A series of tunable emission phosphors of Sm³⁺, Eu³⁺ and Mn²⁺ doped Ba₃Tb(PO₄)₃: luminescence and energy transfer

Abstract

A series of activator Sm³⁺, Eu³⁺, Mn²⁺ ion doped Ba₃Tb(PO₄)₃ phosphors with tunable emitting color were synthesized via the high temperature solid state method. X-ray diffraction, luminescence and fluorescent decay curves were used to characterize the phosphors. The obtained powder crystallizes as a cubic unit cell with the space group Ī43d. Under 377 nm excitation of Tb³⁺, Ba₃Tb(PO₄)₃:Sm³⁺ not only presents ⁵D₄–⁷F_6–3 of Tb³⁺ emission lines but also ⁴G_5/2–⁶H_5/2–9/2 of Sm³⁺ orange emission lines, Ba₃Tb(PO₄)₃:Eu³⁺ contains the emission lines of Tb³⁺ and Eu³⁺ (⁵D₀–⁷F_1–4), and Ba₃Tb(PO₄)₃:Mn²⁺ exhibits the emission lines of Tb³⁺ and ⁴T₁–⁶A₁ orange emission band of Mn²⁺. In addition, the intensities of the red or orange-red emission can be enhanced by tuning the Sm³⁺, Eu³⁺ and Mn²⁺ contents. The intense emission intensities of Sm³⁺, Eu³⁺ and Mn²⁺ ions are attributed to the efficient energy transfer from Tb³⁺ to Sm³⁺, Eu³⁺ and Mn²⁺ ions, respectively, which have been justified through the luminescence spectra and fluorescence decay dynamics. The energy transfer mechanism was demonstrated to be the electric dipole–dipole interaction. For Ba₃Tb(PO₄)₃:Sm³⁺, Ba₃Tb(PO₄)₃:Eu³⁺ and Ba₃Tb(PO₄)₃:Mn²⁺, the best quantum efficiencies are 41.6%, 70.3% and 49.8%, respectively. The properties of the phosphors indicate that they may have potential application in UV-pumped white light emitting diodes.

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1. Introduction

In recent years, the most common commercial white light emitting diodes (white LEDs) fabricated with a blue chip and the yellow phosphor YAG:Ce³⁺ are un-optimized for indoor use, having an emission spectrum deficient in the red region with the result of a high correlated color temperature (CCT ∼ 7750 K) and low color rendering index (CRI ∼ 70–80).¹ Therefore, as an alternative, a combination of near-ultraviolet (n-UV) LED or ultraviolet (UV) LED with red, green and blue emitting phosphors have been developed to improve the CRI, color stability and to tune the CCT value. However, one of the key factors limiting the progress of the white LEDs is the general lack of red emitting phosphors that can improve the performance of white light blends in terms of color rendering and lumen equivalency.^2,3 Generally, the trivalent europium (Eu³⁺) ion has been recognized as an excellent activator in many red phosphors due to the ⁵D₀–⁷F_{J (J=0,1,2,3,4)} transitions of Eu³⁺.^4–6 For the Eu³⁺-doped phosphors, the excitation lines via the characteristic f–f transitions of Eu³⁺ locate at ultraviolet region. However, in the region of 300–380 nm, Eu³⁺-doped phosphors have a weak absorption, thus, it is necessary to find a sensitizer for Eu³⁺ luminescence.^7–10 In fact, taking account of Sm^{3+ 4}G_5/2–⁶H_J emission in the red region, it is interesting to develop Sm³⁺ activated samples since such phosphors are expected to possess superior red color purity.^11–13 Nevertheless, for Sm³⁺, the low oscillator strength and narrow line width of Sm³⁺ 4f–4f absorption transitions lead to a weak absorption in 300–380 nm UV region.^11–13 Generally, the host-sensitizer sensitization effect has been utilized to sensitize Sm³⁺ emission, where V⁵⁺–O²⁻ charge transfer (CT) transitions are involved.¹⁴ However, for CT emission, the Stokes shift is very large, therefore, this sensitization effect is limited by the low efficiency of charge transfer transitions in UV excitation. Moreover, it is known that emission of Mn²⁺ ion varies from green to red, depending on the influence of crystal field.¹⁵ Owing to the forbidden ⁴T₁–⁶A₁ transition of Mn²⁺ ion, the emission intensity of Mn²⁺ ion singly doped phosphor is low under UV excitation, and its emission intensity can be considerably enhanced by introducing the efficient sensitizer. Eu²⁺ or Ce³⁺ ions with 4f–5d allowed transition, are normally used as sensitizer to enhance the emission intensity of Mn²⁺ due to the high transition efficiency.^16,17

Tb³⁺, a well-known activator, can emit a green color owing to its general ⁵D₄–⁷F₅ transition, and can transfer its energy to another ions.¹⁸ However, the Tb³⁺ ions have a weak absorption in n-UV and visible region, which cannot meet the requirement for the phosphor for white LEDs. Therefore, to overcome this drawback, a host lattice with high concentration of Tb³⁺, without causing serious concentration quenching, is needed. According the above reason, in the present work, we selected the eulytite-type ortho-phosphate, Ba₃Tb(PO₄)₃, as the mother structure, and we aim to develop a novel n-UV convertible phosphor based on the efficient Tb³⁺-activators (Eu³⁺, Sm³⁺, Mn²⁺) energy transfer and have an insight into the related mechanism. Eulytite-type orthophosphates with the general formula M₃^IM^II(PO₄)₃ (M^I = Ca, Sr, Ba and Pb; M^II = La, Y, Sc, Bi, Tb and In) have attracted extensive attention as host materials for lanthanide activators because of their excellent thermal stability and optical property.^19–25 In the eulytite-type orthophosphate structure, RE³⁺ (RE = La, Tb, Eu) ions are isolated by the surrounding PO₄ groups, a lower concentration quenching effect can be expected from this crystal character, making the corresponding compound, Ba₃Tb(PO₄)₃, suitable as the host for the aim of the work.

In the present work, a series of Ln³⁺ (Ln = Eu, Sm) and Mn²⁺ doped Ba₃Tb(PO₄)₃ were synthesized to evaluate the potential for white LEDs application and analyze the Tb³⁺-activators energy transfer, and the luminescence and the color hue tuning and quantum efficiency mechanism were analyzed.

2. Experimental

2.1 Sample preparation

BaCO₃ (Analytical Reagent, A. R.), NH₄H₂PO₄ (A. R.), Tb₄O₇ (99.99%), Sm₂O₃ (99.99%), Eu₂O₃ (99.99%) and MnCO₃ (A. R.) were used as the raw materials. A series of Ba₃Tb(PO₄)₃, Ba₃Eu(PO₄)₃, Ba₃Sm(PO₄)₃, Ba₃Tb_1−x(PO₄)₃:xSm³⁺, Ba₃Tb_1−y(PO₄)₃:yEu³⁺, Ba_3−zTb(PO₄)₃:zMn²⁺ (x, y and z, molar concentration) samples were synthesized by the high temperature solid-state method. The stoichiometric amount of raw materials was thoroughly mixed and ground by an agate mortar and pestle for more than 30 min till they are uniformly distributed. For Ba₃Tb(PO₄)₃, and Sm³⁺ (Eu³⁺) doped samples, the obtained mixtures are heated at 1150 °C for 4 h in an air, however, for Mn²⁺ doped samples, the obtained mixtures are heated at 1150 °C for 4 h in the presence of reducing atmosphere (5% H₂/95% N₂), and then these obtained samples were cooled to room temperature and ground again in an agate mortar.

2.2 Materials characterization

Phase formation of phosphors is carefully checked by powder X-ray diffraction (XRD) analysis (Bruker AXS D8 advanced automatic diffractometer (Bruker Co., German)), with Ni-filtered Cu Kα1 radiation (λ = 0.15405 nm) operating at 40 kV and 40 mA, and a scan rate of 0.02° s⁻¹ is applied to record the patterns in the 2θ range from 10° to 70°. The photoluminescence spectra, luminescence decay curves and quantum efficiency are detected by a FLS920 fluorescence spectrometer, the scanning wavelength range from 200 to 700 nm, a spectral resolution of 0.2 nm, and the exciting sources are a 450 W Xe lamp. Photoluminescence absolute quantum efficiency (QE) was measured by an absolute PL quantum yield measurement system (HORIBA, FL-1057). The diffuse reflection spectra were measured with a Hitachi U4100 UV-VIS-NIR Spectroscopy, scanning at 240 nm min⁻¹. Commission International de I'Eclairage (CIE) chromaticity coordinates of sample are measured by a PMS-80 spectra analysis system. All measurements are carried out at room temperature.

3. Results and discussion

3.1 Phase formation

For Ba₃Tb(PO₄)₃, Ba₃Tb_1−x(PO₄)₃:xSm³⁺, Ba₃Tb_1–y(PO₄)₃:yEu³⁺ and Ba_3–zTb(PO₄)₃:zMn²⁺, the phase formation were identified by XRD patterns, and a similar XRD patterns are observed for each sample. As a representative, Fig. 1 shows the XRD patterns of Ba₃Tb_1−x(PO₄)₃:xSm³⁺ (x = 0–0.15). All the diffraction peaks match well with that of the cubic Ba₃Bi(PO₄)₃ according to the standard reference of JCPDS card no. 33-0137 (ICSD card no. 91803), and no traces of impurity phases are observed. For Ba₃Tb_1−y(PO₄)₃:yEu³⁺ and Ba_3−zTb(PO₄)₃:zMn²⁺, Fig. S1 and S2† depict that there are the similar results. The results indicate the little change of this crystal structure when introduced such Sm³⁺, Eu³⁺ or Mn²⁺ ions into Ba₃Tb(PO₄)₃. According to JCPDS card no. 33-0137 (ICSD card no. 91803), Ba₃Tb(PO₄)₃ should belong to the eulytite-type compounds, which crystallize in the cubic system with space group Ī43d, and the unit cell dimensions are a = b = c = 10.4484(2) Å, α = 90°, Z = 4, volume (V) = 1140.6(2) ų.²⁶ As shown in the upper of Fig. 1, the general feature of Ba₃Tb(PO₄)₃ structure should be regarded as a three-dimensional packing of [PO₄]³⁻ anionic tetrahedra and Tb/Ba octahedra, arranged in a manner to share common apices. It is interesting that all the [PO₄]³⁻ tetrahedral are totally independent while the Tb/Ba octahedra share edges with each other and form a three-dimensional network.²⁶

Fig. 1 XRD patterns of Ba₃Tb_1−x(PO₄)₃:xSm³⁺ (x = 0–0.15). Standard data of Ba₃Bi(PO₄)₃ (JCPDS 33-0137) is shown as reference.

3.2 Luminescence properties

As depicted in Fig. 2, the reflectance spectra of Ba₃Tb(PO₄)₃, Ba₃Tb(PO₄)₃:0.02Sm³⁺, Ba₃Tb(PO₄)₃:0.02Eu³⁺, Ba₃Tb(PO₄)₃:0.03Mn²⁺ phosphors show the similar profiles, which illustrate that the absorptions of phosphors derive from the host. The reflectance spectra present strong energy absorption bands in the region of 200–300 nm. In order to assess the absorption edge from the reflectance spectra, the Kubelka–Munk absorption coefficient (K/S) relationship was availed as follows²⁷

(1 − R)²/2R = K/S (1)

where K refers to the absorption coefficient, R represents the reflectivity, and S is the scattering coefficient. From the reflectance spectrum of Ba₃Tb(PO₄)₃ host, a roughly evaluated optical band gap is 5.06 eV. Moreover, for the Sm³⁺, Eu³⁺ or Mn²⁺ doped Ba₃Tb(PO₄)₃, the band gap is approximately 5.01 eV.

Fig. 2 Diffuse reflection spectra of Ba₃Tb(PO₄)₃ (a), Ba₃Tb(PO₄)₃:0.02Sm³⁺ (b), Ba₃Tb(PO₄)₃:0.02Eu³⁺ (c) and Ba₃Tb(PO₄)₃:0.03Mn²⁺ (d).

In order to explain the possibility of the Tb³⁺–Sm³⁺, Tb³⁺–Eu³⁺ and Tb³⁺–Mn²⁺ energy transfer, the luminescent properties of Ba₃Tb(PO₄)₃ and Ba₃Eu(PO₄)₃ were studied.

Fig. 3a described the emission and excitation spectra of Ba₃Tb(PO₄)₃. Under 377 nm excitation, the spectrum display a series of emission lines ascribed to the intra-4f^{8 5}D₄–⁷F₆₋₃ electronic transitions of Tb³⁺ ions. No emission lines from the ⁵D₃ energy level is observed in the emission spectrum, and the reason can be attributed to high concentration of Tb³⁺ ions, which leads to the occurrence of cross-relaxation. The excitation spectrum of Ba₃Tb(PO₄)₃ monitored with the ⁵D₄–⁷F₅ transition (545 nm) is analyzed in detail. The bands centered at 235 and 250–275 nm are assigned to the spin-allowed (low-spin, LS) and spin-forbidden (high-spin, HS) inter-configurational Tb³⁺ f–d transitions, respectively, and the remaining peaks are assigned to intra-4f⁸ transitions between the ⁷F₆ and the ⁵F_5,4, ⁵H_7–4, ⁵D_1,0, ⁵L_10–7, ⁵G_6–2, and ⁵D_2–4 levels.²² The inset of Fig. 3a is a image of Ba₃Tb(PO₄)₃ under 365 nm excitation, in which an intense green light can be observed.

Fig. 3 Spectral characteristics of Ba₃Tb(PO₄)₃ (a), Ba₃Eu(PO₄)₃ (b). Inset: image of Ba₃Tb(PO₄)₃ (a) and Ba₃Eu(PO₄)₃ (b) (λ_ex = 365 nm).

Fig. 3b presented the emission and excitation spectra of Ba₃Eu(PO₄)₃. Monitored at 615 nm, the excitation spectrum of Ba₃Eu(PO₄)₃ displays a weak broad band ranging from 200 to 280 nm, and some narrow peaks (287, 297, 319, 362, 382, 393, 415, 444 and 465 nm, respectively) resulting from the combined absorptions of host and Eu³⁺–O²⁻ charge transfer (CT) transition, as well as 4f–4f characteristic transitions of Eu³⁺, respectively. Upon the excitation of f–f transition centered at 393 nm, the emission spectrum presents the characteristic emission lines deriving from the 4f–4f transitions of Eu³⁺ ions in Ba₃Eu(PO₄)₃, namely, ⁵D₀ excited states to the ⁷F_J ground states including ⁵D₀–⁷F₁ (596 nm), ⁵D₀–⁷F₂ (615 nm), ⁵D₀–⁷F₃ (656 nm) and ⁵D₀–⁷F₁ (707 nm), respectively.^4–6 And the electric dipole transition ⁵D₀–⁷F₂ around 615 nm is much stronger than that of the other transitions of Eu³⁺. Therefore, the inset of Fig. 3b depicts Ba₃Eu(PO₄)₃ shows an obvious red light.

Generally, Sm³⁺ doped phosphors can exhibit several orange-red emission peaks which are assigned to the ⁴G_5/2 → ⁶H_J/2 (J = 5, 7 and 9) transitions of Sm³⁺. However, there is no emission of Ba₃Sm(PO₄)₃ in our experiment. The reason should be attributed to high concentration of Sm³⁺ ions, which leads to the occurrence of cross-relaxation.²⁸

Actually, Mn²⁺ ions doped eulytite-type orthophosphate can create orange-red emission under UV excitation, for example, our previous results show that Sr₃La(PO₄)₃:Mn²⁺ present the orange-red light under 402 nm excitation, and the emission peak locates at 605 nm, therefore, we think that Mn²⁺ doped eulytite-type orthophosphate Ba₃Tb(PO₄)₃ should emit orange-red or red light.²³

In order to explore the energy transfer from Tb³⁺ to Sm³⁺, Eu³⁺ and Mn²⁺, a series of samples with the chemical composition of Ba₃Tb_1−x(PO₄)₃:xSm³⁺, Ba₃Tb_1−y(PO₄)₃:yEu³⁺ and Ba_3−zTb(PO₄)₃:zMn²⁺ were synthesized and their luminescent properties were investigated.

Fig. 4 shows the emission and excitation spectra, and decay curves of Ba₃Tb_1−x(PO₄)₃:xSm³⁺. Upon excitation of the Tb³⁺ ion f–f transition peak at 377 nm, Fig. 4a depicts Ba₃Tb(PO₄)₃:Sm³⁺ shows the typical Tb³⁺ and Sm³⁺ ion f–f emission lines. Moreover, with the lower Sm³⁺ concentration, there are both Tb³⁺ and Sm³⁺ emission peaks, and the emission intensities of Tb³⁺ decrease with increasing Sm³⁺ concentration, and the inset of Fig. 4a shows the emission intensities of Sm³⁺ have an obvious enhancement. When the Sm³⁺ concentration is higher than x = 0.02, not only the Tb³⁺ emission is very weak, but also the emission intensities of Sm³⁺ begin to decrease, the reason is the occurrence of cross-relaxation. And the results can also be seen from the upper image of phosphors. While the excitation spectrum of Sm³⁺ ions consists of typical Tb³⁺ and Sm³⁺ f–f excitation bands, and the excitation intensities, corresponding to Tb³⁺, directly decrease, however, for the Sm³⁺ excitation intensities (inset of Fig. 4b), there are an optimal value at 0.02 Sm³⁺. For the 545 nm emission of Ba₃Tb_1−x(PO₄)₃:xSm³⁺, Fig. 4c shows the corresponding excitation spectra, which show the excitation spectrum is similar to that of Ba₃Tb(PO₄)₃ sample monitored with the ⁵D₄–⁷F₅ transition, however, the excitation intensities directly decrease with increasing Sm³⁺ concentration. The above results indicate the Tb³⁺–Sm³⁺ energy transfer in Ba₃Tb(PO₄)₃ are valid.

Fig. 4 Emission (a, λ_ex = 377 nm) and excitation (b, λ_em = 603 nm; c, λ_em = 545 nm) spectra of Ba₃Tb_1−x(PO₄)₃:xSm³⁺, decay curves of Ba₃Tb_1−x(PO₄)₃:xSm³⁺ (d, λ_ex = 377 nm, λ_em = 545 nm). Inset: emission spectra of Sm³⁺ (a), excitation spectra of Sm³⁺ (b). Upper: image of Ba₃Tb_1−x(PO₄)₃:xSm³⁺ (λ_ex = 365 nm).

To further validate the process of energy transfer from Tb³⁺ to activators, under the 377 nm excitation of Tb³⁺, monitored at 545 nm emission of Tb³⁺, the decay curves of Ba₃Tb_1−x(PO₄)₃:xSm³⁺ are measured and shown in Fig. 4d. It is known when a donor transfers its energy to an acceptor, the temporal decay of the donor become fast. Fig. 4d shows fluorescence decay curve of Tb³⁺ with different Sm³⁺ concentration. If Tb³⁺ and Sm³⁺ work independently and there is no energy transfer from Tb³⁺ to Sm³⁺, the luminescence lifetime of both activators will be as same as in single doped samples. If there is energy transfer, the decay of Tb³⁺ excitation state will be accelerated by the energy transfer, and consequently the lifetime of Tb³⁺ will be shortened.

Fig. 5 presents the emission and excitation spectra, and decay curves of Ba₃Tb_1−y(PO₄)₃:yEu³⁺. Under 377 nm excitation of Tb³⁺, it can be observed that the emission spectra are composed of red (615 nm), orange (596 nm) and green (545 nm) light-emitting peaks, which agree well with the results of ref. 21. The orange and red emission peaks are attributed to the typical ⁵D₀–⁷F_J (J = 1, 2) transition emission of Eu³⁺. The green emission peaks at 545 nm originate from the ⁵D₄–⁷F₅ transition of Tb³⁺. With increasing Eu³⁺ concentration, through there are both Tb³⁺ and Sm³⁺ emission peaks in the emission spectra, however, the emission intensities of Tb³⁺ straightly decrease, and that of Eu³⁺ directly increase (as shown in the inset of Fig. 5a). Viz., there is no concentration quenching effect in the region of our experiment. And the results can also be seen from the upper image of phosphors. For the 615 nm emission of Eu³⁺, Fig. 5b presents the excitation spectrum of Eu³⁺ ions consists of typical Tb³⁺ and Eu³⁺ f–f excitation bands, in the region of 200–300 nm, which agrees well with the results of ref. 21. Moreover, there is an obvious decrease of Tb³⁺ excitation intensity with increasing Eu³⁺ concentration. Differently, for the 545 nm emission of Ba₃Tb_1−y(PO₄)₃:yEu³⁺, as shown in Fig. 5c, the excitation spectrum presents a similar characteristics to that of Ba₃Tb(PO₄)₃ sample monitored with the ⁵D₄–⁷F₅ transition, however, the excitation intensities directly decrease with increasing Eu³⁺ concentration. Therefore, This can prove an energy transfer from Tb³⁺ to Eu³⁺ in Ba₃Tb(PO₄)₃:Eu³⁺. Moreover, Fig. 5d shows the decay curves of Ba₃Tb_1−y(PO₄)₃:yEu³⁺, and the results show that the lifetimes of Tb³⁺ become shorten with increasing Eu³⁺ concentration. As mentioned above, the results indicate that there should be the energy transfer from Tb³⁺ to Eu³⁺.

Fig. 5 Emission (a, λ_ex = 377 nm) and excitation (b, λ_em = 615 nm; c, λ_em = 545 nm) spectra of Ba₃Tb_1−y(PO₄)₃:yEu³⁺, decay curves of Ba₃Tb_1−y(PO₄)₃:yEu³⁺ (d, λ_ex = 377 nm, λ_em = 545 nm). Inset: emission spectra of Eu³⁺ (a). Upper: image of Ba₃Tb_1−y(PO₄)₃:yEu³⁺ (λ_ex = 365 nm).

Fig. 6 presents the emission and excitation spectra, and decay curves of Ba_3−zTb(PO₄)₃:zMn²⁺. Under 377 nm excitation of Tb³⁺, the emission spectra show the emission characteristics of Tb³⁺ and Mn²⁺, and the emission intensities of Tb³⁺ decrease with increasing Mn²⁺ doping content, however, that of Mn²⁺ reaches a maximum as z equals 0.03 and then begins to decrease due to concentration quenching. And the results can also be seen from the upper image of phosphors. For the Mn²⁺ 596 nm emission, Fig. 6b shows the excitation spectrum presents the excitation characteristics of Tb³⁺, there is no an obvious excitation characteristics of Mn²⁺. Moreover, for the 545 nm emission of Ba_3−zTb(PO₄)₃:zMn²⁺, as shown in Fig. 6c, there are the same excitation peaks between this samples and Ba₃Tb(PO₄)₃, however, the excitation intensities directly decrease with increasing Mn²⁺ concentration. Therefore, this may be a direct evidence to demonstrate the energy transfer Tb³⁺–Mn²⁺ in the Ba₃Tb(PO₄)₃ host. As shown in Fig. 6d, under the 377 nm excitation of Tb³⁺, monitored at 545 nm emission of Tb³⁺, the decay curves of Ba_3−zTb(PO₄)₃:zMn²⁺ presents the decrease trend with increasing Mn²⁺ concentration, the results also prove the energy transfer from Tb³⁺ to Mn²⁺.

Fig. 6 Emission (a, λ_ex = 377 nm) and excitation (b, λ_em = 596 nm; c, λ_em = 545 nm) spectra of Ba_3−zTb(PO₄)₃:zMn²⁺, decay curves of Ba_3−zTb(PO₄)₃:zMn²⁺ (d, λ_ex = 377 nm, λ_em = 545 nm). Upper: image of Ba_3−zTb(PO₄)₃:zMn²⁺ (λ_ex = 365 nm).

Fig. 4–6a depicts the emission spectra for Ba₃Tb_1−x(PO₄)₃:xSm³⁺ (x = 0–0.15), Ba₃Tb_1−y(PO₄)₃:yEu³⁺ (y = 0–0.15) and Ba_3−zTb(PO₄)₃:zMn²⁺ (z = 0–0.20) with different concentration of Sm³⁺, Eu³⁺ and Mn²⁺, respectively. We can observe the emission intensities of Tb³⁺ decrease with increasing Sm³⁺, Eu³⁺ and Mn²⁺ concentrations under 377 nm excitation, and the emission intensities of Sm³⁺ and Mn²⁺ in the Ba₃Tb(PO₄)₃ systems clearly increase to the maximum at x = 0.02 and z = 0.03, respectively, and subsequently descend with respectively further concentration of Sm³⁺ and Mn²⁺ ions resulting from the concentration effects in Fig. 4a and 6a. Differently, the emission intensity of Eu³⁺ keeps an obvious increase with increasing Eu³⁺ concentration in Fig. 5a. In order to illuminate the results, these detailed variations of Ba₃Tb(PO₄)₃ and activators emission intensities are presented in Fig. 7.

Fig. 7 Variation of emission intensity of Tb³⁺ and activators on concentrations in Ba₃Tb_1−x(PO₄)₃:xSm³⁺ (a), Ba₃Tb_1−y(PO₄)₃:yEu³⁺ (b) and Ba_3−zTb(PO₄)₃:zMn²⁺ (c) (λ_ex = 377 nm).

The energy transfer efficiencies (η_T) from the Tb³⁺ to activators in Ba₃Tb_1−x(PO₄)₃:xSm³⁺, Ba₃Tb_1−y(PO₄)₃:yEu³⁺ and Ba_3−zTb(PO₄)₃:zMn²⁺ systems were calculated using the equation^29–31

η_T = 1 − (I/I₀) (2)

where η_T refers to the energy transfer efficiency, I₀ and I are luminescent intensities of sensitizer Tb³⁺ in the absence and presence of activator (Sm³⁺, Eu³⁺ or Mn²⁺). As shown in Fig. 8, the η_T values from Tb³⁺ to activators in Ba₃Tb_1−x(PO₄)₃:xSm³⁺, Ba₃Tb_1−y(PO₄)₃:yEu³⁺ and Ba_3−zTb(PO₄)₃:zMn²⁺ systems are plotted as a function of concentrations of Sm³⁺, Eu³⁺ and Mn²⁺ ions, which show that the η_T values monotonously increase to reach maximums at 98.7%, 95.3% and 91.9% under 377 nm radiation of Tb³⁺, respectively, indicating the energy transfer from the Tb³⁺ to activators becomes more and more efficient with the increasing concentration of activators ions.

Fig. 8 Energy transfer efficiencies (η_T) from Tb³⁺ to activators on concentrations in Ba₃Tb_1−x(PO₄)₃:xSm³⁺ (a), Ba₃Tb_1−y(PO₄)₃:yEu³⁺ (b) and Ba_3−zTb(PO₄)₃:zMn²⁺ (c) (λ_ex = 377 nm).

As mentioned above, the shorten lifetime of Tb³⁺ confirms the energy transfer from Tb³⁺ to activators (Sm³⁺, Eu³⁺ and Mn²⁺) ions. The decay curves for Ba₃Tb(PO₄)₃ can be fitted into a single exponential function as³²

I = I₀ exp(−t/τ) (3)

where I₀ and I are the luminescence intensities at time 0 and t, respectively, and t is decay lifetime. On the basis of eqn (3), the lifetime values were determined to be 2.864 ms. However, as illustrated in Fig. 4–6d, when incorporated of activators (Sm³⁺, Eu³⁺ and Mn²⁺) ions, double-exponential decay behaviour of the activator is observed when the excitation energy is transferred from the Tb³⁺ to activator. Therefore, all of them can be well fitted using a second-order exponential function, and their corresponding decay times can be calculated by the following equation^33–35

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (4)

where I is luminescence intensity; A₁ and A₂ are constants; t is time, and τ₁ and τ₂ are lifetimes for rapid and slow decays, respectively. Average lifetimes (τ*) can be obtained by the formula as follows^33–35

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (5)

For Ba₃Tb_1−x(PO₄)₃:xSm³⁺, Ba₃Tb_1−y(PO₄)₃:yEu³⁺ and Ba_3−zTb(PO₄)₃:zMn²⁺, the calculated average lifetimes (τ*) are listed in Table S1.† The results present that the temporal decay of Tb³⁺ become fast with increasing the activators concentration. In the case of energy transfer, the luminescence lifetime of a sensitizer is shortened, because there are additional decay channels that shorten the lifetime of the excited state. Therefore, the decay lifetime of Tb³⁺ ions can be found to decrease with increase activators ions content, which is strong evidence for the energy transfer from the Tb³⁺ to activators.

3.3 Energy transfer mechanism

In order to determine the energy transfer mechanisms from Tb³⁺ to activators in Ba₃Tb(PO₄)₃:Sm³⁺, Ba₃Tb(PO₄)₃:Eu³⁺ and Ba₃Tb(PO₄)₃:Mn²⁺ samples, it is necessary to know the critical distance (R_c) between activators such as Sm³⁺, Eu³⁺ and Mn²⁺. With the increasing Sm³⁺, Eu³⁺ and Mn²⁺ contents, the energy transfer from Tb³⁺ to activators becomes more efficient, and the probability of energy migration between activators increases simultaneously. When the distance is small enough, the concentration quenching occurs and the energy migration is hindered. The R_c value can be roughly assessed by the calculation pointed out by Blasse³⁶

R_c = 2[3V/(4πX_cN)]^1/3 (6)

where V corresponds to the volume of the unit cell, N is the number of host cations in the unit cell, and X_c is the critical concentration of dopant ions. For the Ba₃Tb(PO₄)₃, N = 4, V = 1140.6 Å and X_c is 0.02, 0.15, 0.03 for Sm³⁺, Eu³⁺ (in our experiment, there is no concentration quenching of Eu³⁺ in Ba₃Tb(PO₄)₃, therefore, we select the maximum value) and Mn²⁺ respectively; accordingly, the R_c are estimated to be about 30.08, 15.37, and 26.28 Å. In general, there are three mechanisms for noradiate energy transfer including exchange interaction, radiation reabsorption and electric multipolar interactions. The R_c obtained above indicate the little possibility of exchange interaction since the exchange interaction is predominant only for about 5 Å.³⁷ The mechanism of radiation reabsorption is only efficacious when the fluorescence and absorption spectra are widely overlapping, which also does not intend to occur in these cases. As a result, we can infer that the electric multipolar interactions would be responsible for the energy transfer mechanisms from the Tb³⁺ to activators. On account of Dexter's energy transfer formula of multipolar interactions and Reisfeld's approximation, the following relationship can be attained^38–41

(η_S0/η) ∝ C^α/3 (7)

where η₀ and η are luminescence quantum efficiency of Tb³⁺ in the absence and presence of activators (Sm³⁺, Eu³⁺ and Mn²⁺), respectively; C is the concentration of Sm³⁺, Eu³⁺ or Mn²⁺. The value for α = 6, 8, 10 corresponds to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. However, the value of η_S0/η is hard to obtain, and thus it can be approximately estimated from the correlated intensity ratio (I_S0/I_S), where I_S0 and I_S stand for the luminescence intensity of Tb³⁺ without and with the Sm³⁺, Eu³⁺ and Mn²⁺ ions, respectively, the following relation can be achieved^38–41

(I_S0/I) ∝ C^α/3 (8)

For Ba₃Tb(PO₄)₃:Sm³⁺, Ba₃Tb(PO₄)₃:Eu³⁺ and Ba₃Tb(PO₄)₃:Mn²⁺, the relationship between I_S0/I and C^α/3 based on the above equation is illustrated in Fig. 9a–j, which can be respectively fitted using straight line. One can find that all the biggest R² values of the linear fittings occur when α = 6 in Fig. 9a–j, corresponding to the best linear behaviours (a, d, h). Therefore, the energy transfers from the Tb³⁺ to Sm³⁺, Eu³⁺ and Mn²⁺ ions take place through the dipole–dipole mechanisms.

Fig. 9 Dependence of I_S0/I_S of Tb³⁺ (a, d, h) C^6/3, (b, e, i) C^8/3, and (c, f, j) C^10/3.

Fig. 10 shows a simple model expressing the energy transfer from Tb³⁺ to Sm³⁺, Eu³⁺ and Mn²⁺ ions and the characteristic emission energy levels of Sm³⁺, Eu³⁺ and Mn²⁺ in Ba₃Tb(PO₄)₃. In Sm³⁺, Eu³⁺ and Mn²⁺ doped Ba₃Tb(PO₄)₃ samples, upon 377 nm radiation, Ba₃Tb(PO₄)₃ absorb UV radiation and then it transfer the energy to Sm³⁺, Eu³⁺ and Mn²⁺ ions, which results in the increases of characteristic emission intensities of activators.

Fig. 10 A simple model expressing the energy transfer from Tb³⁺ to activators (Sm³⁺, Eu³⁺, Mn²⁺).

Table 1 Variation of Quantum Efficiency (QE) and CIE chromaticity coordinate of samples (λ_ex = 377 nm)

Samples	CIE (X, Y)	CCT (K)	QE (%)
Ba3Tb(PO4)3	(0.3196, 0.6009)	5889	51.2
Ba3Eu(PO4)3	(0.6469, 0.3511)	1048	59.2
x = 0.001	(0.3794, 0.5652)	4759	20.7
x = 0.005	(0.3993, 0.5519)	4420	27.6
x = 0.01	(0.4695, 0.4977)	3163	36.9
x = 0.02	(0.4951, 0.4780)	2727	41.6
x = 0.03	(0.5225, 0.4557)	2301	37.1
x = 0.05	(0.5352, 0.4452)	2124	33.8
x = 0.07	(0.5380, 0.4406)	2073	30.6
x = 0.10	(0.5489, 0.4324)	1940	27.3
x = 0.15	(0.5586, 0.4220)	1813	22.6
y = 0.001	(0.3897, 0.5544)	4398	35.7
y = 0.005	(0.4313, 0.5242)	3837	40.3
y = 0.01	(0.4794, 0.4832)	2948	44.3
y = 0.02	(0.5079, 0.4582)	2460	47.9
y = 0.03	(0.5545, 0.4210)	1835	48.6
y = 0.05	(0.5692, 0.4073)	1665	52.1
y = 0.07	(0.6025, 0.3900)	1401	57.9
y = 0.10	(0.6255, 0.3706)	1212	65.6
y = 0.15	(0.6455, 0.3528)	1060	70.3
z = 0.001	(0.3898, 0.5598)	4519	19.6
z = 0.005	(0.4106, 0.5462)	3912	23.8
z = 0.01	(0.4497, 0.5062)	3481	29.5
z = 0.02	(0.4877, 0.4792)	2821	41.3
z = 0.03	(0.5410, 0.4445)	2072	49.8
z = 0.05	(0.5519, 0.4364)	1941	45.6
z = 0.07	(0.5705, 0.4213)	1731	40.1
z = 0.10	(0.5839, 0.4139)	1613	32.3
z = 0.15	(0.5696, 0.4268)	1767	27.9
z = 0.20	(0.5502, 0.4204)	1862	25.3

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3.4 CIE chromaticity coordinate and quantum efficiency

Table 1 depicts the corresponding CIE (Commission Internationale de I'Eclairage 1931 chromaticity) coordinates positions and quantum efficiencies for the as-prepared Ba₃Tb_1−x(PO₄)₃:xSm³⁺ (x = 0–0.15), Ba₃Tb_1−y(PO₄)₃:yEu³⁺ (y = 0–0.15) and Ba_3−zTb(PO₄)₃:zMn²⁺ (z = 0–0.20) samples under 365 nm UV excitation (the emission color of phosphors can be seen from Fig. 4–6). It can be found that the Ba₃Tb_1−x(PO₄)₃:xSm³⁺ can emit green to orange-red light, and their chromaticity coordinate varies from (0.3794, 0.5652) (x = 0.001) to (0.5586, 0.4420) (x = 0.15). The Ba₃Tb_1−y(PO₄)₃:yEu³⁺ can emit green to bright red, whose chromaticity coordinate changes from (0.3897, 0.5544) (y = 0.001) to (0.6455, 0.3528) (y = 0.15). The Ba_3−zTb(PO₄)₃:zMn²⁺ (z = 0–0.20) show the luminescence color varies from green to orange-red, for example, the chromaticity coordinate is from (0.3898, 0.5598) (z = 0.001) to (0.5502, 0.4204) (z = 0.20). Moreover, in all phosphors, the maximum quantum efficiency is 70.3% for Ba₃Tb_0.85(PO₄)₃:0.15Eu³⁺.

4. Conclusions

In summary, a series of Eu³⁺, Sm³⁺ and Mn²⁺ ion doped Ba₃Tb(PO₄)₃ phosphors were synthesized via the conventional high temperature solid state method. Ba₃Tb(PO₄)₃ presents a cubic unit cell with space group Ī43d. Under ⁷F₆–⁵D₃ of Tb³⁺ excitation 377 nm, the emission intensities of Ba₃Tb(PO₄)₃:Sm³⁺, Ba₃Tb(PO₄)₃:Eu³⁺ and Ba₃Tb(PO₄)₃:zMn²⁺ can be adjusted by changing the activators concentrations, and their emission color can also tuned from green to red or orange-red, which are attributed to the efficient energy transfer from Tb³⁺ to Sm³⁺, Eu³⁺ and Mn²⁺, respectively. The energy transfer mechanism was demonstrated to be the electric dipole–dipole interaction, and the best quantum efficiencies are 41.6%, 70.3% and 49.8% for Ba₃Tb(PO₄)₃:Sm³⁺, Ba₃Tb(PO₄)₃:Eu³⁺ and Ba₃Tb(PO₄)₃:Mn²⁺ respectively. The results indicate that the phosphor may serve as potential red or orange-red emitting material for n-UV based white LEDs.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (No. 50902042), the Funds for Distinguished Young Scientists of Hebei Province, China (No. A2015201129), the Natural Science Foundation of Hebei Province, China (Nos A2014201035, E2014201037), the Education Office Research Foundation of Hebei Province, China (Nos ZD2014036, QN2014085), the Midwest Universities Comprehensive Strength Promotion Project.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12619c

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