1. Introduction

Transition metal Mn²⁺ ion has 3d⁵ configuration and shows typical d-d transitions, which is an excellent luminescence center of phosphors. For example, the green-emitting Zn₂SiO₄: Mn²⁺ is a commercially available phosphor in plasma display panels (PDPs) at present. Since crystal field has strong influence on the emission of Mn²⁺ ions, the luminescent color of Mn²⁺ can be changed from green to red depending on the coordination surroundings of Mn²⁺ in different host lattices [1]. Accordingly, much attention has been paid to the luminescence of Mn²⁺ in different compounds such as phosphates, sulfides, aluminates, silicates [2–6]. In these phosphors, Mn²⁺ ion can act as an activator or a sensitizer [5–8].

Ce³⁺ is a typical rare-earth ion which exhibits f-d transition. Because the 4f-5d transition is parity-allowed, it has a large optical absorption cross-section. Phosphors doped with Ce³⁺ ions have gained wide applications [9–12]. For example, Y₃Al₅O₁₂: Ce³⁺ is commonly used as a yellow-emitting component in InGaN/GaN-based LEDs (light emitting diodes), Y₂SiO₅: Ce³⁺ is recommended to be a potential blue-emitting phosphor in FEDs (field emission displays), and Lu₂SiO₅: Ce³⁺ is a commercially available scintillator. In addition, Ce³⁺ ion can sensitize many other luminescent ions such as Tb³⁺, Eu²⁺, Pr³⁺ [13–15] due to energy transfer (ET) from Ce³⁺ to these ions.

The alpha-phase strontium pyrophosphate (α-Sr₂P₂O₇) is an important host for luminescence of lanthanides and transition metals [16, 17], the luminescence properties of Ce³⁺-doped α-Sr₂P₂O₇ have been investigated in our previous work [18]. In this paper, because of the spectral overlaps between the Ce³⁺ emission bands and Mn²⁺ excitation bands, we analyzed the luminescence spectra of Mn²⁺ doped and Ce³⁺-Mn²⁺ co-doped α-Sr₂P₂O₇, and investigated the sensitization and enhancement of Mn²⁺ red emission through energy transfer from Ce³⁺ to Mn²⁺.

2. Experimental

A series of Mn²⁺ doped and Ce³⁺-Mn²⁺ co-doped powder samples with nominal chemical formulae α-Sr_2-xMn_xP₂O₇, α-Sr_1.94-xCe_0.03Na_0.03Mn_xP₂O₇ and α-Sr_1.9-xCe_xNa_xMn_0.1P₂O₇ were synthesized by a traditional high temperature solid-state reaction route, respectively. The raw materials are analytical reagents SrCO₃, (NH₄)₂HPO₄, MnCO₃, Na₂CO₃ and CeO₂ (99.99%). Na⁺ ions were added as compensators for charge defects resulting from Sr²⁺ ions substituted by Ce³⁺ ions. The raw materials were weighed in stoichiometric proportions, after mixing and thoroughly grinding, the mixtures were first preheated at 450 °C for 1.5 h and cooled down to room temperature. Then the mixtures were kept at 1100 °C for 2 h in a thermal carbon reducing atmosphere, and final products were achieved after cooled down to room temperature.

The phase purity of final products was checked by powder X-ray diffraction (XRD) with Cu K_α (λ = 1.5405 Å) radiation on a BRUKER D8 ADVANCE type powder X-ray diffractometer operating at 40 kV and 40 mA at room temperature. High quality XRD data for Rietveld refinement was collected over a 2θ range from 10° to 100° at an interval of 0.02°. Structural refinement of XRD data was performed using the TOPAS-Academic [19] program.

The steady-state excitation and emission spectra in the UV-vis range were characterized by a FLS 920 steady-state spectrometer and a 450 W xenon lamp was used as the excitation source. For luminescence decay spectra, a 60 W μF flash lamp (pulse width 1.5 μs, repetition rate 9 Hz) and a 150 W nF900 ns flash lamp (pulse width 1 ns, repetition rate 40 kHz) were used, respectively.

3. Results and discussion

3.1. X-ray diffraction

X-ray diffractions for all samples were characterized at room temperature (RT), five samples are chosen as examples to display in Fig. 1 . The standard card of compound α-Sr₂P₂O₇ is also shown in Fig. 1. The diffraction peaks for all the samples are similar to each other and all agree well with the Joint Committee for Powder Diffraction Standard file 24-1011 (α-Sr₂P₂O₇). We additionally recorded the XRD patterns of three samples α-Sr_1.99Mn_0.01P₂O_7, α-Sr_1.90Mn_0.10P₂O_7, α-Sr_1.8Mn_0.2P₂O₇ with a slow scan rate (0.4 °/min). A careful inspection suggests that the powders contain a tiny amount of Mn₃Sr₁₈(PO₄)₁₂ impurity phase, this will be further discussed later. In fact, such a small amount of impurity phase does not influence the luminescence spectra.

 

Fig. 1 X-ray diffraction patterns for part of α-Sr_1.94-xCe_0.03Na_0.03Mn_xP₂O₇ (x = 0, 0.1, 0.2) and α-Sr_1.9-xCe_xNa_xMn_0.1P₂O₇ (x = 0, 0.03, 0.05) samples at room temperature.

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3.2. Luminescence properties of Mn²⁺ doped samples

The excitation and emission spectra of α-Sr_1.90Mn_0.1P₂O₇ sample in UV-vis range are shown in Fig. 2 . Under 404 nm excitation, an asymmetric broad emission band between 500 and 750 nm with a peak at ~595 nm can be observed in curve a, which is attributed to the transition from ⁴T₁(⁴G) to ⁶A₁(⁶S) of Mn²⁺ ions in the host lattice. When monitoring emission at 595 nm, the corresponding excitation spectrum was recorded. Three obvious excitation bands B, C and D in the wavelength range 300-450 nm (curve b) can be observed, which are related to the transitions of Mn²⁺ ions from ground state ⁶A₁(⁶S) to excited states ⁴E(⁴D), ⁴T₂(⁴D) and [⁴A₁(⁴G), ⁴E(⁴G)].

 

Fig. 2 Excitation and emission spectra of α-Sr_1.90Mn_0.1P₂O₇ sample at RT (λ_ex = 404 nm, λ_em = 595 nm).

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The emission and excitation spectra for the samples with different Mn²⁺ doping concentrations were also investigated. Figure 3 displays the normalized emission spectra under 403 nm and 406 nm excitation, respectively. It can be found that the emission bands shift to long-wavelength side with the increasing of Mn²⁺ concentration in Fig. 3(a, c). This may relate to two factors: First, the influence of crystal field. A Mn²⁺ ion has five 3d electrons, the energy levels of these electrons are strongly affected by crystal field strength. The effect radius of Sr²⁺ is evidently larger than that of Mn²⁺, when more smaller Mn²⁺ ions enter larger Sr²⁺ sites, the volume of crystal cell will decrease and Mn²⁺ ions are expected to experience stronger crystal field effect with the increase of the doping content. When we suppose that the 3d energy centroid and the Stokes shift of Mn²⁺ keep nearly invariable, this effect should lead to the long-wavelength shift of emission band. Second, the site occupancy of Mn²⁺ in the host. The host compound α-Sr₂P₂O₇ crystallizes in orthorhombic structure with space group Pnma, all the Sr²⁺ ions can be divided into two different types with the similar SrO₉ polyhedron. Both polyhedra may be visualized as being derived from a cube. Six oxygen atoms are close to six corners of the cube, which define three edges in the y direction of the structure. The other three O^2- are very roughly arranged along the fourth parallel cube edge. Both Sr²⁺ sites have C_s symmetry but the site sizes are slightly different. The Sr²⁺(1) ion has the larger site size with average distance of 0.2721 nm for the Sr(1)-O bonds. The average distance is about 0.2679 nm for the Sr(2)-O bonds [18, 20, 21]. Because two types of Sr²⁺ ions share similar crystal surroundings, Mn²⁺ ions are expected to enter both sites. Since the two Sr²⁺ sites have similar coordination polyhedra but the site size of Sr²⁺(1) is larger than that of Sr²⁺(2), it is plausible that the shorter wavelength emission may relate to the larger Sr²⁺(1) site [marked as Mn²⁺(1)] and the longer wavelength emission relates to the smaller Sr²⁺(2) site [marked as Mn²⁺(2)]. In addition, when we consider the two sites occupancy of Mn²⁺ in the host lattice, the energy transfer (ET) between two sites should also have influence on the luminescence spectra. The spectra we observed may be the whole results of these influences.

 

Fig. 3 (a, c) Normalized emission spectra for different doping concentration samples chemical formulae upon 403 nm and 406 excitation, (b, d) Normalized emission spectra for α-Sr_1.97Mn_0.03P₂O₇ and α-Sr_1.8Mn_0.2P₂O₇ samples under 403 nm and 406 nm excitation.

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The normalized emission spectra of two typical samples α-Sr_1.97Mn_0.03P₂O₇ and α-Sr_1.8Mn_0.2P₂O₇ under 403 nm and 406 nm excitation are displayed in Fig. 3(b, d). When doping concentration is relatively lower (x = 0.03), emission spectra have similar shape and maximum under either 403 nm or 406 nm excitation. But when doping concentration is relatively higher (x = 0.2), we can see that, the emission spectrum shifts to long-wavelength side upon 406 nm excitation in comparison with than upon 403 nm excitation. For a specific sample, the influence of crystal field strength on the 3d energies and luminescence is definite, so the spectral difference in Fig. 3(b) or Fig. 3(d) should relate to the two different Mn²⁺ ions and the ET between them.

The Rietveld refinement in Fig. 4 was performed for Sr_2-xMn_xP₂O₇:xMn²⁺ (x = 0.01, 0.10, 0.20) using the Pnam structure model reported by J. Barbier et al. [21], all of the observed peaks are consistent with the reflection conditions, lattice constants and cell volumes are shown in Table 1 . The results show that the unit cell volume decreases with increasing of Mn²⁺ concentration. A small amount of Mn₃Sr₁₈(PO₄)₁₄ were detected for all samples in high quality XRD data. The PL spectra of Mn₃Sr₁₈(PO₄)₁₄ have not been reported, so its influences on PL spectra of Sr_2-xMn_xP₂O₇:xMn²⁺ are unknown. Since the amounts are extremely small, the influences may be little. From Table 1, it can be seen that most of Mn²⁺ ions are incorporated into Sr²⁺(2) sites when doping concentration is higher. The concentration ratio of Mn²⁺(2)/Mn²⁺(1) increases with the increase of doping content. All the results agree well with the spectra we observed.

 

Fig. 4 The experimental (crosses) and calculated (red solid line) XRD patterns and their difference (blue solid line) for Sr_1.90Mn_0.10P₂O₇. The second row of Brag positions belongs to the second phase Mn₃Sr₁₈(PO₄)₁₂.

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Table 1. Crystallographic Data and structure parameters of α-Sr_2-xMn_xP₂O₇ (x = 0.01, 0.10, 0.20)

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According to above analysis, Mn²⁺ ions enter both Sr²⁺ sites, emission of Mn²⁺(1) locates at a relatively shorter wavelength and Mn²⁺(2) at a longer wavelength. Two different wavelengths 540 nm and 640 nm which may mainly relate to Mn²⁺(1) and Mn²⁺(2) were chosen to get the excitation spectra. When the doping concentration is low [see Fig. 5(a) ], the two excitation spectra have no significant difference except for intensity. When concentration increases, band B shifts from 403 nm to 406 nm, the relative intensities for bands C and D gradually increase and shift to long-wavelength side as shown in curves b and c. Herein, 403 nm and 406 nm are attributed to transition absorptions from ⁶A₁(⁶S) to excited states [⁴A₁(⁴G), ⁴E(⁴G)] for Mn²⁺(1) and Mn²⁺(2). The observation indicates that site occupation and the energy transfer may the main factor for the change of emission spectra in Fig. 3, the crystal field splitting at different Mn²⁺ doping contents probably has smaller influence.

 

Fig. 5 Excitation spectra for α-Sr_2-xMn_xP₂O₇ (x = 0.03, 0.10, 0.20) by monitoring at 540 nm and 640 nm.

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The luminescence decay curves for all Mn²⁺ doped samples were measured at RT. Figure 6 shows the decay curves for Mn²⁺(1) and Mn²⁺(2), the luminescence of Mn²⁺ in the host shows a nearly exponential decay for the two Mn²⁺ ions. The decay time of Mn²⁺(1) slightly decreases from 14.63 ms to 13.23 ms, that of Mn²⁺(2) slightly increases from 14.06 ms to 16.59 ms with a fast rising process, indicating energy transfer between the two Mn²⁺ ions exists, but it is inefficient. Here, the measured lifetime τ is controlled by the radiative rate τ_R and nonradiative decay rate τ_NR. The energy transfer (ET) within same type of Mn²⁺ and that between two different types of Mn²⁺ may show influence on the lifetime τ_R. From the tendency of above fitted decay constants, it seems that ET of Mn²⁺(1)→Mn²⁺(2) is dominant and those of Mn²⁺(2)→Mn²⁺(1), Mn²⁺(1)→Mn²⁺(1) and Mn²⁺(2)→Mn²⁺(2) are even quite weak though all these processes are inefficient.

 

Fig. 6 Decay curves for α-Sr_2-xMn_xP₂O₇ (x = 0.01, 0.03, 0.05, 0.08, 0.10, 0.15, 0.20) (a) λ_ex = 403 nm, λ_em = 540 nm (b) λ_ex = 406 nm, λ_em = 640 nm.

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3.3. Energy transfer from Ce³⁺ to Mn²⁺ in co-doped samples

The excitation and emission spectra of Mn²⁺ and Ce³⁺ singly doped α-Sr₂P₂O₇ phosphors are shown in Fig. 7 . The excitation spectrum of Mn²⁺ has been assigned in above section. Three emission bands range from 290 to 450 nm can be observed for Ce³⁺ ion upon 268 nm excitation. Comparing the two curves in Fig. 7, it is obvious that there is a significant spectral overlap between the excitation spectrum of Mn²⁺ and the emission spectrum of Ce³⁺, indicating the possibility of energy transfer from Ce³⁺ to Mn²⁺ in α-Sr₂P₂O₇.

 

Fig. 7 Excitation spectrum of Mn²⁺ and emission spectrum of Ce³⁺ in α-Sr₂P₂O₇.

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As shown in Fig. 8 , by monitoring Mn²⁺ emission at 595 nm, the excitation spectra for Ce³⁺-Mn²⁺ co-doped samples α-Sr₂P₂O₇: xCe³⁺, 0.1Mn²⁺ were measured at RT. According to our previous work, the excitation bands at ~275 nm, ~296 nm, ~313 nm are attributed to the transition absorptions of Ce³⁺ in the host [18]. Weak bands B, C and D are related to the transition absorptions of Mn²⁺ from ground state ⁶A₁(⁶S) to excited states ⁴E(⁴D), ⁴T₂(⁴D) and [⁴A₁(⁴G), ⁴E(⁴G)], which are consistent well with excitation spectrum in Fig. 2. Since there is no emission for Ce³⁺ at 595 nm, the results indicate energy transfer (ET) from Ce³⁺ to Mn²⁺ in the system.

 

Fig. 8 Excitation spectra for α-Sr₂P₂O₇: xCe³⁺, 0.1Mn²⁺samples by monitoring at 595 nm at room temperature.

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Luminescence properties of Ce³⁺ doped α-Sr₂P₂O₇ have been reported in our previous work [18], Ce³⁺ ions can enter both Sr²⁺ sites, three emission bands locate at about ~310 nm [mainly from Ce³⁺(1)], ~330 nm [overlapping of Ce³⁺(1) and Ce³⁺(2)] and ~350 nm [mainly from Ce³⁺(2)]. The lowest 5d absorptions for Ce³⁺(1) and Ce³⁺(2) are at ~296 nm and ~311 nm. Figure 9 shows the emission spectra for Ce³⁺ and Mn²⁺ co-doped α-Sr₂P₂O₇: xCe³⁺, 0.1Mn²⁺ samples upon 296 and 315 nm excitations, respectively. Two groups of emission bands can be observed: Emission bands between 300 and 400 nm are attributed to Ce³⁺ ions, three bands are located at ~308, ~329 and ~350 nm, which agree well with our previous work [18]. The band range from 500 to 750 nm is the emission of Mn²⁺ ions. The emission intensity of Mn²⁺ first increases with the increasing doping concentration and reaches a maximum when x = 0.03, then the emission intensity decreases. Because we didn’t find concentration quenching of Ce³⁺ or Mn²⁺ in this concentration range for single doping samples, the reason for this observation is unknown.

 

Fig. 9 Emission spectra for sample α-Sr₂P₂O₇: xCe³⁺, 0.1Mn²⁺ at RT under 296 and 315 nm excitation.

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From Fig. 2 we can see that, there is no absorptions for Mn²⁺ ions at about 296 and 315 nm, but when under 296 nm and 315 nm excitation, the emission of Mn²⁺ appears, indicating energy transfer from Ce³⁺ to Mn²⁺ occurs. When co-doped with Ce³⁺ ion, the emission of Mn²⁺ is enhanced due to the energy transfer from Ce³⁺ to Mn²⁺. For a rough estimation, the luminescence intensities of Mn²⁺ ions for x = 0.03 and 0.002 samples in α-Sr₂P₂O₇: xCe³⁺, 0.1Mn²⁺ under 315 and 296 nm excitation were integrated, respectively. Under 315 nm excitation the Mn²⁺ emission intensity of sample x = 0.03 is about 15 times in comparison with that of x = 0.002, this value is just 9 under 296 nm excitation. This observation may indicate energy transfer from Ce³⁺(2) is more efficient than from Ce³⁺(1) to Mn²⁺ in the host. Here, it is also interesting to note that the emission intensity ratio of Mn²⁺ to Ce³⁺ in Fig. 9(a) is larger than that in Fig. 9(b). We think this phenomenon is because of the co-existence of Ce³⁺(1)→Ce³⁺(2) and Ce³⁺→Mn²⁺ ET. Energy can be transferred to Ce³⁺(2) and Mn²⁺ upon excitation the Ce³⁺(1) centers at 296 nm, then Ce³⁺(2) transfers the energy to Mn²⁺, but when under 315 nm excitation, only energy transfer process Ce³⁺(2)$\to$Mn²⁺ occurs, so the emission intensity of Mn²⁺ under 296 nm excitation is relatively higher than directly upon excitating Ce³⁺(2) at 315 nm. Because partial energy can be transferred to Ce³⁺(2) under 296 nm excitation, the ET efficiency of Ce³⁺(1)→Mn²⁺ decreases in comparison with that of Ce³⁺(2)→Mn²⁺.

Under 296 nm excitation, both Ce³⁺ ions can be excited at the same time. Figure 10(a) shows the emission intensity of Ce³⁺ ions decreases gradually with the increase of Mn²⁺ concentration, which indicates energy transfer from Ce³⁺ to Mn²⁺ [22]. Thus the whole Ce³⁺→Mn²⁺ energy transfer efficiency can be calculated using the following formula [23]:

 

Fig. 10 (a) The emission spectra of α-Sr₂P₂O₇: 0.03Ce³⁺, xMn²⁺(x = 0-0.2) under 296 nm excitation (b) Relative emission intensity of Ce³⁺ and energy transfer efficiency from Ce³⁺ to Mn²⁺ with the increase Mn²⁺ concentration; The inset shows CIE chromaticity diagram for samples α-Sr₂P₂O₇: 0.03Ce³⁺, xMn²⁺ under 296 nm excitation

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Exchange interaction and multipolar interaction may result non-radiative energy transfer [23]. In general, exchange interaction is response for forbidden transitions and the critical distance between luminescence centers is about 5 Å. The critical distance R_c for energy transfer between from Ce³⁺ to Mn²⁺ ions can be calculated according to the critical concentration using the following equation [23, 24]:

The inset of Fig. 10 shows the CIE coordinates of α-Sr₂P₂O₇: 0.03Ce³⁺, xMn²⁺ samples under 296 nm UV excitation. We can see that, the CIE coordinate gradually moves from yellow to orange region with the increase of Mn²⁺ concentration.

The luminescence decays of Mn²⁺ ions in α-Sr₂P₂O₇: 0.03Ce³⁺, xMn²⁺ samples were measured, the results are shown in Fig. 11 . When monitoring wavelength at 540 nm [see Figs. 11(a) and 11(c)], the decay curves did not show significant change, the luminescence decay of Mn²⁺(1) ranges from 12.03 to 13.40 ms. In Figs. 11(b) and 11(d), upon 296 nm and 315 nm excitation, the decay curves show somewhat larger deviation, the decay time of Mn²⁺(2) varies between 12.72 and 15.14 ms.

 

Fig. 11 Decay curves for α-Sr₂P₂O₇: 0.03Ce³⁺, xMn²⁺ samples (λ_ex = 296 nm, 315 nm, λ_em = 540 nm, 640 nm).

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To further investigate the process of energy transfer, the fluorescence decay curves for both Ce³⁺(1) and Ce³⁺(2) with different Mn²⁺ concentrations in samples α-Sr₂P₂O₇: 0.03Ce³⁺, xMn²⁺ were measured and presented in Fig. 12 . Without Mn²⁺ ions, the luminescence of Ce³⁺ in the host shows a nearly exponential decay for the two Ce³⁺ ions. When Mn²⁺ ions are introduced, the decay curves deviate from exponential. With the increase of the Mn²⁺ concentration, this deviation is more evident and the decay becomes faster and faster, which is due to the energy transfer from Ce³⁺ to Mn²⁺. The deviation for Ce³⁺(1) is larger than Ce³⁺(2). Two factors may influence the results: energy transfer from Ce³⁺(1) to Ce³⁺(2) and energy transfer from Ce³⁺(1) to both Mn²⁺ ions. An efficient energy transfer form Ce³⁺(1) to Ce³⁺(2) can be observed clearly in our previous work [18]. Ce³⁺(1) centers may transfer excitation energy to Ce³⁺(2), Mn²⁺(1) and Mn²⁺(2) centers simultaneously, so the decays in Fig. 12(a) show more evident changes in comparison with Fig. 12(b).

 

Fig. 12 Decay curves for Ce³⁺ ions with different Mn²⁺ concentrations in samples α-Sr₂P₂O₇: 0.03Ce³⁺, xMn²⁺ (a) λ_ex = 296 nm, λ_em = 310 nm (b) λ_ex = 315 nm, λ_em = 350 nm.

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4. Conclusion

The photoluminescence properties of Mn²⁺ doped and Ce³⁺-Mn²⁺ co-doped α-Sr₂P₂O₇ phosphors were investigated. The excitation spectra of Mn²⁺ doped samples exhibit some differences for different doping samples by monitoring wavelengths at 540 and 640 nm, respectively, indicating Mn²⁺ ions enter two different sites in the host. The emission band of Mn²⁺ shifts to longer wavelength with the increasing of doping concentration mainly due to site occupation and energy transfer. The emission wavelength for Mn²⁺(2) is longer than Mn²⁺(1). Energy transfer from Ce³⁺ to Mn²⁺ in the host lattice is demonstrated by luminescence spectra, luminescence decay curves in α-Sr₂P₂O₇: 0.03Ce³⁺, xMn²⁺ and α-Sr₂P₂O₇: xCe³⁺, 0.1Mn²⁺ powders. Energy can transfer from both Ce³⁺(1) and Ce³⁺(2) to Mn²⁺(1) and Mn²⁺(2) in the host, but transfer efficiency may be different.