The luminescence and energy transfer of cerium and terbium codoped Ba2Mg(PO4)2 were investigated. The phosphor was prepared by a solid-state reaction at high temperature. The photoluminescence properties were investigated under UV excitation. From a powder X-ray diffraction analysis, the formation of single-phased Ba2Mg(PO4)2 with a monoclinic structure was confirmed. The photoluminescence excitation spectrum of Ba2Mg(PO4)2:Ce3+ shows that the excitation band from 220 to 350 nm and the broad-band UV-blue emission are attributed to the allowed 4f ↔ 5d transitions of Ce3+. Upon excitation of Ce3+ band, Ba2Mg(PO4)2:Ce3+,Tb3+ phosphors exhibit both Ce3+ emission band the characteristic Tb3+ emission lines. Also, efficient energy transfer from Ce3+ to Tb3+ in Ba2Mg(PO4)2 was verified by observing the excitation spectra of Ba2Mg(PO4)2:Ce3+,Tb3+. An energy level scheme is constructed to depict the energy transfer process.
© 2014 Optical Society of America
In 1971, Thornton, Koedam and Opstelten predicted that a luminescent lamp with very high color rendering and efficiency can be obtained by blending three narrow band phosphors with emission maxima at 450, 550, and 610 nm [1, 2]. Such tricolor lamps had been achieved in 1974 with a combining an efficacy of 80 lm/W and a color rendering index of 85 .
The 4f electrons of rare earth ions are well shielded by the 5s and 5p shells, therefore their 4f ↔ 4f transitions are insensitive to the host lattice. The ground and excited states share same equilibrium position and force constant. The resulting emissions of 4f ↔ 4f transitions show sharp lines in the spectra. The 5D4→7FJ transitions of Tb3+ are mainly in the green region. Usually, the 5D4→7F5 (~540 nm) transition dominates. Besides, due to the large gap between 5D4 to 7F0, Tb3+ -activated phosphors are characterized by its high quantum yields. Therefore, Tb3+ is the best candidate for the green component.
However, in LS coupling, electric dipole transitions within the 4f configurations of Tb3+ are both parity and spin forbidden. Actually, the transitions from the ground state 7F6 level to any higher 4f configurations of Tb3+ are neglectable. Although the 4f ↔ 5d transitions are allowed, the absorption band often lies at very high energy . A sensitizer is always necessary for Tb3+ activated phosphors. From the very beginning to recent years, Ce3+ ion has been codoped for this purpose in LaMgAl11O19 [5, 6], LaPO4 [7–9], Sr2LiSiO4F , Na2Ca4(PO4)2(SiO4) , Sr3AlO4F , Ba2Y(BO3)2Cl , etc.
In the present paper, we report our study on Ce3+and Tb3+-activated Ba2Mg(PO4)2. The structure of Ba2Mg(PO4)2 was reported to be crystallized in a monoclinic crystal system with space group P21/n (Z = 4) by Lucas et al.  and Faza et al. . In the crystal lattice, there are two Ba2+ sites, 7- or 8-coordinated (CN), which can be replaced by rare earth dopants. Figure 1 presents the crystal structure of Ba2Mg(PO4)2.
The synthesis, luminescent properties of Eu2+-activated Ba2Mg(PO4)2 have been reported [16, 17]. In this work, we report our studies on Ce3+ and/or Tb3+-doped Ba2Mg(PO4)2. The luminescent properties as well as the energy transfer (ET) in Ba2Mg(PO4)2:Ce3+,Tb3+ were systematically investigated by means of photoluminescent excitation (PLE) and emission (PL) spectra.
Ce3+ and/or Tb3+ -activated Ba2Mg(PO4)2 powders were synthesized using a high temperature solid state reaction method. Stoichiometric amounts of starting materials BaCO3, 4MgCO3∙Mg(OH)2∙5H2O, (NH4)2HPO4, Eu2O3 CeO2 and Tb4O7 were weighed and thoroughly mixed in an agate mortar. The obtained mixtures were given a preliminary firing at 500 °C in a stagnant air atmosphere for 2 h to prevent spewing and also to decompose the diammonium phosphate and carbonates. After regrinding, the samples were sintered at 1200 °C for 5 h under a flowing atmosphere of forming gas (90% N2, 10% H2). We also synthesized undoped Ba2Mg(PO4)2 and Eu3+ -activated Ba2Mg(PO4)2. The final sintering procedure of these two samples was conducted in air. After cooling in a natural way, the powders were recuperated from the crucibles and ground in a mortar for next characterizations.
Phase purity of the prepared samples was checked by powder X-ray diffraction (XRD) using a diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 36 kV tube voltage and 20 mA tube current. Diffuse reflection spectra were obtained by a UV-visible spectrophotometer (Shimadzu UV-2450) using BaSO4 as a reference. The PLE and PL spectra of the samples were recorded by a Hitachi F-7000 fluorescence spectrophotometer. All the measurements were carried out at room temperature.
3. Results and discussion
3.1 XRD phase
The XRD patterns of the prepared samples were checked, and a typical example is shown in Fig. 2.From the XRD pattern it was found that the notable phase formed is Ba2Mg(PO4)2, after the diffraction peaks are well indexed based on the reference PDF#87-0419 [Ba2Mg(PO4)2]. It is clearly observed that the as-synthesized sample is well coincident with the standard pattern, indicating that our samples are of single phase. The rare-earth ions are supposed to reside in Ba2+ sites rather than Mg2+ sites, because they have similar ionic radii and valences.
3.2 Luminescence properties
Under UV light irradiation, the Ce3+-activated Ba2Mg(PO4)2 samples exhibit strong near UV-blue emission. Figure 3(a) shows the PL and PLE spectra of Ba1.97Mg(PO4)20.03Ce3+. The Ce3+ ion has a very simple electron configuration either in the ground or in the excited states. The 4f1 ground state configuration is split in two levels, viz. 2F5/2 and 2F7/2, by spin-orbit coupling. The energy difference between the two levels is about 2000 cm−1 . Therefore, the Ce3+ emission spectrum usually consist of two separated bands. However, in some cases, due to the strong coupling with lattice vibrational modes, the emission bands were expanded, which may be comparable to the 2000 cm−1 separation. In addition, there are two Ba2+ sites that can be substituted by Ce3+, therefore it is impossible to analyze by authentic peak deconvolution. The emission spectrum of Ba2Mg(PO4)2:Ce3+ consists of a broad band from 320 to 470 nm with a single maximum wavelength at about 365 nm. The 5d1 excited state is split by the crystal field into several components. Monitored by the emission at 365 nm, the PLE spectrum exhibits an unresolved broad band in the range from 200 to 360 nm, which can be assigned to the transitions from the ground state 2F5/2 (4f1) to the crystal-field-split components of the 5d1 configuration. The Stokes shift of Ba2Mg(PO4)2:Ce3+ was estimated to be 4000 cm−1. The resolution of our spectra is not very high, it not easy to determine the low energy side of the PLE spectra. This may introduce some uncertainty in the Stokes shift value.
Although most Eu2+-activated phosphate phosphors emit in blue or near UV , Ba2Mg(PO4)2:Eu2+ is a yellow phosphor [16, 17]. Dorenbos has found the relation between Eu2+ and Ce3+ f ↔ d transition energies in inorganic compounds . According to the constructed relation, long-wavelength (~405 nm) emission is expected in Ba2Mg(PO4)2:Ce3+. However, the result shows otherwise. Although Eu2+ and Ce3+ both reside in Ba2+ sites, Eu2+ and Ce3+ have different coordination polyhedra. Cerium is trivalent while barium is divalent, in order to keep electrically neutral in every micro-area, interstitial oxygen is supposed to neutralize the substitution. The oxygen may come from the doping process in origin. When CeO1.5 replaces BaO, half oxygen must find interstitial spots to reside in. Apparently, the first anion coordination sphere of Ce3+ is the lowest energy position. The charge compensating interstitial oxygen reduces crystal field splitting so that the d → f emission energy of Ce3+ increases.
The dependence of emission intensity on Ce3+ concentration is displayed in Fig. 3(b). The optimum Ce3+ concentration was found to be 0.03. The critical distance (Rc) for concentration quenching can be estimated by the crystallographic method .
Where, V is unit cell volume, xc = 0.03 is the critical concentration of Ce3+ and Z is the number of Ba2Mg(PO4)2 in the unit cell. For the Ba2Mg(PO4)2 host, Z = 4 and V = 0.755 nm . According to Eq. (1), the critical distance for concentration quenching is estimated to be about 2.3 nm. The f↔d transitions of Ce3+ are totally allowed electric-dipole type with a considerable spectral overlap (see Fig. 3(a)), accordingly, the critical distance is large.
Figure 4 gives the PLE and PL spectra of Ba1.98Mg(PO4)2:0.02Tb3+. The broad band centered at ~226 nm in the PLE spectrum is ascribed to the transition from the 4f8(7F6) ground state to the crystal-field-split components of the 4f75d1 configuration of Tb3+ ion. The lines corresponding to excitation of the forbidden 4f–4f transitions of the Tb3+ ion are too weak to be observed in the PLE spectrum. The UV and blue emission lines of the Tb3+ ion at 456, 436, 412 and 382 nm are assigned to the 5D3 → 7FJ (J = 3-6) transitions, respectively, while the green emission lines at 583, 543 and 487 nm are assigned to the 5D4 → 7FJ (J = 4-6) transitions, respectively .
3.3 Energy transfer from Ce3+ to Tb3+
Although the spectral overlap between the PLE spectrum of Ba2Mg(PO4)2:Tb3+ and the PL spectrum of Ba2Mg(PO4)2:Ce3+ seems to be absence (see Fig. 3 and 4), effective resonance-type ET from Ce3+ to Tb3+ in Ba2Mg(PO4)2 occurs. Actually, this kind of ET is quite common and has been observed in many systems, as we introduced in the first section. Tb3+ ion has 4f8 configuration which yields various levels . Some of these levels locate 25,000-30,000 cm−1 above the ground level (7F6), the corresponding transition energies match the emission spectrum of Ba2Mg(PO4)2:Ce3+. These energy levels can play a role in the ET processes.
Figure 5 gives the PLE and PL spectra of Ba1.97-yMg(PO4)2:0.03Ce3+, yTb3+. Upon excitation of Ce3+ band (260-320 nm), the PL spectrum of Ba1.97-yMg(PO4)2:0.03Ce3+, yTb3+ consists of the 5d →4f UV-blue emission broad band assigned to Ce3+ ions and the blue (5D3→7FJ) and green (5D4→7FJ) emissions corresponding to Tb3+ ions. It is clear that the addition of Tb3+ ions in the Ce3+ activated Ba2Mg(PO4)2 result in a decrease of the overall Ce3+emission, which indicate that partial Ce3+ ions nonradiatively transfers their energy to Tb3+ before they can fluoresce. The PLE spectrum monitored by the Tb3+ 5D4→7FJ transition (546 nm) includes both the excitation bands of Ce3+ and Tb3+. The PLE and PL spectra in Fig. 5 imply an efficient ET from Ce3+ to Tb3+ in Ba2Mg(PO4)2.
With increasing Tb3+ concentration, the green emission intensity of Ba1.97-yMg(PO4)2:0.03Ce3+, yTb3+ was found to increase gradually until y exceeds 0.11 under 272 nm excitation. The quenching of Tb3+ emission at high concentration is due to the energy migrating over Tb3+sublattice via exchange interaction. It is well known that the energy difference between the 5D3 and 5D4 levels is the same as that between the 7F0 and 7F6 levels, so the emission form 5D3 multiplet at high Tb3+ concentration are quenched by the cross relaxation process. Therefore, only 5D4 → 7FJ transitions are observed.
The average Ce3+→Tb3+ distance can also be estimated by Eq. (1), if we assume: (1) Tb3+ ions randomly distribute in the host lattice and (2) compared with Tb3+ concentration, Ce3+ concentration is very small. The average Ce3+→Tb3+ distance in Ba1.86Mg(PO4)2:0.03Ce3+, 0.11Tb3+ is calculated to be 1.49 nm. The critical distance for ET by exchange interaction is restricted to 0.5–0.8 nm . At first sight the large discrepancy between the calculated result and typical distance for exchange interaction is expected to rule out the contribution to ET of exchange interaction. However, the rare earth dopants not necessarily randomly distribute in the Ba2+ sublattice [22–24], Ce3+–Tb3+ clusters may be formed in the lattice. Blasse and Bril argued that nonradiative ET occurs mainly by exchange interaction if the sensitizer emission band overlaps only 4f ↔ 4f absorption bands of acceptor . This is exactly the present case. However, it is often reported that multipole-multipole interactions are the responsible mechanism for the ET from Ce3+ to Tb3+ [26–33]. The multipole-multipole interactions can also take place between a Ce3+–Tb3+ pair which is too far for exchange interaction.
3.4 Energy level scheme
The reflectance spectra of Ba2Mg(PO4)2 and Ba2Mg(PO4)2:Eu3+ are shown in Fig. 6.The spectra of the two samples show platforms of high reflection in the wavelength range of 400–800 nm and steep slopes in 300 to 400 nm. These absorption bands result from the host absorption. The body colors of the two samples are white which agrees well with the spectra. The near edge relation between absorption coefficient (α) and bandgap Eg for a direct bandgap material can be described by the equation [34–36]Eq. (2) can be substituted with [37, 38]Figure 6(b) gives the Tauc plot of Ba2Mg(PO4)2 that is widely used for the determination of bandgap. A red line is drawn in the near edge region tangent to the point of inflection on the curve. The hν value at the point of intersection of the tangent line and the hν = 0 line gives the bandgap Eg ≈3.5 eV.
When Eu3+ ions are doped into the host, new band occurs. The broad absorption band centered at ~250 nm can be attributed to charge-transfer (CT) band arising from the top of valence band (VB) to Eu3+ ion. The CT band overlaps the host absorption band; therefore, the absorption band in the 220–320 nm area increases a little. The intra-configurational 4f ↔ 4f transitions of Eu3+ are both parity and spin forbidden; therefore, they can hardly be seen.
The PLE and PL spectra of Ba1.95Mg(PO4)2:0.05 Eu3+ are shown in Fig. 7.The excitation spectrum consists of a broad band and a group of lines in the visible and UV regions. The broad excitation band centered at ~250 nm (ECT = 4.96 eV) is attributed to the CT transition. The energy of end state of CT is higher than that of the ground state of Eu2+ resulting from the repulsion of the charge-compensating defects. Besides the charge-compensating defects, in order to reduce the energy of the substitutional defect, some Eu3+ ion may be reduced to Eu2+ by losing oxygen [39,40]. The absorption of Eu2+ may also contribute to the band. The lines are ascribed to the intraconfigurational 4f ↔ 4f transitions of Eu3+. The lines are assigned according to Ref . The emission spectrum is composed of groups of lines from the characteristic emission of Eu3+ intraconfigurational 4f ↔ 4f transi-tions (5D0 →7FJ). Because of efficient multi-phonon emission, any transition starting higher than 5D0 is quenched.
According to the considerations in Ref  and method in Ref , once the CT band of Eu3+ is known, the 4fn and 4fn-15d1 levels of each trivalent lanthanide ion can be drawn relative to the bottom of the conduction band (CB) and the top of the VB. The main idea is that the CT energy of Eu3+ provides a good measure of the location of the Eu2+ ground state. However, the deviation is usually large when a trivalent lanthanide is on a divalent cation site. The CT energies are usually close to the bandgaps of the hosts [44–46]. In the present work, a reliable energy level scheme of relevant lanthanide ions with respect to host bands is difficult to achieve. On the basis of the spectral analysis, as well as the energy transfer processes (Ce3+→Tb3+) in this host, an energy level scheme is shown in Fig. 8.
In this work, phosphate phosphors Ba2Mg(PO4)2:Ce3+,Tb3+ were prepared and characterized. The spectroscopic study has revealed fundamental information about the luminescence features of Ce3+ and Tb3+ ions in Ba2Mg(PO4)2. The emission intensity of the Tb3+ singly activated sample was faint, even though its 4f → 5d transition is allowed. Owing to considerable energy overlap of the emission of Ba2Mg(PO4)2:Ce3+ with the 5DJ, 5GJ, and 5LJ levels of Tb3+ ion, efficient energy transfer from Ce3+ to Tb3+ occurs in the Ba2Mg(PO4)2:Ce3+,Tb3+ system. The enhanced green light of Tb3+ with sharp emission lines could be obtained by the broad band excitation in 220-350 nm region from the allowed 4f → 5d transitions of Ce3+ ions. The optimum concentration of Ce3+ exhibiting emission was 0.03, while the optimum concentration of Tb3+ with a fixed Ce3+ concentration of 0.03 in Ba2Mg(PO4)2:Ce3+,Tb3+ was roughly 0.11.
This work is financially supported by the National Natural Science Foundation of China (No. 21271049); The Special Funds for University Discipline and Specialty Construction of Guangdong Province, China (No. 2013KJCX0066).
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