1. Introduction

White light-emitting diodes (w-LEDs) have attracted much interest because of their attractive advantages in recent years. LED-based white light sources have a longer lifetime, higher energy efficiency, greater reliability, and more environmental friendly characteristics than conventional incandescent and fluorescent lamps [1,2]. For the phosphor-converted LED (pc-LED), phosphors are the key materials used to down-convert near-ultraviolet (NUV)/blue pump light from InGaN LEDs into visible light [3]. At present, the most convenient way to achieve white light is coupling yellow emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor with blue LED chips [4]. But this type of white light suffers low color-rendering index (CRI, Ra ~70-80) and high correlated color temperature (CCT ~7750 K) due to the deficiency of red emission component [4]. To solve this problem, another approach by combining NUV LEDs with a blend of blue, tunable green-to-yellow and red-emitting phosphors has been suggested and paid much attention in recent years [5]. The w-LEDs fabricated in this way could present tunable CCT and excellent CRI values. Thus, it is important to explore new phosphors with long-wavelength visible emission, which can be effectively excited by NUV light.

To obtain excellent CRI for w-LEDs, broad excitation and emission bands of phosphors are favorable. The electronic transition of Eu²⁺ with the 4f⁷ configuration is associated with the 4f⁶5d¹–4f⁷ parity allowed transition so that the excitation and emission spectra of Eu²⁺ are broad at room temperature [6,7]. Besides Eu²⁺, Mn²⁺ can also give green or red broad emission, depending on the crystal field condition in phosphors. If Mn²⁺ lies in octahedral surrounding with large crystal field, the emission is usually red; if in tetrahedral surrounding with a much smaller crystal field, a green emission is usually obtained [8]. To enhance the Mn²⁺ emission, Eu²⁺ ions are usually codoped with Mn²⁺ and work as sensitizer in the energy transfer (ET) process. The emission-tunable phosphors can be obtained in this way, such as NaCaBO₃:Eu²⁺,Mn²⁺ and Y₇O₆F₉:Eu²⁺,Mn²⁺ [9,10]. Besides, the thermal quenching is also a key quality factor for high-power LEDs. Ci et al. pointed out that the doping of Ba²⁺ in the Sr₃SiO₅:Eu²⁺ made a contribution to the thermal stability [11]; Liu et al. studied the thermal stability of Ba₅Cl(PO₄)₃:Eu²⁺ by using the activation energy E_a [12].

For the host materials of phosphors, phosphate compounds are widely used currently owing to the high physical and chemical stabilities, facile synthesis, and environmental-friendly characteristics, such as Ba₃Y(PO₄)₃:Sm³⁺ and Ca₈MgLa(PO₄)₇:Eu²⁺, Mn²⁺ [3,13]. It was reported the Ba₂Mg(PO₄)₂ compound crystallizes in monoclinic system with space group of P2₁/n, and two Ba²⁺ sites (Ba(1) and Ba(2)) exist in this structure [14], so Eu²⁺ that is sensitive to the crystal-field environment is suitable to be doped in this host material. In this work, to explore new phosphors with long-wavelength visible emission, a series of Eu²⁺/Mn²⁺ doped Ba₂Mg(PO₄)₂ samples were prepared by solid-state reaction method, and their photoluminescence properties were investigated.

2. Experimental

Powder samples of Ba_2(1-x)Mg_1-y(PO₄)₂:xEu²⁺,yMn²⁺ (BMP:xEu²⁺,yMn²⁺, 0.2% ≤ x ≤ 5%, 0 ≤ y ≤ 15%) were prepared by solid-state reaction method. The starting materials included BaCO₃ (99%), (MgCO₃)₄·Mg(OH)₂·5H₂O (99%), (NH₄)₂HPO₄ (99%), Eu₂O₃ (99.99%), and MnCO₃ (99%). Stoichiometric amounts of the starting reagents were thoroughly mixed and ground together by an agate mortar. The mixture was first fired at 600 °C for 3 h in air, regound, and calcined in a reduction atmosphere (N₂: H₂ = 95: 5) at 1010 °C for 5 h.

The phase purity was determined by using an ARL X'TRA powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 35 mA. The photoluminescence (PL) spectra were recorded on an FS5 fluorescence spectrophotometer with 150 W xenon lamp as the light source. The decay curves were measured with a 360 nm pulsed EPLED on the Edinburgh FS5 spectrofluorometer system.

3. Results and discussion

Figure 1 presents the XRD patterns of BMP:xEu²⁺ (0.2% ≤ x ≤ 5%) and BMP:2.2%Eu²⁺,yMn²⁺ (5% ≤ y ≤ 15%). The patterns show that all of the diffraction peaks are identified to the BMP host lattice (JCPDS # 87-0419), indicating single-phase samples can be successfully obtained when Eu²⁺/Mn²⁺ ions are introduced.

 

Fig. 1 XRD patterns of (a) BMP:xEu²⁺ (0.2% ≤ x ≤ 5%) and (b) BMP:2.2%Eu²⁺,yMn²⁺ (5% ≤ y ≤ 15%).

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The excitation spectrum of the typical BMP:2.2%Eu²⁺ (for y = 0) by monitoring 598 nm is shown in Fig. 2(a). It can be found that a broad excitation band covers the region of 240-450 nm, which well matches with the emission wavelength of the NUV LED chip. This excitation band could be attributed to the 4f⁷-4f⁶5d¹ transition of Eu²⁺. To learn the emission spectral character, Fig. 2(b) depicts the PL spectra of BMP:xEu²⁺ (0.2% ≤ x ≤ 5%) upon 365 nm excitation. All the spectra cover a broad range from 450 to 800 nm, ascribed to the 4f⁶5d¹-4f⁷ transition of Eu²⁺. The profiles of the emission spectra change little for different Eu²⁺ contents, and the optimal Eu²⁺ content is for x = 2.2% as can be seen from the inset of Fig. 2(b) which shows the emission intensity as a function of Eu²⁺ concentration.

 

Fig. 2 (a) Excitation spectra of BMP:2.2%Eu²⁺,yMn²⁺ (y = 0 and 10%); (b) emission spectra of BMP:xEu²⁺ (0.2% ≤ x ≤ 5%), inset shows the emission intensity of Eu²⁺ as a function of Eu²⁺ concentration x.

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To tune the emitting light color, Mn²⁺ ions are codoped with Eu²⁺ in the BMP host, and Fig. 3(a) represents the emission spectra of BMP:2.2%Eu²⁺,yMn²⁺ (0 ≤ y ≤ 15%) under 365 nm excitation. It can be found the emission band intensity decreases gradually with increasing Mn²⁺ concentration. To observe the change of the spectral shape clearly, Fig. 3(b) shows the normalized emission spectra of BMP:2.2%Eu²⁺,yMn²⁺ (0 ≤ y ≤ 15%). It is obvious that a continuous red-shift of the emission bands occurs with the Mn²⁺ content increased. This phenomenon indicates that the Mn²⁺ has played a main role on the spectral shape. Figure 3(c) presents the excitation and emission spectra of BMP:10%Mn²⁺. By monitoring 655 nm, the main excitation peaks at 342, 370, 388, 414, and 450 nm could be assigned to the transitions from ⁶A₁(⁶S) to ⁴E(⁴D), ⁴T₂(⁴D), ⁴E, ⁴T₂(⁴G), and ⁴T₁(⁴G) energy levels, respectively [15,16]. Upon 365 nm excitation, a broad emission band peaking at 655 nm is found in the region of 580-775 nm, which belongs to the ⁴T₁(⁴G)-⁶A₁(⁶S) transition of Mn²⁺ [15]. Based on this, it can be imagined that the red-shift of the emission bands in Fig. 3(b) is mainly owing to the blend of the Mn²⁺ red emission into the Eu²⁺ emission. However, the emission intensity of Mn²⁺-single-doped BMP is low, about 10% of that of BMP:2.2%Eu²⁺ (see inset of Fig. 3(c)). Thus, an ET from Eu²⁺ to Mn²⁺ may occur to generate the obvious red-shift of whole emission bands since the Eu²⁺ emission exhibits continuous decrease with increasing Mn²⁺ content. The spectral overlap between the Eu²⁺ emission band and the Mn²⁺ excitation band also seems to support this point (see Fig. 3(d)). But, this ET process is not easy to perceive just by the emission spectra of Eu²⁺ and Mn²⁺ because their emission spectra overlap is so much that the emission bands of Eu²⁺ and Mn²⁺ can’t be distinguished clearly. To interpret the occurrence of this ET, the decay curves for BMP:2.2%Eu²⁺,yMn²⁺ (0 ≤ y ≤ 15%) with 365 nm excitation and 570 nm emission were measured, as shown in Fig. 3(e). The monitoring wavelength of 570 nm was selected for the sake that this wavelength belongs to the Eu²⁺ emission and excludes the Mn²⁺ emission. All the measured decay curves can be well-reproduced by a double-exponential function as , where τ₁ and τ₂ are the fast and slow components of the luminescent lifetimes, and A₁ and A₂ are the fitting parameters, respectively. The measured lifetimes and the A_i (i = 1 and 2) values are summarized in Table 1. The average lifetimes, defined as $<\tau >=(A_1{\tau }_1^2+A_2{\tau }_2^2)/(A_1{\tau }_1+A_2{\tau }_2)$ [17], could be calculated to be 1403.8, 1351.6, 1283.0, 1171.1, 1120.9 ns for y = 0, 5%, 7%, 10%, and 15%, respectively. With increasing Mn²⁺ concentration, the decay lifetime decreases gradually owing to the ET between Eu²⁺ and Mn²⁺. The ET efficiency can be determined by the formula [18], where τ_s0 and τ_s are the decay lifetimes of the sensitizer (Eu²⁺) in the absence and presence of the activator (Mn²⁺), respectively. The obtained η_T values are 3.7%, 8.6%, 16.6%, and 20.2% for y = 5%, 7%, 10%, and 15%, respectively. Thus, the ET between Eu²⁺ and Mn²⁺ is not very efficient. The corresponding ET mechanism could be understood from the schematic in the inset of Fig. 3(d). When the electrons in Eu²⁺ are pumped into the 4f⁶5d¹ energy level, they can jump to the ground state 4f⁷ level and the energy is released via emitting visible photons. Meanwhile, the ET from Eu²⁺ to Mn²⁺ could occur and the Mn²⁺ ions will be excited, resulting in the red emission of Mn²⁺ finally. Nevertheless, we can’t remove the reason for the spectral red-shift phenomenon also owing to the change of the crystal field. The d-orbital splitting of a metal center such as Eu²⁺ can be described as , where Dq( = Δ) corresponds to the energy level separation, R is the distance between the central ion and its ligands, Z is the charge or valence of the anion, e is the charge of the electron, and r is the radius of the d wavefunction [19]. As Dq ∝ R⁻⁵, when the R changes, the crystal field of the metal center will also change, and thus leads to the shift of spectral position. In this work, the substitution of Mn²⁺ on the Mg²⁺ sites changes the R value, which further affluences the crystal field strengthen around Eu²⁺ and Mn²⁺. Maybe, it can be say the above spectral red-shift could result from both the change of the crystal field and the blend of the Eu²⁺ and Mn²⁺ emissions. Based on these, the emitting light color of BMP:2.2%Eu²⁺,yMn²⁺ (0 ≤ y ≤ 15%) has been tuned. The Commission International de l’Eclairage (CIE) chromaticity coordinates for BMP:2.2%Eu²⁺,yMn²⁺ were calculated to be (0.506, 0.472), (0.511, 0.466), (0.515, 0.464), (0.515, 0.460), and (0.519, 0.456) for y = 0, 5%, 7%, 10%, and 15%, respectively. Figure 3(f) presents the CIE chromaticity diagram for the typical BMP:2.2%Eu²⁺,yMn²⁺ (y = 0 and 10%) samples, and their emitting-light-colors in the digital photographs under 365 nm UV lamp irradiation (see insets (a) and (b)) match with their chromaticity coordinates. Thus, warm white emission could be further gained if the above samples are combined with a certain blue phosphor. To learn the excitation spectral feature after doping Mn²⁺, the excitation spectrum of the typical BMP:2.2%Eu²⁺,10%Mn²⁺ sample by monitoring at 623 nm is shown in Fig. 2(a). It can be found this excitation band shows a shift towards the short-wavelength direction compared with that of the original BMP:2.2%Eu²⁺ sample, but still covers a broad range of 240-450 nm.

 

Fig. 3 (a) Emission spectra of BMP:2.2%Eu²⁺,yMn²⁺ (0 ≤ y ≤ 15%); (b) normalized emission spectra of BMP:2.2%Eu²⁺,yMn²⁺ (0 ≤ y ≤ 15%); (c) excitation and emission spectra of BMP:10%Mn²⁺, inset shows emission spectra of BMP:2.2%Eu²⁺,yMn²⁺ (y = 0 and 10%); (d) excitation (BMP:10%Mn²⁺) and emission (BMP:2.2%Eu²⁺) spectra, inset shows the schematic of ET from Eu²⁺ to Mn²⁺; (e) decay curves of BMP:2.2%Eu²⁺,yMn²⁺ (0 ≤ y ≤ 15%); (f) CIE chromaticity diagram for BMP:2.2%Eu²⁺,yMn²⁺ (y = 0 and 10%), insets (a) and (b) show the digital photographs under 365 nm UV lamp irradiation for y = 0 and 10%, respectively.

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Table 1. The values of A_i and τ_i (i = 1 and 2) for decay curves of BMP:2.2%Eu²⁺,yMn²⁺ (0 ≤ y ≤ 15%)

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To investigate the thermally stable luminescence of the samples, the typical BMP:2.2%Eu²⁺,yMn²⁺ (y = 0 and 10%) phosphors were measured upon 365 nm excitation by heating the samples in a temperature range from 25 to 200 °C. Figure 4(a) shows the emission spectra of BMP:2.2%Eu²⁺. It can be seen that the Eu²⁺ emission intensity decreases gradually with increasing temperature. In order to learn the change of the spectral position, Fig. 4(b) shows the normalized emission spectra of BMP:2.2%Eu²⁺. Obviously, the emission band of Eu²⁺ shows a continuous blue-shift with increasing temperature. This observation could be due to the different thermal quenching abilities of the different Eu²⁺ centers in Ba(1) and Ba(2) sites, which is similar to the interpretation for the Ba₂Ln(BO₃)₂Cl:Eu²⁺ (Ln = Y, Gd and Lu) phosphors in Ref [8]. Figure 4(c) depicts the emission spectra of BMP:2.2%Eu²⁺,10%Mn²⁺. Similarly, the emission intensity exhibits a continuous decrease with increasing temperature. Figure 4(d) shows their normalized emission spectra. Also, a blue-shift phenomenon is observed, which is owing to not only the different thermal quenching abilities of Eu²⁺ centers but also the different thermal quenching abilities between Eu²⁺ and Mn²⁺. To compare the thermal quenching properties between BMP:2.2%Eu²⁺ and BMP:2.2%Eu²⁺,10%Mn²⁺, the insets of Fig. 4(a) and 4(b) demonstrates their relative emission intensities as a function of temperature, respectively. It can be found that when the temperature rises, the emission intensity of BMP:2.2%Eu²⁺,10%Mn²⁺ shows a faster decay than that of BMP:2.2%Eu²⁺. This result indicates that the thermal quenching ability of Eu²⁺ is superior to that of Mn²⁺ in the BMP host.

 

Fig. 4 Fig. 4. (a) Emission spectra of BMP:2.2%Eu²⁺ at different temperatures, inset shows its relative intensity as a function of temperature; (b) normalized emission spectra of BMP:2.2%Eu²⁺ at different temperatures; (c) emission spectra of BMP:2.2%Eu²⁺,10%Mn²⁺ at different temperatures, inset shows its relative intensity as a function of temperature; (d) normalized emission spectra of BMP:2.2%Eu²⁺,10%Mn²⁺ at different temperatures.

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4. Conclusions

In this paper, a series of BMP:xEu^2+,yMn²⁺ (0.2% ≤ x ≤ 5%, 0 ≤ y ≤ 15%) phosphors were synthesized by solid-state reaction method. The Eu²⁺-single-doped BMP samples show broad orange-yellow emission and the optimal Eu²⁺ doping concentration was determined for 2.2mol%. The excitation spectrum for BMP:2.2%Eu²⁺ covers the whole NUV region, which agrees well with the emission wavelength of the NUV LED chips. By introducing Mn²⁺ into BMP:2.2%Eu²⁺, the Eu²⁺ emission band shows an obvious red-shift, mainly owing to the blend of the Mn²⁺ and Eu²⁺ emissions. The ET from Eu²⁺ to Mn²⁺ has been verified by the fluorescent lifetimes. The thermal quenching investigation reveals the BMP:2.2%Eu²⁺ and BMP:2.2%Eu²⁺,10% Mn²⁺ samples possess different thermal stability.