba₃Y(PO₄)₃:Ce³⁺/Tb³⁺/Mn²⁺: luminescence, energy transfer and thermal stability

Abstract

A series of Ce³⁺/Tb³⁺, Tb³⁺/Mn²⁺ and Ce³⁺/Tb³⁺/Mn²⁺ doped Ba₃Y(PO₄)₃ were synthesized by the high temperature solid state method. Phase formation, energy transfer, luminescence properties and thermal quenching properties of phosphors were analyzed in detail. For the co-doped samples, the energy transfer from Ce³⁺ to Tb³⁺ and Tb³⁺ to Mn²⁺ was proved by analyzing the spectra and fluorescence lifetime, and the energy transfer mechanism was calculated to be dipole–dipole interaction. A series of color-tunable phosphors were obtained by the energy transfer from Ce³⁺ to Tb³⁺ and Tb³⁺ to Mn²⁺. For the tri-doped samples, it was confirmed that the energy transfers from Ce³⁺ to Tb³⁺, Tb³⁺ to Mn²⁺ and Ce³⁺ to Mn²⁺ exist at the same time by analyzing the spectra properties, and it can emit warm-white light with extensive color temperature regulability. In addition, the thermal stability was abnormal and outstanding because the defects exist in the samples. The results show that the phosphors may be novel warm white emitting phosphors for white light emitting diodes.

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1 Introduction

In recent years, white-light-emitting diodes (LEDs) have received more and more attention for their many advantages¹ Compared with traditional light sources, white LEDs have higher efficiency, higher brightness, longer service time and are more environmentally friendly, hence they are considered as the fourth generation light source.^1–7 The most common method to synthesize white LEDs is to combine the GaN blue LED chip with yellow phosphor YAG:Ce³⁺. However, this type of white LED has many drawbacks due to lack of a red ingredient, such as the poor color rendering index and the high correlated color temperature.^3,4,8 Because white LEDs can not only be used in lighting field, but also in display and medical fields, hence they should show specific optical performance in different application scenarios which means that phosphors with high tunability are needed.^7,9 Nowadays, the ultraviolet-excited single-phased phosphors have been taken seriously, which can achieve white light with good color rendering index, suitable correlated color temperature and tunable emission by adjusting the doping ratio of different ions.^10–13 Therefore, it is urgent to find novel and high performance single-phased phosphors. Generally, when doping rare earth ions or transition metal ions into compound, the white emitting phosphor may be achieved by the energy transfer between different activator ions,¹⁴ such as Ca₁₀Na(PO₄)₇:Ce³⁺/Tb³⁺/Mn²⁺,¹⁵ Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺/Eu²⁺/Tb³⁺/Mn²⁺.¹⁶ Among many host compounds, the eulytite-type orthophosphates with the type of M₃^IM^II(PO₄)₃ (M^I = Ba, Sr, Ca, and Pb, M^II = La, Sc, Y, Bi, and In) have attracted great interests, for example, when doped Ce³⁺, Tb³⁺, and Mn²⁺ in Ba₃Y(PO₄)₃ (BYP), the phosphors can produce blue, green and red light, respectively.^17–19 Thus, it is possible to achieve white emission when the above three ions are tri-doped in BYP. Moreover, the thermal stability of phosphor is an essential characteristic. It has been reported in our previous work that BYP:Ce³⁺,Mn²⁺ shows extraordinary thermal stability due to the existence of defect in the samples.¹⁷ It is expectable that the defect is to be utilized to gain a series of phosphors with high temperature stability in Ce³⁺/Tb³⁺/Mn²⁺ doped BYP. In this work, series of Ce³⁺/Tb³⁺, Tb³⁺/Mn²⁺ or Ce³⁺/Tb³⁺/Mn²⁺ doped BYP are obtained. Phase formation, energy transfer, luminescence properties and thermal property were investigated in detail. Finally, series of single-phased white emitting phosphors are obtained by the energy transfer Ce³⁺ → Tb³⁺, Tb³⁺ → Mn²⁺ and Ce³⁺ → Mn²⁺.

2 Experimental

2.1 Sample preparation

Series of Ba₃Y_0.95−x(PO₄)₃:0.05Ce³⁺,xTb³⁺, Ba_3−yY_0.95(PO₄)₃:0.05Ce³⁺,yMn²⁺ and Ba_2.86Y_0.7−z(PO₄)₃:zCe³⁺,0.3Tb³⁺,0.14Mn²⁺ were synthesized by a high temperature solid state method. Raw materials BaCO₃ (analytical reagents, A.R.), NH₄H₂(PO₄)₃ (A.R.), Y₂O₃ (99.99%), CeO₂ (99.99%), Tb₄O₇ (99.99%) and MnCO₃ (A.R.) were weighed according to stoichiometric proportions, and then mixed them in an agate mortar pestling for 20 min to ensure the materials are distributed uniformly. The mixed materials were put into crucibles and heated at 1300 °C for 3 h in a carbon reducing atmosphere. After firing, the samples cooled to room temperature slowly and were grounded into powder for subsequent use.

2.2 Materials characterization

XRD patterns of sample are measured by powder X-ray diffraction (XRD) using a D8-A25 Focus diffractometer at 40 kV and 40 mA with step size of 0.05° and a scan speed 0.1 s per step. Spectral property of sample was measured by Hitachi F-4600 fluorescence spectrophotometer with a 450 W Xe lamp as the excitation source. Decay curves were measured by Horiba FL-4600 fluorescence spectrophotometer with a 320 nm pulse laser radiation (nano-LED) for Ce³⁺ and 450 W Xe lamp as the excitation resource for Mn²⁺ at room temperature. Thermoluminescence spectra of samples were measured by a FJ-427A1 TL dosimeter at room temperature with a fixed heating rate of 1 k s⁻¹ within the range 300–450 K. Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of samples were obtained by the PMS-80 spectral analysis system.

3 Results and discussion

3.1 Phase formation

The XRD patterns of BYP:0.05Ce³⁺,xTb³⁺ (x = 0.02, 0.45), BYP:0.05Ce³⁺,yMn²⁺ (y = 0.08, 0.14) and BYP:zCe³⁺,0.3Tb³⁺,0.14Mn²⁺ (z = 0.07, 0.15) were shown in Fig. 1. It can be seen that no apparent impurity phase appears and all the diffraction peaks of samples can be indexed to the standard card of BYP (JCPDS No. 44-0318) appropriately. The XRD patterns illustrates that Ce³⁺, Tb³⁺ and Mn²⁺ did not cause any significant change to the crystal structure of BYP and series of pure phase phosphors were obtained.

Fig. 1 XRD patterns of the representative samples and the standard patterns of Ba₃Y(PO₄)₃ (JCPDS No. 44-0318).

3.2 Luminescence and energy transfer properties of BYP:Ce³⁺,xTb³⁺

Fig. 2a–c display the emission and excitation spectra of BYP:0.05Ce³⁺ (λ_ex = 330 nm, λ_em = 400 nm), BYP:0.45Tb³⁺ (λ_ex = 377 nm, λ_em = 550 nm) and BYP:0.05Ce³⁺,0.15Tb³⁺ (λ_ex = 330 nm, λ_em = 400 nm, 550 nm), respectively. The emission spectrum of BYP:0.05Ce³⁺ shows a blue emission band peaking at 400 nm, which originates from the electron transition 5d → 4f of Ce³⁺. The excitation band of BYP:0.05Ce³⁺ ranges from 200 nm to 360 nm (λ_max = 330 nm) due to the 4f → 5d electron transition. For BYP:0.45Tb³⁺, a series of characteristic emission peaks of Tb³⁺ appear in the emission spectrum which originates from the electron transition ⁵D₄ → ⁷F₆ (500 nm), ⁵D₄ → ⁷F₅ (550 nm), ⁵D₄ → ⁷F₄ (590 nm) and ⁵D₄ → ⁷F₃ (630 nm) of Tb³⁺, respectively. The excitation spectrum of BYP:0.45Tb³⁺ shows a broad excitation band from 210 nm to 400 nm because of the allowed 4f⁸–4f⁷5d transition and intra-configuration transition of Tb³⁺. Comparing the emission spectrum of BYP:0.05Ce³⁺ with the excitation spectrum of BYP:0.45Tb³⁺, an obvious spectral overlapping can be observed which means an effective energy transfer from Ce³⁺ to Tb³⁺ (ET_Ce–Tb) may be occur.^19,20 In addition, the emission spectrum of BYP:0.05Ce³⁺,0.15Tb³⁺ contains both the emission band of Ce³⁺ and Tb³⁺, the excitation spectrum monitored at 550 nm (emission peak of Tb³⁺) has the similar profile with that monitored at 400 nm (emission peak of Ce³⁺), it also means that there may be energy transfer from Ce³⁺ to Tb³⁺.

Fig. 2 Emission and excitation spectra of (a) BYP:0.05Ce³⁺; (b) BYP:0.45Tb³⁺; (c) BYP:0.05Ce³⁺,0.15Tb³⁺.

In order to further investigate the luminescence properties, a series of BYP:Ce³⁺,xTb³⁺ were synthesized. Fig. 3a shows the emission spectra of BYP:0.05Ce³⁺,xTb³⁺ (x = 0–0.45) under 330 nm UV excitation. Fig. 3b reveals the emission intensity of Ce³⁺ (λ_em = 400 nm) and Tb³⁺ (λ_em = 550 nm), it can be seen that the emission intensity of Ce³⁺ declines constantly as the concentration of Tb³⁺ increasing, meanwhile, the emission intensity of Tb³⁺ goes up ceaselessly. According to the change of emission intensity, it means that an effective energy transfer from Ce³⁺ to Tb³⁺ is expectable.

Fig. 3 (a) Emission spectra of BYP:0.05Ce³⁺,xTb³⁺ (x = 0–0.45). (b) Emission intensity of Ce³⁺ and Tb³⁺ as a function of Tb³⁺ concentration.

To confirm the energy transfer from Ce³⁺ to Tb³⁺, the decay curves of Ce³⁺ monitored at 400 nm under 320 nm pulse laser radiation (nano-LED) excitation and decay curves of Tb³⁺ monitored at 550 nm under 377 nm excitation for BYP:0.05Ce³⁺,xTb³⁺ are shown in Fig. 4a and b, respectively. The decay curves of Ce³⁺ and Eu²⁺ can be matched well with second-order exponential function, which can be expressed as the following expression:^21,22

I_(t) = A₁ exp(t/τ₁) + A₂ exp(t/τ₂) (1)

where I_(t) is the luminescence intensity, A₁ and A₂ are constants, t is time, τ₁ and τ₂ are the lifetimes for the exponential components. The average lifetime τ* can be calculated by the following formula^21,23

τ* =(A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (2)

Fig. 4 (a) Decay curves of Ce³⁺ in BYP:0.05Ce³⁺,xTb³⁺ (λ_em = 400 nm, λ_ex = 320 nm). (b) Decay curves of Tb³⁺ in BYP:0.05Ce³⁺,xTb³⁺ (λ_em = 550 nm, λ_ex = 377 nm). (c) Lifetime of Ce³⁺ and Tb³⁺ as function of Tb³⁺ concentration. (d) Energy transfer efficiency from Ce³⁺ to Tb³⁺ as function of Tb³⁺ concentration.

The calculated average lifetime of Ce³⁺ and Tb³⁺ are shown in Fig. 4c, from which one can see that the lifetime of Ce³⁺ decreases consistently and that of Tb³⁺ increases consistently with increasing Tb³⁺ concentration. The lifetime can strongly prove the energy transfer from Ce³⁺ to Tb³⁺, and the energy transfer efficiency can be calculated by the following equation^9,22

η_T = 1 − τ/τ₀ (3)

where τ and τ₀ are the decay time of the sensitizer Ce³⁺ in the presence and absence of activator Tb³⁺, respectively. Fig. 4d shows that the energy transfer efficiency rises up constantly with increase Tb³⁺ concentration, and the maximum value is 69.9% when the doping concentration of Tb³⁺ reaches the maximum x = 0.45.

According to Dexter's theory, there are two types of interactions between sensitizers and activators: multipolar interaction and exchange interaction.^21,24 The critical distance between Ce³⁺ and Tb³⁺ (R_Ce–Tb) is a critical factor to judge which type it belongs to, and the critical distance R_Ce–Tb can be calculated by the following formula²⁵

where N is the number of the cations in host, V is the volume of the unit cell. χ_c is the sum of Ce³⁺ and Tb³⁺ concentration when the emission intensity of Ce³⁺ in the presence of Tb³⁺ is half that in the absence of Tb³⁺. Herein, V equals to 1165.61 ų and N equals to 4, χ_c is calculated to be 0.2. Thus, the R_Ce–Tb is calculated to be 17.72 Å which is larger than the maximum distance of 5 Å for the exchange interaction, it means the energy transfer mechanism of Ce³⁺–Tb³⁺ belongs to multipolar interaction.

There are three kinds of multipolar interactions: dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interaction. The mechanism of ET_Ce–Tb can be discussed by Dexter's theory:^24,26

where I_s0 and I_s are the emission intensity of sensitizer Ce³⁺ in the absence and presence of the activator Tb³⁺, respectively. The sum concentration of Ce³⁺ and Tb³⁺ is represented by C. Herein, n = 6, 8 and 10 correspond to dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively.^24,26,27 The linear relationship between I_s0/I_s and C^n/3 for BYP:0.05Ce³⁺,xTb³⁺ are illustrated in Fig. 5, it can be seen that a well fitted curve can be gotten when n = 6 (R² = 0.94952), hence it can be concluded that the dipole–dipole interaction plays an essential role in the energy transfer from Ce³⁺ to Tb³⁺.

Fig. 5 Dependence of I_s0/I_s of Ce³⁺ and Tb³⁺ on C^6/3, C^8/3 and C^10/3 in BYP:0.05Ce³⁺,xTb³⁺.

The Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of BYP:0.05Ce³⁺,xTb³⁺ (x = 0–0.45) was calculated basing on the relevant emission spectra under 330 nm excitation. The results along with luminescent photographs of related phosphors are depicted in Fig. 6, the emission color of BYP:0.05Ce³⁺,xTb³⁺ can be modulated from blue (0.1622, 0.0406) to yellow-green (0.2946, 0.4169). Hence, series of color-tunable phosphors were synthesized successfully.

Fig. 6 CIE chromaticity coordinates of BYP:0.05Ce³⁺,xTb³⁺ (x = 0–0.45) along with their luminescent photographs.

To analyze the thermal stability of BYP:0.05Ce³⁺,xTb³⁺, the temperature-dependent emission spectra of BYP:0.05Ce³⁺,0.45Tb³⁺ excited by 330 nm were measured which is shown in Fig. 7a, and the emission intensity of Ce³⁺ and Tb³⁺ with increasing the temperature is depicted in Fig. 7b. It can be seen from the pictures that the emission intensity of Ce³⁺ shows a first increases and then decreases with increasing the temperature, but it remains 107.7% of the initial value (300 K) when the temperature reaches 425 K. As for Tb³⁺, its emission intensity increases constantly and reaches to 138.8% of the initial value when the temperature reaches 425 K. The abnormal change of emission intensity of Ce³⁺ and Tb³⁺ with temperature may be caused by the defects in BYP:0.05Ce³⁺,0.45Tb³⁺.^28–30Fig. 8a shows the thermoluminescence spectrum of BYP:0.05Ce³⁺,0.45Tb³⁺, it can be seen that there are two emission bands ranging from 400 K to 420 K and from 460 K to 480 K, respectively. The thermoluminescence spectrum confirms the existence of defects. Fig. 8b depicts how defects affect the thermal stability of BYP:0.05Ce³⁺,xTb³⁺: the shallow trap D1 and deep trap D2 can store the electrons, but the electrons transmit to the conduction band when BYP:0.05Ce³⁺,xTb³⁺ are heated. After that, a part of electrons transmit to ⁵D₄ energy level of Tb³⁺ and 5d energy band of Ce³⁺ from conduction band, then more electrons transmit from ⁵D₄ to ⁷F_J energy level and 5d to 4f energy band by radiative transition meaning more energy would be released. Thus, the emission intensity of Ce³⁺ and Tb³⁺ rise up with increasing temperature.²⁹

Fig. 7 (a) Temperature-dependent emission spectra of BYP:0.05Ce³⁺,xTb³⁺ excited by 330 nm. (b) Relative intensity of Ce³⁺ and Tb³⁺ in the temperature-dependent emission spectra.

Fig. 8 (a) Thermoluminescence spectrum of BYP:0.05Ce³⁺,0.45Tb³⁺. (b) Schematic diagram of the interaction between the defect and temperature-dependent emission intensity in BYP:0.05Ce³⁺,0.45Tb³⁺.

3.3 Luminescence and energy transfer of BYP:0.4Tb³⁺,yMn²⁺

The emission spectra of BYP:0.4Tb³⁺,yMn²⁺ excited by 377 nm UV-light are shown in Fig. 9a and the relative emission intensities at 550 nm and 600 nm are depicted in Fig. 9b. The emission intensity of Tb³⁺ (550 nm) decreases constantly with increasing Mn²⁺ concentration, it means that Mn²⁺ may absorb energy from Tb³⁺. It is worth noting that the emission peaks of Tb³⁺ and Mn²⁺ coexist at 600 nm, as increasing Mn²⁺ concentration the source of the peak at 600 nm change from Tb³⁺ (⁵D₄–⁷F₄) to both Mn²⁺ (⁴T₁–⁶A₁) and Tb³⁺ and Mn²⁺ is a main source. The intensity of the peak at 600 nm shows a first decrease, then increase and decrease in the end. The possible energy transfer maybe the main reason for the intensity changing: as increasing Mn²⁺ concentration the energy transfer from Tb³⁺ to Mn²⁺ started to work, thus the emission intensity shows a first decrease; after that, the concentration of Mn²⁺ is enough, hence the source of the emission peak mainly originates from Mn²⁺ and the emission intensity of Tb³⁺ is too low, thus the emission intensity of the peak at 600 nm started to pick up. In the end, the concentration of Mn²⁺ reaches the quenching point, so its emission intensity started to decrease again.

Fig. 9 (a) Emission spectra of BYP:0.4Tb³⁺,yMn²⁺ (y = 0–0.12). (b) Relative emission intensity at 550 nm and 600 nm.

To confirm the energy transfer from Tb³⁺ to Mn²⁺ in BYP:0.4Tb³⁺,yMn²⁺, the decay curves of Tb³⁺ (λ_em = 550 nm, λ_ex = 377 nm) and monitored at 600 nm (including Tb³⁺ and Mn²⁺, λ_ex = 377 nm) are shown in Fig. 10a and b, respectively. The decay curves can be matched well with second-order exponential function expression (2), by means of expression (2) the average lifetimes can be calculated and the results are shown in Fig. 10c. It can be seen that the lifetime of Tb³⁺ decreases constantly as increasing Mn²⁺ concentration, this can prove the energy transfer from Tb³⁺ to Mn²⁺ (ET_Ce–Tb). In addition, the lifetime monitored at 600 nm shows a first decrease when y = 0 rises to y = 0.02, then increase when y = 0.02 rises to y = 0.08. This phenomenon is due to the low concentration of Mn²⁺ at the beginning, the measured and calculated lifetimes are mainly originated from Tb³⁺, hence the lifetime decrease. When increased Mn²⁺ concentration, it passed into the lifetime of Mn²⁺, thus the lifetime starts to increase.

Fig. 10 (a) Decay curves of Tb³⁺ in BYP:0.4Tb³⁺,yMn²⁺ (λ_em = 550 nm, λ_ex = 377 nm). (b) Decay curves of Tb³⁺ and Mn²⁺ in BYP:0.4Tb³⁺,yMn²⁺ (λ_em = 600 nm, λ_ex = 377 nm). (c) Lifetime of Tb³⁺ and Mn²⁺ as function of Mn²⁺ concentration. (d) Energy transfer efficiency from Tb³⁺ to Mn²⁺ as function of Mn²⁺ concentration.

The energy transfer efficiency of ET_Tb–Mn had been calculated by formula (3) and the results are shown in Fig. 10d. It can be seen that the energy transfer efficiency increases as Mn²⁺ concentration increasing and the maximum value is 52.3% when y = 0.12. The critical distance from Tb³⁺ to Mn²⁺ (Rc_Tb–Mn) can be calculated by expression (4), for BYP:0.4Tb³⁺,yMn²⁺, χ_c is calculated to be 0.42 and the Rc_Tb–Mn is calculated to be 13.84 Å which is obviously larger than the maximum distance of 5 Å for the exchange interaction. Thus it can be confirmed that the type of interaction between Tb³⁺ and Mn²⁺ is multipolar interaction. The linear relationship between I₀/I and C^n/3 for BYP:0.4Tb³⁺,yMn²⁺ are illustrated in Fig. 11a, and a well fitted curve can be gotten when n = 10 and R² = 0.97838, therefore, it can be confirmed that the ET_Tb–Mn mechanism is quadrupole–quadrupole interaction.

Fig. 11 Dependence of I_s0/I_s of Ce³⁺ and Tb³⁺ on C^6/3, C^8/3 and C^10/3 in BYP:0.4Tb³⁺,yMn²⁺yMn²⁺.

The Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of BYP:0.4Tb³⁺,yMn²⁺ (y = 0.02–0.12) was calculated basing on the relevant emission spectra under 377 nm excitation. The calculated results along with luminescent photographs of related phosphors are depicted in Fig. 12, the emission color of BYP:0.4Tb³⁺,yMn²⁺ can be modulated from yellow-greenish (0.3329, 0.5275) to white (0.3936, 0.3562). Thus, BYP:0.4Tb³⁺,yMn²⁺ can act as a color-tunable material which can be used in near UV based white LEDs.

Fig. 12 CIE chromaticity coordinates of BYP:0.4Tb³⁺,yMn²⁺ (y = 0.02–0.12) along with their luminescent photographs.

The temperature-dependent emission spectra of BYP:0.4Tb³⁺,0.12Mn²⁺ excited by 377 nm was measured and shown in Fig. 13a, and the emission intensity of Tb³⁺ and Mn²⁺ is presented in Fig. 13b. It can be seen from Fig. 13a that the emission peak of Mn²⁺ shifts from 600 nm to 588 nm due to the thermally active phonon-assisted excitation from lower energy levels to higher energy levels in the excited state of Mn²⁺.²⁴ Because of the thermal quenching, the emission intensity of Mn²⁺ decreases continually with the increase of temperature and remains 80.5% of the initial value (300 K) when the temperature reaches 425 K. However, the emission intensity of Tb³⁺ increases continually, and when the temperature reaches 425 K the emission intensity rises to 111.6% of the initial value.

Fig. 13 (a) Temperature-dependent emission spectra of BYP:0.4Tb³⁺,0.12Mn²⁺ excited by 377 nm. (b) Relative intensity of Tb³⁺ and Mn²⁺ in the temperature-dependent emission spectra.

The abnormal change of emission intensity of Tb³⁺ is also caused by the defects in BYP:0.4Tb³⁺,0.12Mn²⁺. Fig. 14a depicts the thermoluminescence spectrum of BYP:0.4Tb³⁺,0.12Mn²⁺, as mentioned in Chapter 3.2: the emission intensity change of Ce³⁺ and Tb³⁺ in temperature-dependent emission spectra of BYP:0.05Ce³⁺,0.45Tb³⁺. Fig. 14b is the schematic diagram on how the defects affect the emission intensity of Tb³⁺ in BYP:0.4Tb³⁺,0.12Mn²⁺.

Fig. 14 (a) Thermoluminescence spectrum of BYP:0.4Tb³⁺,0.12Mn²⁺. (b) Schematic diagram of the interaction between the defect and temperature-dependent emission intensity in BYP:0.4Tb³⁺,0.12Mn²⁺.

3.4 Luminescence and energy transfer of BYP:zCe³⁺,0.4Tb³⁺,0.12Mn²⁺

Fig. 15a shows the emission spectra of BYP:0.15Ce³⁺,0.4Tb³⁺,0.12Mn²⁺ excited by 330 nm, it can be seen that the emission peaks of Ce³⁺ (390 nm), Tb³⁺ (550 nm) and Mn²⁺ (600 nm) coexist in the emission spectra covering blue, yellow, green and red region, thus, the efficient white emission is expectable. Fig. 15b depicts the excitation spectra of BYP:0.15Ce³⁺,0.4Tb³⁺,0.12Mn²⁺ monitored at 390 nm, 550 nm and 600 nm, from which one can see that the obtained three spectra have the similar profile, it means that there is energy transfer among Ce³⁺, Tb³⁺ and Mn²⁺. To further investigate the luminescence and energy transfer, the emission spectra of BYP:zCe³⁺,0.4Tb³⁺,0.12Mn²⁺ under 330 nm excitation are shown in Fig. 15c, the relative intensity of Ce³⁺, Tb³⁺ and Mn²⁺ is depicted in Fig. 15d. It can be seen that the emission intensity of Ce³⁺, Tb³⁺ and Mn²⁺ increase continuously with increasing Ce³⁺ concentration, it indicates that there are ET_Ce–Tb, ET_Ce–Mn and ET_Tb–Mn in BYP:zCe³⁺,0.4Tb³⁺,0.12Mn²⁺.

Fig. 15 (a) Emission spectrum of BYP:0.15Ce³⁺,0.4Tb³⁺,0.12Mn²⁺ (λ_ex = 330 nm). (b) Excitation spectra of BYP:0.15Ce³⁺,0.4Tb³⁺,0.12Mn²⁺ (λ_em = 390 nm, 550 nm and 600 nm). (c) Emission spectra of BYP:zCe³⁺,0.4Tb³⁺,0.12Mn²⁺ (λ_ex = 330 nm). (d) Relative intensity of Ce³⁺, Tb³⁺ and Mn²⁺ in BYP:zCe³⁺,0.4Tb³⁺,0.12Mn²⁺.

Fig. 16 shows the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of BYP:zCe³⁺,0.4Tb³⁺,0.12Mn²⁺ (z = 0.01–0.11) and luminescent photographs of related phosphors when excited by 330 nm UV excitation. The emission color of BYP:zCe³⁺,0.4Tb³⁺,0.12Mn²⁺ can be modulated from white (0.3030 , 0.2893) with high color temperature to white (0.4096 , 0.3770) with low color temperature, viz., a series of single-phased white light-emitting phosphors are realized successfully, and it maybe a potential application in white LEDs.

Fig. 16 CIE chromaticity coordinates of BYP:zCe³⁺,0.4Tb³⁺,0.12Mn²⁺ (z = 0.01–0.11) along with their luminescent photographs.

As a representative, BYP:0.03Ce³⁺,0.4Tb³⁺,0.12Mn²⁺ was chosen to explore the thermal stabilities. Fig. 17a depicts the temperature-dependent emission spectra of BYP:0.03Ce³⁺,0.4Tb³⁺,0.12Mn²⁺ under 330 nm UV excitation, Fig. 17b shows the emission intensity of Ce³⁺, Tb³⁺ and Mn²⁺. The emission peak of Mn²⁺ shifts from 600 nm to 587 nm with increasing the temperature and the reason is the same as that mentioned above, it is because of the thermally active phonon-assisted of Mn²⁺. With rising of temperature, the emission intensity of Mn²⁺ decreases due to thermal quenching effect and it still remains 80.8% of the initial value when the temperature rises to 425 K. The emission intensity of Ce³⁺ shows a first increase and then decrease, the emission intensity of Tb³⁺ increases constantly with increasing temperature. Fig. 17c presents the thermoluminescence spectrum of BYP:0.03Ce³⁺,0.4Tb³⁺,0.12Mn²⁺. The change in emission intensity of Ce³⁺ and Tb³⁺ is ascribed to the same theory as mentioned in chapter 3.2: the emission intensity change of Ce³⁺ and Tb³⁺ in temperature-dependent emission spectra of BYP:0.05Ce³⁺,0.45Tb³⁺. Fig. 17d is the schematic diagram on how the defects affect the emission intensity of Ce³⁺ and Tb³⁺ in BYP:0.03Ce³⁺,0.4Tb³⁺,0.12Mn²⁺.

Fig. 17 (a) Temperature-dependent emission spectra of BYP:0.4Tb³⁺,0.12Mn²⁺ under 330 nm UV excitation. (b) Relative intensity of Ce³⁺, Tb³⁺ and Mn²⁺. (c) Thermoluminescence spectrum of BYP:0.03Ce³⁺,0.4Tb³⁺,0.12Mn²⁺. (d) Schematic diagram of the interaction between the defect and temperature-dependent emission intensity in BYP:0.03Ce³⁺,0.4Tb³⁺,0.12Mn²⁺.

4 Conclusions

In summary, a series of eulytite-typed and single-phased phosphors BYP:0.05Ce³⁺, xTb³⁺, BYP:0.05Ce³⁺, yMn²⁺ and BYP:zCe³⁺,0.3Tb³⁺,0.14Mn²⁺ were synthesized successfully. The energy transfer from Ce³⁺ to Tb³⁺ and Tb³⁺ to Mn²⁺ were confirmed by analyzing its spectra and fluorescence lifetime. The thermal stability of samples is extraordinary, and it had been proved that the unusual temperature performance is caused by the defects in the samples. Finally, a series of single-phased color-tunable with high thermal stability phosphors were obtained, especially BYP:zCe³⁺,0.3Tb³⁺,0.14Mn²⁺, it can emit warm white light and the color temperature can be modulated by the energy among Ce³⁺, Tb³⁺ and Mn²⁺. Thus, it may have application potential in white LEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (Nos. 51672066, 51902080), the Funds for Distinguished Young Scientists of Hebei Province, China (No. A2018201101), and the Natural Science Foundation of Hebei Province, China (No. E2019201223).

References

1. J. W. Qiao, Z. Zhao, Q. L. Liu and Z. G. Xia, Recent advances in solid-state LED phosphors with thermally stable luminescence, J. Rare Earths, 2019, 37, 565–572.
2. C. Li, H. W. Zheng, H. W. Wei, S. J. Qiu, L. Xu, X. M. Wang and H. Jiao, Color tunable and white light emitting Ca₂Si₅N₈:Ce³⁺, Eu²⁺ phosphor via efficient energy transfer for near-UV white LEDs, Dalton Trans., 2018, 47(19), 6860–6867.
3. F. B. Xiong, S. X. Liu, H. F. Lin, X. G. Meng, S. Y. Lian and W. Z. Zhu, A novel white-light-emission phosphor Dy³⁺-doped CaLaB₇O₁₃ under UV excitation, Opt. Laser Technol., 2018, 106, 29–33.
4. M. Zhao, H. Liao, M. S. Molokeev, Y. Zhou, Q. Zhang, Q. Liu and Z. Xia, Emerging ultra-narrow-band cyan-emitting phosphor for white LEDs with enhanced color rendition, Light: Sci. Appl., 2019, 8, 38.
5. X. Y. Huang and H. Guo, LiCa₃MgV₃O₁₂:Sm³⁺: A new high-efficiency white-emitting phosphor, Ceram. Int., 2018, 44, 10340–10344.
6. N. T. K. Chi, N. T. Tuan, N. T. K. Lien and H. Nguyen, Red Emission of SrAl₂O₄:Mn⁴⁺ Phosphor for Warm White Light-Emitting Diodes, J. Electron. Mater., 2018,(10), 1–8.
7. J. X. Hu, T. H. Huang, Y. P. Zhang, B. Lu, H. Q. Ye, B. J. Chen, H. P. Xia and C. Y. Ji, Enhanced deep-red emission from Mn⁴⁺/Mg²⁺ co-doped CaGdAlO₄ phosphors for plant cultivation, Dalton Trans., 2019, 48, 2455–2466.
8. J. T. Zhao, X. Y. Sun and Z. Q. Wang, Ce³⁺/Eu²⁺ doped SrSc₂O₄ phosphors: Synthesis, luminescence and energy transfer from Ce³⁺ to Eu²⁺, Chem. Phys. Lett., 2018, 691, 68–72.
9. Z. Y. Mao, J. J. Chen, J. Li and D. J. Wang, Dual-responsive Sr₂SiO₄:Eu²⁺-Ba₃MgSi₂O₈:Eu²⁺,Mn²⁺ composite phosphor to human eyes and plant chlorophylls applications for general lighting and plant lighting, Chem. Eng. J., 2016, 284, 1003–1007.
10. S. Wang, Y. F. Li, L. G. Feng, L. Z. Zhang, Y. Zhang, X. L. Su, W. Ding and F. Yun, Laser patterning of Y₃Al₅O₁₂:Ce³⁺ ceramic phosphor platelets for enhanced forward light extraction and angular color uniformity of white LEDs, Opt. Express, 2016, 24(15), 17522–17531.
11. Y. Zhang, X. J. Zhang, H. R. Zhang, L. L. Zheng, Y. Zeng, Y. Lin, Y. L. Liu and B. F. Lei, Tunable emission from green to red in the GdSr₂AlO₅:Tb³⁺,Eu³⁺phosphor via efficient energy transfer, RSC Adv., 2018, 8(7), 3530–3535.
12. T. Senden, E. J. V. Harten and A. Meijerink, Synthesis and narrow red luminescence of Cs₂HfF₆:Mn⁴⁺ a new phosphor for warm white LEDs, J. Lumin., 2018, 194, 131–138.
13. H. P. You, Y. H. Song, G. Jia and G. Y. Hong, Energy transfer from Tb³⁺ to Mn²⁺ in LaMgAl₁₁O₁₉:Tb, Mn phosphors, Opt. Mater., 2008, 31(2), 342–345.
14. Z. M. Zhao, Z. G. Xia, X. X. Huang, L. X. Ning, G. Romain, M. S. Molokeev, Y. Y. Zhou, Y. C. Chuang, Q. Y. Zhang, Q. L. Liu and K. R. Poeppelmeier, Li substituent tuning of LED phosphors with enhanced efficiency, tunable photoluminescence, and improved thermal stability, Sci. Adv., 2019, 5, eaav0363.
15. J. R. Sun, P. Huang, Y. F. Liu, L. Wang, C. E. Cui, Q. F. Shi and Y. Tian, Color-tunable Ca₁₀Na(PO₄)₇:Ce³⁺/Tb³⁺/Mn²⁺, phosphor via energy transfer, J. Rare Earths, 2018, 36(6), 567–574.
16. W. Lü, H. W. Xu, M. M. Hao, H. C. Wang and X. J. Kang, Tunable full-color emitting Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺, Eu²⁺, Tb³⁺, Mn²⁺, phosphor for UV white LEDs: Photoluminescence and energy transfer, J. Alloys Compd., 2018, 752, 231–237.
17. Y. Chen, Z. J. Wang, W. G. Ding, X. Li, Q. Bao, J. J. Liu, K. L. Qiu, X. Y. Meng, Z. P. Yang and P. L. Li, A single-phase white light emitting phosphor Ba₃Y(PO₄)₃:Ce³⁺/Eu²⁺/Mn²⁺: Luminescence, energy transfer and thermal stability, J. Lumin., 2019, 210, 322–334.
18. C. H. Huang, T. W. Kuo and T. M. Chen, Thermally stable green Ba₃Y(PO₄)₃:Ce³⁺,Tb³⁺ and red Ca₃YAlO₃(BO₃)₄:Eu³⁺ phosphors for white-light fluorescent lamps, Opt. Express, 2011, 19(S1), A1–A6.
19. N. Guo, C. Z. Jia, J. Li, Y. F. Zhao, R. Z. Oyang and W. Lü, Color Tuning and Energy Transfer in Eu²⁺/Mn²⁺-Doped Ba₃Y(PO₄)₃ Eulytite-Type Orthophosphate Phosphors, RSC Adv., 2015, 5(58), 46517–46524.
20. Y. B. Chen, K. W. Cheah and M. L. Gong, Low thermal quenching and high-efficiency Ce³⁺, Tb³⁺-co-doped Ca₃Sc₂Si₃O₁₂ green phosphor for white light-emitting diodes, J. Lumin., 2011, 131(8), 1589–1593.
21. C. Wang, P. L. Li, Z. J. Wang, Z. P. Yang, Z. L. Li, M. M. Tian, J. G. Cheng and Y. S. Sun, Crystal structure, luminescence properties, energy transfer and thermal properties of a novel color-tunable, white light-emitting phosphor Ca_9-x-yCe(PO₄)₇:xEu²⁺,yMn²⁺, Phys. Chem. Chem. Phys., 2016, 18(41), 28661–28673.
22. S. C. Xu, Z. J. Wang, P. L. Li, T. Li, Q. Y. Bai, J. Sun and Z. P. Yang, White-emitting phosphor Ba₂B₂O₅:Ce³⁺, Tb³⁺, Sm³⁺: luminescence, energy transfer, and thermal stability, J. Am. Ceram. Soc., 2017, 100, 2069–2080.
23. X. Li, P. L. Li, Z. J. Wang, S. M. Liu, Q. Bao, X. Y. Meng and K. L. Qiu, Color-tunable luminescence properties of Bi³⁺ in Ca₅(BO₃)₃F via changing site occupation and energy transfer, Chem. Mater., 2017, 29(20), 8792–8803.
24. X. G. Zhang, J. G. Xu and M. L. Gong, Site-occupancy, luminescent properties and energy transfer of a violet-to-red color-tunable phosphor Ca₁₀Li(PO₄)₇:Ce³⁺, Mn²⁺, J. Lumin., 2017, 183, 348–354.
25. Y. Zhang, X. J. Zhang, H. R. Zhang, L. L. Zheng, Y. Zeng, Y. Lin, Y. L. Liu and B. F. Lei, Tunable emission from green to red in the GdSr₂AlO₅:Tb³⁺, Eu³⁺ phosphor via efficient energy transfer, RSC Adv., 2018, 8, 3530–3535.
26. G. G. Li, Y. Tian, Y. Zhao and J. Lin, Recent progress in luminescence tuning of Ce³⁺ and Eu²⁺-activated phosphors for pc-WLEDs, Chem. Soc. Rev., 2016, 47(1), 8688–8713.
27. F. Xiao, R. X. Yi, H. L. Yuan, G. J. Zang and C. N. Xie, Color tunable emission and energy transfer of Ce³⁺/Dy³⁺ codoped Ba₃La₂(BO₃)₄ phosphor for UV white LEDs, Spectrochim. Acta, Part A, 2018, 202, 352–358.
28. H. J. Guo, Y. T. Wang, G. Li, J. Liu, P. Feng and D. W. Liu, Cyan emissive super-persistent luminescence and thermoluminescence in BaZrSi₃O₉:Eu²⁺, Pr³⁺ phosphors, J. Mater. Chem. C, 2017, 5(11), 2844–2851.
29. Z. L. Li, Z. J. Wang, P. L. Li, J. G. Cheng, M. M. Tian, C. Wang and Z. P. Yang, Improvement of thermally stable and photoluminescence in Sr_0.8Ca_0.2Al₂Si₂O₈:Eu²⁺ by the substitution of Si-Na # Al-Sr and Ca # Sr on structural modifications, Dalton Trans., 2017, 41, 14310–14317.
30. Q. Y. Bai, Z. J. Wang, P. L. Li, S. C. Xu, T. Li, J. G. Cheng and Z. P. Yang, Utilizing Tb³⁺ as the energy transfer bridge to connect Eu³⁺-Zn2GeO4 host: Realization of efficient Eu³⁺ red emission, Mater. Des., 2016, 108, 597–607.

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