Substituting Different Cations in Tuning of the Photoluminescence in

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Substituting Different Cations in Tuning of the Photoluminescence in Ba3Ce(PO4)3 Ting Li, Panlai Li,* Zhijun Wang,* Shuchao Xu, Qiongyu Bai, and Zhiping Yang College of Physics Science & Technology, Hebei Key Lab of Optic-Electronic Information and Materials, Hebei University, Baoding 071002, China ABSTRACT: An attempt has been made to explore how the luminescence properties change when rare-earth elements are substituted for different cations in the host. We synthesized Eu2+-doped Ba3Ce(PO4)3 via a high-temperature solid-state reaction process, substituting for Ba2+ and Ce3+ ions and naming them Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+, respectively. The structure, X-ray diffraction with Rietveld refinements, reflectance spectra, and luminescence characterization of the phosphor are measured to explore the difference of substituting for different ions. In order to explain why all of the emission peaks containing the highest peak and the fitting values of Ba3Ce(1−x)(PO4)3:xEu2+ are shorter than those of Ba(3−y)Ce(PO4)3:yEu2+ (when x= y), we built a model by N, which represents the surrounding environment. This mechanism is predicted to be general to Eulytite-type orthophosphates and will be useful in tuning optical and other properties whose structural disorder influences the crystallization and is sensitive to local coordination environments. Substituting different cations in tuning of the red shift, widening of the full width at half-maxima (fwhm), and thermal quenching were also observed.

1. INTRODUCTION Rare-earth elements substitute for cations in certain hosts acting as important activators for application in modern lighting and display fields due to their abundant emission colors to achieve spectral conversion.1−5 Luminescent lanthanide ions can be divided into two categories, that is, 4f−4f and 5d−4f transitions. Because the 4f−4f transitions are strictly forbidden, the widths of their emission peaks are very narrow and fixed.6 External factors and the host have little effect on their emission peaks because a 4f electron locates in the inner shell. For 5d−4f transitions, the 5d orbitals are exposed on the outside and their emission spectra are greatly affected by the host, for example, the crystal-field strength, substituting different sites and different cations if any. As we all know, Ce3+ and Eu2+ ions are the most precious 4f−5d transitions, and they show broad near-ultraviolet (n-UV) excitation and visible emission in a specific host.7−12 If Ce3+ and Eu2+ ions locate in the same host, the emission phenomena should be very complicated because of the host effect and their mutual influence on each other.13−16 Recently, Eulytite-type orthophosphates with the general formula M3IMII(PO4)3 (MI = Ca, Sr, Ba, and Pb; MII = 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 properties.17−24 The cations are distributed in a statistical manner on the same sites, while the P atoms are fixed on other similar sites and the Eulytite-type orthophosphates end with divalent and trivalent cations. Ji et al. reported the different crystal structures and photoluminescence (PL) properties of Eulytite-type Ba3Eu(PO4)3 and Sr3Eu(PO4)3.25 Ba3Ce(PO4)3 is one of the Eulytite-type phosphates, with Ce3+ as a structure building ion, in which there are two kinds of cations, i.e., Ba2+ and Ce3+. Ba3Ce(PO4)3 is a perfect © XXXX American Chemical Society

host to analyze the issue of substituting different cations on the same site, which has not been discussed up to now. In this work, we doped Eu2+ into Ba3Ce(PO4)3, substituting Ba2+ and Ce3+ ions and calling them Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+, respectively. Also, these phosphors are synthesized by a high-temperature solid-state reaction process to explore how the luminescence properties change when the rare-earth elements are substituted with different cations in Ba3Ce(PO4)3. We first determined the difference of their lattice parameters by Rietveld refinement. We measured the diffuse-reflectance spectra, PL spectra, decay lifetime curves, and thermal quenching spectra. Close attention was also paid to the explanation of different peak positions, tuning of the red shift, widening of the fwhm, and thermal quenching.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. A series of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphor samples were prepared by a high-temperature solid-state reaction process. The starting materials BaCO3 (analytical reagents, A.R.), NH4H2PO4 (A.R.), CeO2 (99.99%), and Eu2O3 (99.99%) were weighed by an electronic scales with a 0.0001 g accuracy. The doping concentrations of Eu2+ were selected as 0.005−0.20 mol in Ba3Ce(PO4)3. Typically, stoichiometric amounts of BaCO3, NH4H2PO4, CeO2, and Eu2O3 were thoroughly mixed in an agate mortar for 30 min. The as-obtained mixing solution was transferred to an alumina crucible and then sintered at 1150 °C for 4 h in a reductive atmosphere (20% H2 + 80% N2). After firing, the samples were cooled to room temperature in the furnace and ground again into a powder for subsequent use. Received: June 2, 2016

A

DOI: 10.1021/acs.inorgchem.6b01282 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry The specific chemical equations that obtained Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors can be expressed as follows: x 3BaCO3 + 3NH4H 2PO4 + (1 − x)CeO2 + Eu 2O3 2

→ Ba3Ce(1 − x)(PO4 )3 : x Eu 2 + (3 − y)BaCO3 + 3NH4H 2PO4 + CeO2 +

y Eu 2O3 2

→ Ba(3 − y)Ce(PO4 )3 : y Eu 2 + 2.2. Material Characterization. The crystal structure of the assynthesized samples was determined by X-ray diffraction (XRD) profiles using a powder X-ray diffraction spectrometer (D/max-rA, Cu Kα, 40 kV, and 40 mA). The XRD patterns, which were submitted for Rietveld refinement, were acquired at a step size of 0.05° with a counting time of 2 s step−1. The Findit and Crystalmaker software were used for the structure. The PL and PL excitation (PLE) spectra of the samples were analyzed by a F-4600 spectrofluorometer equipped with a 450 W xenon light source. The temperature-dependent luminescence properties were measured on the same spectrophotometer, which was assembled with a TAP-02 high-temperature fluorescence controller. The Commission International de I’Eclairage (CIE) chromaticity coordinates for all samples were measured by a PMS80 UV−vis−near-IR spectral analysis system. Diffuse-reflectance spectra on the phosphors were surveyed on a Hitachi U4100 machine, at a scanning wavelength range of 200−800 nm. The decay curves were recorded using a 450 W xenon lamp as the excitation source (Horiba FL-1057). All of the luminescent characteristics of the phosphors were investigated at room temperature.

Figure 1. (a) XRD patterns of the Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors as a function of the Eu2+ content, where x and y = 0.01 and 0.10. As a reference, the standard XRD data for Ba3La(PO4)3 (PDF card no. 85-2448) are shown. (b) Magnified patterns between 26 and 28° of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+, where x and y = 0.01 and 0.10 samples.

(integer), and λ is the wavelength of X-rays. In addition, k and λ are fixed, so d and θ are inversely proportional. For Ba3Ce(1−x)(PO4)3:xEu2+, d becomes bigger and θ becomes smaller with increasing x. Correspondingly, d becomes smaller and θ becomes bigger with increasing y for Ba(3−y)Ce(PO4)3:yEu2+. This observation indicates that substitution of Eu2+ for Ba3+ or Ce3+ changes the lattice constants, which can be calculated by structure refinement based on the powder XRD data. Figure 2 shows the powder XRD patterns (×) of the Ba3Ce1−x(PO4)3:xEu2+ (x = 0.01, 0.10) (parts a and b), Ba3−yCe(PO4)3:yEu2+ (y = 0.01, 0.10) (parts c and d), and Ba3Ce(PO4)3 host (part e) samples with the corresponding Rietveld refinement (red) and residuals (blue).27 The main refined parameters of the processing and refined results are presented in Table 1. The results of the refinement further demonstrate that the series of phosphors are single phase without any impurity or secondary phases. Meanwhile, we also find that the lower the concentration of Eu2+, the higher the quality of their XRD data. The changes of the lattice parameters and volumes are shown in Figure 2e,f, which are obtained from the results of the refinement, demonstrating that the doped Eu2+ influenced Ba3Ce1−x(PO4)3:xEu2+ and Ba3−yCe(PO4)3:yEu2+ samples regularly with x and y, respectively. The lattice parameters and unit cell volumes are bigger and bigger in Ba3Ce1−x(PO4)3:xEu2+ with x and smaller and smaller in Ba3−yCe(PO4)3:yEu2+ with y, which is attributed to the substitution of Eu2+ ions for the small Ce3+ and large Ba2+ ions, suggesting that the coordination environment of the cations becomes more unconsolidated and compact with x and y increasing, respectively.28 Figure 3 shows the crystal structure of Ba3La(PO4)3. The basic structural features of the Ba3La(PO4)3 crystal include (PO4)3− and Ba/La/Ce−O groups. The cations together with the Ba2+, La3+, Ce3+, and Eu2+ ions are distributed in a statistical manner on the 16c sites, while the P atoms are fixed on the 12a sites. According to the ionic radii of Eu2+, Ba2+, and Ce3+

3. RESULTS AND DISCUSSION 3.1. Crystal Structures and XRD Patterns of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+. For the purpose of observing whether the crystal structures of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ are different, for which the XRD and Rietveld refinements are direct data, we measured the XRD patterns and submitted Rietveld refinement. The powder XRD patterns of the obtained Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors as well as the standard card of Ba3La(PO4)3 for comparison are shown in Figure 1a. It can be found that all of the diffraction peaks of these samples can be exactly assigned to the pure cubic phase Ba3La(PO4)3 according to JCPDS 85-2448. No other phase or impurity can be detected, indicating that the Eu2+ ions were completely dissolved in the Ba3La(PO4)3 host without inducing significant changes of the crystal structure. The results show that the series of Ba3−yCe1−x(PO4)3:(x + y)Eu2+ are Eulytite-type structures and different dopant concentrations did not result in any other phase except the main phase in the synthesized Ba 3 Ce ( 1− x ) (PO 4 ) 3 :xEu 2 + and Ba (3 − y ) Ce(PO4)3:yEu2+ phosphors, respectively. Figure 1b shows the magnified patterns between 26 and 28° of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+, where x and y = 0.01 and 0.10 samples. We can find that the diffraction peaks shift to smaller angles with increasing Eu2+ content in Ba3Ce(1−x)(PO4)3:xEu2+, while the diffraction peaks shift to bigger angles with increasing Eu2+ content in Ba(3−y)Ce(PO4)3:yEu2+, for which the reason may be the different ionic radii between Ba2+ (1.34 Å), Ce3+ (1.034 Å), and Eu2+ (1.09 Å). Specifically, this shift can be explained by Bragg’s equation:26 2d sin θ = kλ

(1)

where d is the distance between parallel lattice planes, θ is the diffraction angle (Bragg angle), k is the order of reflection B


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Inorganic Chemistry

Figure 2. Powder XRD patterns (×) of the Ba3Ce1−x(PO4)3:xEu2+ (x = 0.01, 0.10) (a and b), Ba3−yCe(PO4)3:yEu2+ (y = 0.01, 0.10) (c and d), and Ba3Ce(PO4)3 host (e) samples with the corresponding Rietveld refinement (red) and residuals (blue). (f and g) Relative shifts in the lattice parameters of Ba3Ce1−x(PO4)3:xEu2+ and Ba3−yCe(PO4)3:yEu2+ compared with those of the Ba3Ce(PO4)3 host, respectively.

Table 1. Rietveld Refinements of XRD Data of Ba(3−y)Ce(1−x)(PO4)3:(x + y)Eu2+ x = 0, y=0 a=b= c/ Å V/Å3 α=β= γ/deg Rp/% Rwp/% space group

x = 0.01, y=0

x = 0.10, y=0

x = 0, y = 0.01

x = 0, y = 0.10

10.489

10.494

10.506

10.485

10.482

1153.951 90

1155.646 90

1159.600 90

1152.661 90

1151.629 90

3.86 4.33 I4̅3d

5.57 7.71 I4̅3d

6.35 8.14 I4̅3d

4.26 5.88 I4̅3d

5.92 7.65 I4̅3d

mentioned above, it is believed that Eu2+ ions substitute for the sites of Ce3+ and Ba2+ ions in the Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors, respectively. According to Dai et al.,29 the bond lengths for P−On (n = 1− 3) in Sr3Ce(PO4)3:Eu2+ are different from each other, indicating that the Sr3Ce(PO4)3 compound possibly possesses many different orientations of the PO4 tetrahedra. In this way, it can be imagined that the Sr3Ce(PO4)3 phase shows not only cation disorder but also oxygen sublattice disorder.29−31 Because Sr3Ce(PO4)3 and Ba3Ce(PO4)3 have similar structures, Ba3Ce(PO4)3 should have the same phenomenon; that is, the

Figure 3. (a) Crystal structure of Ba3La(PO4)3 in a cubic system with space group I4̅3d of the 2 × 2 × 1 unit cells. Coordination environments of the (c) P5+ and (b) Ba2+/La3+/Ce3+ sites.

doped rare-earth ions substituting for the Ba2+ or Ce3+ ions occupy a distorted dodecahedron of O ions in Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors as well as show oxygen sublattice disorder. C


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Figure 4. Diffuse-reflectance spectra of (a) Ba3Ce(1−x)(PO4)3:xEu2+ (x = 0, 0.01, 0.1) and (b) Ba(3−y)Ce(PO4)3:yEu2+(y = 0, 0.01, 0.1). Insets: SEM micrographs of Ba3Ce0.9(PO4)3:0.10Eu2+ and Ba2.9Ce(PO4)3:0.10Eu2+, respectively.

3.2. Reflectance Spectra of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba ( 3 − y ) Ce(PO 4 ) 3 :yEu 2 + . The reflectance spectra of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:xEu2+ phosphors were shown in Figure 4a,b. From Figure 4a,b, we observed that the reflectance spectral curve trends are essentially the same. The diffuse-reflectance spectrum of the Ba3Ce(PO4)3 host shows a status of high reflection in the wavelength ranging from 380 to 800 nm,and decreasing intensity from 250 to 380 nm that can be attributed to the host absorption band. Because their tendencies are essentially the same, we chose Ba3Ce(PO4)3 to calculate the host absorption band. The Kubelka− Munk absorption coefficient (K/S) relationship is used to calculate the measured reflectance (R) for the host lattice. (1 − R )2 K = S 2R

shows the PL and PLE spectra of the host Ba3Ce(PO4)3. Under excitation at 330 nm, the host Ba3Ce(PO4)3 shows a symmetrical broad-band emission ranging from 370 to 450 nm and centered at 394 nm, which is responsible for the transition of Ce3+ from the 5d excited state to the 4f ground state.32 The corresponding PLE spectra (monitored at λem = 394 nm) covering the range of 300−350 nm are attributed to the 5d−4f transition of Ce3+. We can also see the picture of Ba3Ce(PO4)3 excited under 365 nm UV light emitting a pure blue light, which matches well with the emission spectra. The inset of Ce3+ illustrates excited and emission processes. Parts a and b of Figure 6 describe the PL and PLE spectra of Ba3Ce0.9(PO4)3:0.1Eu2+ and Ba2.9Ce(PO4)3:0.1Eu2+, respec-

(2)

where K represents the absorption coefficient, S the scattering coefficient, and R the reflectivity. The fundamental band-gap energy (absorption edge) of the Ba3Ce(PO4)3 host was calculated to be approximately 2.53 eV (490 nm) from the K/S relation spectrum by extrapolation. Obvious differences appear in the spectral profiles of the ion-doped samples and the host. The strong broad absorption appeared in the 300−450 nm n-UV range, which was attributed to the electronic transition absorption of the ions. As can be seen in the photographs in Figure 4a,b, the integral particle size of Ba3Ce0.9(PO4)3:0.10Eu2+ is bigger than that of Ba2.9Ce(PO4)3:0.10Eu2+, which caused Ba(3−y)Ce(PO4)3:yEu2+ to have better absorption than Ba3Ce(1−x)(PO4)3:xEu2+ in the range 200−300 nm, shown in Figure 4, marked A, A* and B, B*. 3.3. Luminescence Properties of Ba 3 Ce(PO 4 ) 3 , Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+. Figure 5

Figure 6. PL and PLE spectra of Ba3Ce0.9(PO4)3:0.1Eu2+ (a) and Ba2.9Ce(PO4)3:0.1Eu2+ (b).

tively. From Figure 6a,b, we can find that the intensities and positions of the three Gaussian fitting curves obtained by deconvolution of the emission peak are different because the emission spectra are asymmetrical and the fwhm are different, which are the direct result of substituting different cations in the host. In addition, when the PL spectra of Ce3+ (370−420 nm; Figure 5) are compared with the PLE spectra of Eu2+ (350−420 nm), it is found that they have an obvious overlap, which indicates that there may be energy transfer from Ce3+ to Eu2+. In more detail, comparing parts a and b of Figure 6, we can easily find that the values of both the highest and fitting peaks of Ba3Ce0.9(PO4)3:0.10Eu2+ are shorter than those of Ba2.9Ce(PO4)3:0.10Eu2+, which are a result of the radii of Ce3+ and Ba2+ and the environment of Eu2+ in the two phosphors. According to the earlier discussion proposed by Van Uitert, the emission wavelength position of the Eu2+ ion often strongly depends on its local environment and the host cation

Figure 5. PL and PLE spectra of the host Ba3Ce(PO4)3. D


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Inorganic Chemistry substituted by the “active” cation, which can be estimated using the following formula:33−35 ⎡ ⎤ ⎛ V ⎞1/ V E = Q ⎢1 − ⎜ ⎟ × 10−nEar /80⎥ ⎝4⎠ ⎢⎣ ⎥⎦

Ba3Ce0.9(PO4)3:0.1Eu2+ is shorter than that of Ba2.9Ce(PO4)3:0.1Eu2+, we can conclude that N plays a master role in Ba3Ce1−x(PO4)3:xEu2+ rather than in Ba(3−y)Ce(PO4)3:yEu2+. Parts a and b in Figure 8 represent the emission spectra of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors excited under 330 nm excitation, respectively. The intensities of Ce3+ and Eu2+ with increasing x and y are shown in the insets, in which we can see the intensity changes more intuitively. From Figure 8a,b and the insets, we can observe that the peak of Ce3+ (398 nm) is decreased quickly with increasing doping concentration, which may due to energy transfer from Ce3+ to Eu2+. We can find a slight decrease when the concentration of Eu2+ is low, the reason for which may be that the intensity of Ce3+ has a significant influence at 480 nm when the concentration of Eu2+ is low. The maximum emission intensities of Eu2+ in both Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ occur at x = 0.10. The emission intensities of both of the two cases subsequently decrease after the maxima, which is due to the concentration quenching effect of Eu2+ ions. Moreover, because the distance between the nearest adjacent Ba2+/Ce3+/Eu2+ in Ba3Ce(1−x)(PO4)3:xEu2+ is shorter than that in Ba(3−y)Ce(PO4)3:yEu2+, which is due to the different radii of Ba2+, Ce3+, and Eu2+, the energy transfer between Eu2+ ions for the case of Ba3Ce(1−x)(PO4)3:xEu2+ can be more efficient, which has been confirmed by the decline rate in Ba3Ce(1−x)(PO4)3:xEu2+ beyond the quenching concentration, as shown in the inset in Figure 8a. In addition, we can find that the peak of Ce 3+ in Ba 3 Ce (1−x) (PO 4 ) 3 :xEu 2+ phosphors decreased more quickly than that in Ba(3−y)Ce(PO4)3:yEu2+ phosphors, which may be due to the decreasing Ce3+ concentration in Ba3Ce(1−x)(PO4)3:xEu2+ phosphors and fixed Ce3+ concentration in Ba(3−y)Ce(PO4)3:yEu2+ phosphors. In addition, the peak of Eu2+ (475 nm) is essentially the same in Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+. In general, the concentration quenching of luminescence is due to energy migration among the activator ions at high concentrations. The critical distance Rc between Ce3+ and Eu2+ ions can be estimated using the equation given by Blasse:36−38

(3)

where E refers to the position in energy of the d-band edge for the rare-earth ion (cm−1), Q is the position in energy of the lower d-band edge for the free ion (Q = 34000 cm−1 for Eu2+), V corresponds to the valence of the “active” cation (V = 2 for Eu2+), n is the number of anions in the immediate shell around the “active” cation, Ea refers to the electron affinity of the atoms that form anions (eV), which is variant when Eu2+ is introduced into different anion compounds with different coordination numbers while it is constant for the same host, and r is the radius of the host cations Ce3+ and Ba2+ substituted by the “active” cation (Eu 2+ ) in Ba 3 Ce (1−x) (PO 4 ) 3 :xEu 2+ and Ba(3−y)Ce(PO4)3:yEu2+, respectively. Considering the equation above, we can easily infer that the value of E is proportional to the quantity of r and n; i.e., λ is inversely proportional to the quantity of r and n. First, we know that the radius of Ba2+ is bigger than that of Ce3+, which leads to the wavelength position of Ba3Ce(1−x)(PO4)3:xEu2+ being longer than that of Ba(3−y)Ce(PO4)3:yEu2+. It does not meet the spectra in Figure 6a,b. Then, we analyze the parameter n, which is the number of anions in the immediate shell around the “active” cation. As mentioned above, the doped rare-earth ions substituting for the Ba2+ or Ce3+ ions occupy a distorted dodecahedron of O ions. Although the site is distorted, the O ions also have the tendentiousness of revolving in the vicinity of the cation by a certain percentage. Because the exact factor is the surrounding environment rather than the definition of n, we represented it by N. As shown in Figure 7, when Eu2+ is substituted for Ce3+

⎡ 3V ⎤1/3 R c = 2⎢ ⎥ ⎣ 4πXcM ⎦

(4)

where V is the volume of the unit cell, Xc is the total concentration of the Eu2+ ions, optimum concentration, and relative Ce3+ concentration, and M is the number of host cations in the unit cell. In this case, M = 9, V = 1165.61 Å3, and the total concentrations X c are 1.0 and 1.1 for Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+, respectively. Therefore, the critical distance (Rc) was calculated to be about 6.28 and 6.082 Å for Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+, respectively. The results obtained are close to the exchange interaction critical distance, 5 Å, and significantly shorter than the data reported in many reports,39,40 indicating that the exchange interaction is dominant for Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+. 3.4. Red Shift and Mutative fwhm Phenomenon of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+. The normalized intensities of Ba 3 Ce (1−x) (PO 4 ) 3 :xEu 2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors excited under 400 nm and the wavelength change with the concentration of Eu2+ are shown in Figure 9, for which the optimum excitation of Eu2+ is 400 nm. We can see that they both show red-shift phenomena. The insets in parts a and b show the wavelength changes with

Figure 7. Schematic of O ion dispersion chances leading to the N variety in Ba3Ce1−x(PO4)3:xEu2+ and Ba3−yCe(PO4)3:yEu2+, respectively.

in the host, the volume of Ba3Ce1−x(PO4)3:xEu2+ becomes bigger, and when Eu2+ is substituted for Ba2+, the volume becomes smaller, which have been proven in the refinement part. With respect to the cation size, we suppose that there are only 3 Ba2+, 1 Ce3+, 3 P5+, and 12 O2− ions in the host, yet we do not involve the P5+ ion. We can observe that Eu2+ is smaller than Ba2+ in Ba3Ce1−x(PO4)3:xEu2+, shown in the top half of Figure 7, and Eu2+ is bigger than Ce3+ and smaller than Ba2+ in Ba(3‑y)Ce(PO4)3:yEu2+, shown in the lower half. As a result, more O ions are revolving in the vicinity of the Eu2+ ions in Ba3Ce1−x(PO4)3:xEu2+ than in Ba(3−y)Ce(PO4)3:yEu2+, which supports that the number N of Ba3Ce1−x(PO4)3:xEu2+ is bigger than that of Ba(3−y)Ce(PO4)3:yEu2+. Because the peak value of E


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Inorganic Chemistry

Figure 8. Emission spectra of (a) Ba3Ce(1−x)(PO4)3:xEu2+ and (b) Ba(3−y)Ce(PO4)3:yEu2+ phosphors excited under 330 nm. Insets: Intensities of Ce3+ and Eu2+ with increasing x and y, respectively.

Figure 9. Normalized intensities of (a) Ba3Ce(1−x)(PO4)3:xEu2+ and (b) Ba(3−y)Ce(PO4)3:yEu2+ phosphors excited under 400 nm. Insets: Wavelength changes with the concentration of Eu2+.

the concentration of Eu2+. As shown in Figure 9a and its inset, with increasing Eu2+-doped content, a continuous red shift from 450 to 475 nm is observed in normalized spectra under excitation at 400 nm, whereas the observed red shift from 450 to 480 nm is shown in Figure 9b and its inset. Herein, there are three possible reasons for the red shifting of PL spectra, namely, reabsorption, increased crystal-field splitting of Eu2+, and energy transfer among the Eu2+ centers. Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors have the same reasons on the red-shift side, so we talk about Ba3Ce(1−x)(PO4)3:xEu2+ first. However, in the case of increased crystal-field splitting of Eu2+, there is something different between them, and we will talk about it in detail in the following. First of all, in the case of reabsorption, the emission with higher energy generally resonates with the lower-energy part of the excitation spectra, resulting in high-energy emission, which is partially reabsorbed, thus shifting the emission spectrum to the red region.29,41 In this work, we can clearly see that PLE spectra shift to larger wavelength, as shown in Figure 10. In the case of increased crystal-field splitting of Eu2+, the crystal-field splitting of the Eu2+ ions can be determined as obeying42,43 Dq =

ze 2r 2 6R5

Figure 10. Normalized PLE spectra of the Ba3Ce(1−x)(PO4)3:xEu2+ phosphors (x = 0.005, 0.01, 0.015, 0.03, 0.05, 0.10, 0.15, and 0.20) by monitoring the lower-energy emission bands.

the d wave function, and e is the charge of an electron. For the d(Eu−O) orbital, if z, e, and r are equal, then Dq is only a function of 1/R5. When the small Ce3+ ion (1.034 Å) is substituted by the large Eu2+ ion (1.09 Å), the distance between Eu2+ and O2− becomes longer and the magnitude of the crystalfield strength decreases.3,44 Thus, the crystal-field splitting of the Eu2+ ion is reasonably decreased, and this results in a gradual increase of the lowest 5d state.45 As a consequence, the emission wavelength shows a red shift among the as-prepared phosphors and a color variation from blue to green. We have mentioned the difference between Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors in the increased crystal-field splitting of the Eu2+ side. We need

(5)

where Dq is a measure of the energy-level separation, R represents the distance from the central ion to its ligands, z stands for the charge or valence of the anion, r is the radius of F


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Inorganic Chemistry to explain that the radius of Ba2+ is larger than that of Eu2+. It should have a blue shift if they are completely the same as mentioned above when we consider Eu2+ substituted for Ba2+ in Ba(3−y)Ce(PO4)3:yEu2+. In this work, because the radius of Eu2+ (1.09 Å) is closer to that of Ce3+ (1.034 Å) rather than Ba2+ (1.34 Å), we can speculate that Eu2+ replaced Ba2+ and Ce3+, although we reduce the Ba2+ ion proportion when we compute the stoichiometry. Figure 11 shows the energy-level diagram

in Figure 12, and exhibit non-single-exponential decay behavior. All of the decay curves can be well fitted with a second-order exponential equation: I(t ) = I0 + A1e−t / τ1 + A 2 e−t / τ2

(6)

where I(t) is the luminescence intensity, A1 and A2 represent constants, τ is the time, and τ1 and τ2 stand for rapid and slow lifetimes for exponential components, respectively. Moreover, the effective lifetime constant (τ) can be calculated as τ = (A1τ12 + A 2 τ2 2)/(A1τ1 + A 2 τ2)

(7)

On the basis of eq 6, the average lifetimes for the three Eu2+ emissions in Ba3Ce0.9(PO4)3:0.10Eu2+ (Figure 12a) are calculated to be 501, 596, and 641 ns, respectively, and the lifetime values are shown. Variation of the lifetime as a function of the emission wavelength suggests that the local structures around Eu2+ ions are different. Similar phenomena have also been observed in the Ba2.9Ce(PO4)3:0.10Eu2+ sample (Figure 12b), and their average lifetimes for the three emissions are calculated to be 496, 588, and 634 ns, respectively. Herein, it is impossible to distinguish different lifetimes induced by small differences in the local structure. Accordingly, we tentatively assume that the broad-band emission originates from a large number of Eu2+ ions with different local structures, which may result from the random distribution of Ba and Ce ions at an identical crystallographic site. Then we prove energy transfer among Eu2+ centers. Figure 13 shows the decay lifetimes of the Gaussian peak fittings of Ba3Ce(1−x)(PO4)3:xEu2+ at (a) 452, (b) 482, and (c) 510 nm and of Ba(3−y)Ce(PO4)3:yEu2+ at (d) 454, (e) 488, and (f) 530 nm with increasing concentration of Eu2+. We can conclude from Figure 13 that the trend of each peak for Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ is approximately increasing. As we all know, if there is intensive energy transfer among the Eu2+ centers, the lifetime of the center that provides energy should decrease, while the lifetime of the center that accepts energy should increase. In a brief conclusion, there is no or little energy transfer among Eu2+ centers. Comparing parts a and b of Figure 9, we can find that the fwhm of Ba3Ce(1−x)(PO4)3:xEu2+ is narrower than that of Ba(3−y)Ce(PO4)3:yEu2+, which we also mentioned in section 3.3. Because we have proven the small effect of energy transfer among Eu2+ centers, we speculated that the reason is the energy-transfer efficiency from Ce3+ to distinct Eu2+ centers. Figure 14 shows the Gaussian peak fittings of

Figure 11. Energy-level diagram showing the lowest-energy ground state and the first excited 5d state of Eu2+ in Ba(3−y)Ce(PO4)3:yEu2+. The energy change for the 5d state is indicated by ΔE.

with the lowest-energy ground state and the first excited 5d state of Eu2+ in Ba(3−y)Ce(PO4)3:yEu2+, and the energy change for the 5d state is indicated by ΔE. The crystal-field strength arising from Eu2+ substituted for Ce3+ would dominate, resulting in a red shift of the emission band. Briefly, the influence of Eu2+ → Ce3+ (red shift) on the crystal-field strength is much greater than that of Eu2+ → Ba2+ (blue shift), finally leading to a red shift of the emission peak. The case of energy transfer among Eu2+ centers influences the red shift. First of all, we need to validate that the distinct Eu2+ centers are different. We measured the lifetime decay profiles of different wavelengths under excitation at 370 nm. Once the lifetimes corresponding to the different emission wavelengths are determined, the existence of multiple Eu2+ centers can be confirmed. A set of luminescence decay curves are recorded for selected samples with 0.10 for Ba3Ce(1−x)(PO4)3:xEu2+ [Ba(3−y)Ce(PO4)3:yEu2+] by monitoring the emission at 452 [458], 482 [490], and 510 [513] nm, respectively. For the Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ samples, it is clearly seen that the profiles for the three decay curves are different from each other, just as shown

Figure 12. (a) Logarithmic intensities of Ba3Ce0.9(PO4)3:0.10Eu2+ phosphors monitoring the emission at 452, 482, and 510 nm excited under 370 nm. (b) Logarithmic intensities of Ba3Ce0.9(PO4)3:0.10Eu2+ Ba2.9Ce(PO4)3:0.1Eu2+ phosphors monitoring the emission at 454, 488, and 515 nm excited under 370 nm. G


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Inorganic Chemistry

Figure 13. Decay lifetimes of the Gaussian peak fittings of Ba3Ce(1−x)(PO4)3:xEu2+ at (a) 452, (b) 482, and (c) 510 nm and of Ba(3−y)Ce(PO4)3:yEu2+ at (d) 454, (e) 488, and (f) 530 nm with increasing concentration of Eu2+.

Figure 14. Gaussian peak fittings of Ba3Ce(1−x)(PO4)3:xEu2+ at (a) 450, (b) 480, and (c) 506 nm and of Ba(3−y)Ce(PO4)3:yEu2+ at (d) 452, (e) 482, and (f) 510 nm with increasing Eu2+ concentration.

Ba3Ce(1−x)(PO4)3:xEu2+ at (a) 450, (b) 480, and (c) 506 nm and of Ba(3−y)Ce(PO4)3:yEu2+ at (d) 452, (e) 482, and (f) 510 nm, whose data further proved the model of N. It is found that the emission intensity increment of Eu2+ at peak 3 (6) is relatively faster than those at peaks 1 (4) and 2 (5) with an increase of Eu2+ concentration in both Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+. In order to better demonstrate the phenomenon mentioned above, we analyzed the relative intensity of the ratio of peaks 2 and 3 to peak 1 in Ba3Ce(1−x)(PO4)3:xEu2+ and the ratio of peaks 5 and 6 to

peak 4 in Ba (3−y) Ce(PO 4 ) 3 :yEu 2+ with increasing Eu 2+ concentration, as shown in Figure 15. The relative intensity ratios Ipeak 3/Ipeak 1 and Ipeak 6/Ipeak 4 increase faster than Ipeak 2/ Ipeak 1 and Ipeak 5/Ipeak 1 in general, respectively, which leads to the wider and wider peaks. We can also observe that the ratio Ipeak 3/Ipeak 1 in Ba3Ce(1−x)(PO4)3:xEu2+ is bigger than Ipeak 6/ Ipeak 4 in Ba(3−y)Ce(PO4)3:yEu2+, which can explain why Ba3Ce(1−x)(PO4)3:xEu2+ is wider than Ba(3−y)Ce(PO4)3:yEu2+. We cannot know how Ce3+ diverted its energy to the different wavelengths when the Eu2+ doping concentrations were H


Article

Inorganic Chemistry

Figure 15. Relative intensities of the ratio of peaks 2 and 3 to peak 1 in Ba3Ce(1−x)(PO4)3:xEu2+ and the ratio of peaks 5 and 6 to peak 4 in Ba(3−y)Ce(PO4)3:yEu2+ with increasing Eu2+ concentration.

enhanced, but we can analyze the spectra from the Eu2+ concentration. As shown in Figure 16, with increasing Eu2+

Figure 17. CIE coordinates and photographs of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors with increasing Eu2+ concentration.

0.318 and Y = 0.417) with increasing Eu2+ concentration. In conclusion, the color of Ba(3−y)Ce(PO4)3:yEu2+ phosphors tune wider than that of Ba3Ce(1−x)(PO4)3:xEu2+, so we can adjust the color by simply varying the concentration of Eu2+ ions or substituting different ions in the host. However, we find that the color of Ba3Ce(1−x)(PO4)3:xEu2+ changes faster than that of Ba(3−y)Ce(PO4)3:yEu2+ when the concentration of Eu2+ is low, for which the reason may be the different substitution order between the two cases. In Ba3Ce(1−x)(PO4)3:xEu2+, the Eu2+ ions are all substituted for the Ce3+ ions, which has relatively less effect on the crystal structure than that in Ba(3−y)Ce(PO4)3:yEu2+, where Eu2+ are partially substituted for the Ce3+ and Ba2+ ions. Figure 18 shows the structural model for

Figure 16. Schema of how the Eu2+ concentration influences the peaks in (a) Ba3Ce(1−x)(PO4)3:xEu2+ and (b) Ba(3−y)Ce(PO4)3:yEu2+.

concentration, the intensities of all of the peaks increased especially those of peaks 3 and 6, which is because of the increase of the Eu2+ centers and their tendency to balance. Also, there are relatively fewer Eu2+ centers distributed to peak 3 in Ba 3Ce(1−x) (PO 4) 3 :xEu2+ than distributed to peak 6 in Ba(3−y)Ce(PO4)3:yEu2+, which is reflected in the figure marked ΔI1 > ΔI2. In a brief conclusion, the Eu2+ centers’ tendency to balance leads to the wider fwhm in the two cases, while the distribution difference leads to the wider fwhm in Ba(3−y)Ce(PO4)3:yEu2+ than in Ba3Ce(1−x)(PO4)3:xEu2+. 3.5. CIE Chromaticity Coordinates of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+. Figure 17 shows the CIE coordinates and photographs of Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ phosphors with increasing concentration of Eu2+. The CIE coordinates vary systematically from blue (X = 0.155 and Y = 0.188) to white (X = 0.300 and Y = 0.297) to greenish-yellow (X = 0.318 and Y = 0.367) for Ba3Ce(1−x)(PO4)3:xEu2+ phosphors with changes of the Eu2+ ion concentration. Thus, the light color emission for Ba3Ce(1−x)(PO4)3:xEu2+ phosphors can be tuned from blue-greenish-yellow by adjusting the concentration of Eu2+, just as shown in Figure 17, marked red dots. The CIE coordinates for Ba(3−y)Ce(PO4)3:yEu2+ phosphors are shown in Figure 17, marked black dots, and the change of the color is from blue (X = 0.165 and Y = 0.101) to white (X = 0.300 and Y = 0.297) to yellow (X = 0.329 and Y = 0.386) to greenish (X =

Figure 18. Structural model for explaining the substitution order and the influence on the color change in Ba3Ce(1−x)(PO4)3:xEu2+ (a) and Ba(3−y)Ce(PO4)3:yEu2+ (b).

explaining the substituted order and the influence on the color change in Ba 3 Ce (1−x) (PO 4 ) 3 :xEu 2+ (a) and Ba (3−y) Ce(PO4)3:yEu2+ (b). In addition, the green photograph for Ba(3−y)Ce(PO4)3:yEu2+ (y = 0.15) should result in a fast decrease of the relative intensity of peak 6, which is shown in Figures 14f and 15. 3.6. Thermal Quenching Properties of Ba3Ce0.99(PO4)3:0.01Eu2+ and Ba2.99Ce(PO4)3:0.01Eu2+. In order to observe whether the thermal quenching properties of I


Article

Inorganic Chemistry

Figure 19. Emission intensities of (a) Ba3Ce0.99(PO4)3:0.01Eu2+ and (b) Ba2.99Ce(PO4)3:0.01Eu2+ with different temperatures (λex = 330 nm). Insets: 407 nm emission intensities with different temperatures, respectively. Arrhenius fittings of the emission intensity and activation energy (ΔE) for thermal quenching for (c) Ba3Ce(1−x)(PO4)3:xEu2+ and (d) Ba(3−y)Ce(PO4)3:yEu2+.

Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ are different, we measured the temperature stabilities of the samples Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+. The intensities of the main emission peak (407 nm) for the prepared typical Ba3Ce0.99(PO4)3:0.01Eu2+ and Ba2.99Ce(PO4)3:0.01Eu2+ excited by 330 nm changing with the testing temperature are shown in Figure 19a,b. The inset shows the 407 nm emission intensities with different temperatures. The emission intensity declined with increasing temperature, which we can call thermal quenching.46 Comparing parts a and b of Figure 19, we can find that the peak of Ba3Ce(1−x)(PO4)3:0.001Eu2+ decreases more quickly than that of Ba(3−y)Ce(PO4)3:0.01Eu2+, which may be a result of the activation energy (Ea). Then, parts c and d of Figure 19 show Arrhenius fitting of the emission intensities of Ba3Ce0.99(PO4)3:0.01Eu2+ and Ba2.99Ce(PO4)3:0.01Eu2+ and Ea for thermal quenching, respectively. Moreover, Ea can be expressed by the following equation: ln(I0/I ) = ln A( −Ea /kT )

Figure 20. Schematic configuration coordinate diagram for the explanation of temperature quenching and the difference between Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+.

to explain the different decreasing rates by the configuration coordinate diagram. With an increase of the temperature, the phonon vibration is strengthened and more electrons located at the lowest excited state B can absorb phonon energy and be excited to state C or D. Electrons at state C or D can go back to ground state A by relaxation and give no emission, making a number of electrons go back to the ground state through a radiation decrease. Thus, the emission intensity of the phosphor will decrease with an increment of the temperature. As we all know, when the excited-state chemical-bonding ability becomes weaker and weaker, the curvature of excited-state potential energy curve becomes smaller and smaller, which leads to a flattened curve. As a result, we can conclude that the excited-state chemical-bonding ability of Ba3Ce(1−x)(PO4)3:xEu2+ is weaker than that of Ba(3−y)Ce(PO4)3:yEu2+. If we theorize the amount of excited-state

(8)

where I 0 and I are the luminescence intensities of Ba3Ce0.99(PO4)3:0.01Eu2+ and Ba2.99Ce(PO4)3:0.01Eu2+ at room temperature and the testing temperature, respectively, A is a constant, k is the Boltzmann constant (8.617 × 10−5 eV K − 1 ). E a was obtained as 0.25 and 0.25 eV for Ba3Ce0.99(PO4)3:0.01Eu2+ and Ba2.99Ce(PO4)3:0.01Eu2+, respectively. The fitting results shown in Figure 19c,d indicate that the activation energy for Ba3Ce0.99(PO4)3:xEu2+ and Ba2.99Ce(PO4)3:yEu2+ is 0.25 eV. We can find that the Ea of Ba3Ce0.99(PO4)3:xEu2+ differs little from that of Ba2.99Ce(PO4)3:yEu2+. As shown in Figure 20, we listed the temperature quenching mechanism and the difference between Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+ J


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Inorganic Chemistry electrons in both cases, the Ba3Ce(1−x)(PO4)3:xEu2+ phosphor should have more excited-state electrons reaching state C by the absorption of phonon energy than those of Ba(3−y)Ce(PO 4 ) 3 :yEu 2+ reaching state D, which leads to the Ba3Ce(1‑x)(PO4)3:xEu2+ phosphor decreasing more quickly than Ba(3−y)Ce(PO4)3:yEu2+.

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4. CONCLUSION In order to explore how the luminescence properties change when the rare-earth elements substitute different cations in the host, we synthesized a series of single-composition phosphors, Ba3Ce(1−x)(PO4)3:xEu2+ and Ba(3−y)Ce(PO4)3:yEu2+, via a hightemperature solid-state reaction process. The XRD patterns with Rietveld refinement, reflectance spectra, luminescence spectra, decay lifetime curves, and thermal quenching spectra of the phosphors were measured in room temperature. All emission peaks containing the highest peak and the fitting values of Ba3Ce(1−x)(PO4)3:xEu2+ are shorter than those of Ba(3−y)Ce(PO4)3:yEu2+ (when x= y). For the purpose of explaining this phenomenon, we built a model to explain it by N, which represents the surrounding environment. The reason for the red shift is reabsorption, increasing the crystal-field splitting of Eu2+. We proved that the different Eu2+ center tendency leads to different spectra; that is, Ba(3−y)Ce(PO4)3:yEu2+ is wider than Ba3Ce(1−x)(PO4)3:xEu2+. We explained that the peak of Ba 3 Ce (1−x) (PO 4 ) 3 :0.001Eu 2+ decreases more quickly than that of Ba(3−y)Ce(PO4)3:0.01Eu2+ when the temperature is increased with different excited-state chemical-bonding abilities.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.L.). *E-mail: [email protected] (Z.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (Grant 50902042), the Funds for Distinguished Young Scientists of Hebei Province, China (Grant A2015201129), the Natural Science Foundation of Hebei Province, China (Grants A2014201035 and E2014201037), the Education Office Research Foundation of Hebei Province, China (Grants ZD2014036 and QN2014085), a China Postdoctoral Science Foundation funded project (Grant 2015M581311), and the Midwest Universities Comprehensive Strength Promotion Project.

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REFERENCES

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