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Luminescence Properties of Ca19Ce(PO4)14:A (A = Eu3+/Tb3+/Mn2+) Phosphors with Abundant Colors: Abnormal Coexistence of Ce4+/3+− Eu3+ and Energy Transfer of Ce3+ → Tb3+/Mn2+ and Tb3+−Mn2+ Mengmeng Shang,† Sisi Liang,†,‡ Hongzhou Lian,† and Jun Lin*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Science and Technology of China, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: A series of Eu3+/Tb3+/Mn2+-ion-doped Ca19Ce(PO4)14 (CCPO) phosphors have been prepared via the conventional high-temperature solid-state reaction process. Under UV radiation, the CCPO host presents a broad blue emission band from Ce3+ ions, which are generated during the preparation process because of the formation of deficiency. The Eu3+-doped CCPO phosphors can exhibit magenta to red-orange emission as a result of the abnormal coexistence of Ce3+/Ce4+/ Eu3+ and the metal−metal charge-transfer (MMCT) effect between Ce3+ and Eu3+. When Tb3+/Mn2+ are doped into the hosts, the samples excited with 300 nm UV light present multicolor emissions due to energy transfer (ET) from the host (Ce3+) to the activators with increasing activator concentrations. The emitting colors of CCPO:Tb3+ phosphors can be tuned from blue to green, and the CCPO:Mn2+ phosphors can emit red light. The ET mechanism from the host (Ce3+) to Tb3+/Mn2+ is demonstrated to be a dipole−quadrapole interaction for Ce3+ → Tb3+ and an exchange interaction for Ce3+ → Mn2+ in CCPO:Tb3+/Mn2+. Abundant emission colors containing white emission were obtained in the Tb3+- and Mn2+-codoped CCPO phosphors through control of the levels of doped Tb3+ and Mn2+ ions. The white-emitted CCPO:Tb3+/Mn2+ phosphor exhibited excellent thermal stability. The photoluminescence properties have shown that these materials might have potential for UVpumped white-light-emitting diodes.

1. INTRODUCTION Recently, rare-earth ions and transition-metal-ion-doped oxide luminescent materials have attracted much attention and have been extensively used in display devices (e.g., cathode ray tubes, field emission displays), lighting apparatuses [e.g., fluorescent tubes and white-light-emitting diodes (WLEDs)], solid-state lasers, and so on, owing to their good thermal and chemical stability.1−7 Among them, phosphor-conversion WLEDs are considered to be next-generation solid-state lighting sources because of their energy efficiency, high efficiency, environmentally friendliness, and so on. Compared with common WLEDs (InGaN chip + Y3Al5O12:Ce3+ phosphor), coating UV light-emitting-diode (LED) chips with tricolor (red, green, and blue) phosphors or a single-composition white-emitting phosphor is a promising way to achieve WLEDS with low correlated color temperature and high rendering index.8−11 With the development of GaxAl1−xN LED chips, its emitting wavelength can be extended to 330 nm by adjusting the value of x.12 Therefore, it is expected that LED chips with shorter emitting wavelength will be realized in the near future based on the demand of more efficient excitation by higher energy for rare-earth ions. © 2017 American Chemical Society

Generally, it is vital to select appropriate compounds as the hosts for rare-earth ions and transition-metal ion Mn2+ to accommodate the luminescent activators. As a typical Whitlockite-like phosphate, the β-Ca3(PO4)2 structure has five Ca(1)−Ca(5) cationic sites, and the Ca(1), Ca(2), Ca(3), and Ca(5) sites are dominated by Ca atoms, while Ca(4) was a deficiency site.13 Whitlockite-like phosphate compounds containing tetravalent cerium (Ce4+) ions can readily be prepared.14 A typical example is the Ca19Ce(PO4)14 (CCPO) phosphate, which is isostructural to Ca3(PO4)2.15 There are four cationic sites in the CCPO lattice: Ca2+ statistically occupies the Ca(1), Ca(2), Ca(3), and Ca(5) sites, and Ce4+ occupies the Ca(1), Ca(2), and Ca(3) positions. This special structure indicates that the lattice can accommodate other cations with similar radii and charges and does not induce any significant changes of the crystal structure. To the best of our knowledge, the CCPO compound is rarely reported as a luminescent material host. As widely used red- and greenemitting activators, the Eu3+ and Tb3+ ions show their Received: January 12, 2017 Published: May 8, 2017 6131

DOI: 10.1021/acs.inorgchem.7b00092 Inorg. Chem. 2017, 56, 6131−6140

Article

Inorganic Chemistry characteristic emissions originating from the transitions of 4f− 4f with good purity. Moreover, the transition-metal ion Mn2+ can emit a wide range colors from green to red, which is strongly influenced by the host crystal field. So, we chose CCPO as the host material and investigated the luminescence properties of Eu3+-, Tb3+-, and Mn2+-ion-doped CCPO. In our work, the abnormal coexistence of Ce3+−Ce4+−Eu3+ in the CCPO host was observed. The metal−metal charge-transfer (MMCT) effect of Ce3+−Eu3+ was discussed, and the energytransfer (ET) mechanisms Ce3+ → Tb3+, Ce3+ → Mn2+, and Tb3+ → Mn2+ were investigated in detail. Multicolor emissions were obtained in CCPO:Eu3+/Tb3+/Mn2+ phosphors via the MMCT and ET processes.

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. A series of phosphors with the chemical composition of (Ca1−x/y/z)19Ce(PO4)14:19(xEu3+/yTb3+/ zMn2+) (abbreviated as CCPO:xEu3+/yTb3+/zMn2+, where x, y, and z = 0−0.15) were all synthesized via the conventional hightemperature solid-state method. Powder phosphor samples were made using mixtures of high-purity CaCO3, CeO2, (NH4)H2PO4, Li2CO3, Tb4O7, Eu2O3, and MnCO3 presintered in air at 800 °C for 4 h. After that, the samples were reduced at 1350 °C for 6 h in a 5% H2/ 95% N2 gas mixture. 2.2. Characterization. The composition and phase purity of the products were studied by powder X-ray diffraction (XRD) measurements using a D8 Focus diffractometer (Bruker) with Cu Kα radiation (λ = 0.15405 nm). The General Structure Analysis System (GSAS) program was used to conduct the structure refinements.16 X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB MK II electron spectrometer using Mg Kα (1200 eV) as the excitation source. Room temperature photoluminescence (PL) spectra were measured on a Hitachi F-7000 luminescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. PL quantum yields (QYs) were measured directly by the absolute PL QY (internal quantum efficiency) measurement system (C9920-02, Hamamatsu Photonics K.K., Japan). The temperature-dependent (20−250 °C) PL spectra were obtained on a fluorescence spectrophotometer equipped with a 450 W xenon lamp as the excitation source (Edinburgh Instruments FLSP-920) with a temperature controller. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 digital osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns; gate = 50 ns) as the excitation source (Contimuum Sunlite OPO).

Figure 1. (a) Rietveld refinement for the powder XRD profile of the CCPO host. (b) Schematic crystal structures of the CCPO host with one unit cell and the coordination environment of the cation in the host crystal lattice.

are 6, 7, 7, and 6, respectively].15 The Ca2+ ions occupy all of the cationic lattice sites, while the Ce4+ ions only occupy the M1, M2, and M3 sites. Figure 2a compares the XRD patterns of

3. RESULTS AND DISCUSSION 3.1. Phase Identification, Structure, and Morphology. The composition, phase purity, and crystal structure of the asprepared representative CCPO host were first identified by the Rietveld refinement of a powder XRD data profile. The starting model was constructed with the crystal data taken from ICSD83401 in the structure CCPO. The experimental, calculated, and difference XRD patterns for the Rietveld refinement of CCPO at room temperature are presented in Figure 1a. The pure CCPO crystallizes hexagonally with the space group R3c (No. 161) with cell parameters of a = b = 10.445(5) Å, c = 37.474(4) Å, V = 3541.02 Å3, Z = 3, α = β = 90°, and γ = 120°. All atom positions, fraction factors, and thermal vibration parameters were refined by convergence and satisfied well the reflection conditions: Rp = 3.46%, Rwp = 4.97%, and χ2 = 4.487. Figure 1b displays the crystal structure of CCPO and the coordination environments of the cationic sites in the host lattice. Obviously, the CCPO compound is isostructural to βCa3(PO4)2 with four different crystallographic sites, namely, M1, M2, M3, and M4 [where the coordination numbers (CNs)

Figure 2. (a) XRD patterns of the representative CCPO host, CCPO:Eu3+, CCPO:Tb 3+, CCPO:Mn2+, and CCPO:Tb 3+,Mn2+ samples. (b) Typical SEM image of Tb3+- and Mn2+-codoped CCPO samples.

t h e C CP O : Eu 3 + , C C PO : T b 3 + , C C P O : M n 2 + , a n d CCPO:Tb3+,Mn2+ samples to the calculated pattern of the CCPO host obtained by GSAS refinement. As seen from the XRD patterns in Figure 2a, it can be determined that the expected single-phase phosphors with hexagonal structure have been synthesized successfully and the phase could not be changed by altering the activators. The impurity peaks were not 6132

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450 nm in wavelength due to the 5d1 → 4f1 transition of Ce3+. Moreover, the emission band excited with 280 nm UV light has a peak at 368 nm with a full width at half-maximum (fwhm) of 56 nm; the emission band excited with 318 nm UV light has a peak at 376 nm with a fwhm of 46 nm; the emission band excited with 226 nm UV light has a peak at 364 nm with a fwhm of 60 nm. Considering the different excitation wavelengths produced in the different emission spectra, we can infer that there are three types of Ce3+ luminescent centers in CCPO. As discussed earlier, Ce ions occupy the M1, M2, and M3 sites with CNs of 6, 7, and 7, respectively. The spectral results are consistent with its crystal structure. All of the facts indicate that, under the conditions of the synthesis performed in this work, Ce cations partly stabilized in the trivalent state.14 Certainly, they will be accompanied by the emergence of vacancies, which is very likely to occur in the high-temperature solid-state reaction. Figure 3b gives the decay curve of the host emission excited at 300 nm and monitored at 370 nm. The decay curve was analyzed through curve fitting and well fitted with a triple-exponential function. This result is consistent with the fact that the Ce3+ ions substitute three different crystallographic sites in the CCPO host. The crystal structure and spectral characterization are also supported by the above results. The average fluorescence lifetime was defined as the following formula:18

found. However, the positions of the diffraction peak at 2θ = 30.964° of the Eu3+-, Tb3+-, and/or Mn2+-doped CCPO samples shift to the higher diffraction degree compared with that of the calculated CCPO pattern. This result can be attributed to the smaller ionic radii of Eu3+ [r = 0.947 and 0.101 Å for CN = 6 and 7, respectively], Tb3+ (r = 0.923 and 0.980 Å for CN = 6 and 7, respectively), and Mn2+ (r = 0.83 and 0.90 Å for CN = 6 and 7, respectively) with respect to those of Ca2+ (r = 0.100 and 0.106 Å for CN = 6 and 7, respectively).17 Moreover, as the radii of the dopant ions decrease, the diffraction peak position gradually shifts to a higher degree. All of the facts mean that the as-prepared samples are single phase and have the same structure as the CCPO host. Figure 2b gives the scanning electron microscopy (SEM) image of the representative CCPO:Tb3+,Mn2+ samples. The powder sample consists of irregular grains with sizes ranging from 0.5 to 1 μm diameter. 3.2. PL Properties. The luminescence properties of the CCPO:Eu3+/Tb3+/Mn2+ phosphors were then examined. It is worth noting that, considering the charge balance in the CCPO host, Ce ions exist in the form of tetravalent ions. However, a typical blue emission from the CCPO host prepared under air conditions is observed upon excitation with a 300 nm UV lamp. Figure 3a exhibits the excitation and emission spectra and the

∞

τ=

∫0 tI(t ) dt ∞

∫0 I(t ) dt

The calculated average lifetime is 43.3 ns, which is consistent with the typical lifetime of Ce3+ ions. To further verify the blue emission from the trivalent Ce3+ ions, we compare the emission spectra of the CCPO host prepared under air and reduction atmospheres. As shown in Figure S1, the shapes of the emission spectra do not change; only the emission intensities are different. This result proves the conclusion deduced from Figure 3a. Moreover, the emission intensity of the CCPO sample prepared under a reduction atmosphere is much stronger than that of the sample prepared under air conditions. This is because the reduction atmosphere causes more Ce4+ ions to be reduced to Ce3+ ions, which can be supported by the XPS spectra of CCPO samples prepared under air and reduction atmospheres. XPS results shown in Figure S2 indicate that Ce is in mixed-valence states of 3+ (A1 = 880.40 eV, A2 = 884.8 eV, and A3 = 904.6 eV) and 4+ (A4 = 888.2 eV, A5 = 895.4 eV, A6 = 900.3 eV, A7 = 908.7 eV, and A8 = 915.6 eV). A semiquantitative analysis of the integrated peak area can provide the relative concentration of Ce3+ ions as follows: Figure 3. (a) PL excitation and emission spectra of the CCPO host. Inset: Digital photograph of the host sample upon 300 nm UV lamp box excitation. (b) Decay curve of the host emission (Ce3+) at 370 nm.

[Ce3 +] =

A1 + A 2 + A3 ∑ Ai

where Ai is the integrated area for peak “i”.19 The relative concentration of Ce3+ ions is calculated to be 15% and 44% for the CCPO samples prepared under air and reduction atmospheres, respectively. Figure 4a shows the PL excitation and emission spectra of the CCPO:Eu3+ sample. Upon monitoring at 617 nm, it can be seen that the PL excitation spectrum of CCPO:Eu3+ displays a broad band ranging from 200 to 300 nm, with the maximum at 250 nm and some strong narrow peaks (dominated at 396 and 467 nm, respectively) resulting from the absorption of the

luminescence photograph of the CCPO host sample. It can be found that the excitation and emission spectra are derived from the typical luminescence of the Ce3+ ions. The excitation spectrum of the CCPO sample monitored at 370 nm consists of some separated bands from 200 to 350 nm, which correspond to the transitions from the ground state to the different excited states due to the crystal-field splitting of the Ce3+ 5d energy level. Under UV-light excitation, the CCPO host sample gives a broad emission band ranging from 325 to 6133

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Figure 5. (a) PL emission spectra of the representative CCPO:xEu3+ samples (x = 0.02, 0.05, and 0.10) under 254 nm excitation. (b) Luminescence photographs of the CCPO:xEu3+ samples upon 254 nm UV-lamp box excitation: numbers 1, 2, 3, ..., 10 represent x = 0.01, 0.02, 0.03, ..., 0.10, respectively. Figure 4. (a) PL excitation and emission spectra of the CCPO:Eu3+ sample (λex = 396 nm and λem = 617 nm). The inset photographs represent the luminescence photographs of the CCPO:Eu3+ sample under 365 nm UV-lamp irradiation. (b) Variations of the emission intensity with increasing Eu3+ concentration x in CCPO:xEu3+ phosphors.

and the red emission from Eu3+ simultaneously observed in the CCPO:Eu3+ samples? As discussed earlier, the CCPO host crystal accommodates the Ce3+, Ce4+, and some vacancy sites. Doping Eu3+ ions into the CCPO lattice will induce more vacancy sites. So, the Ce4+ ions and vacancy sites in the lattice would overcome MMCT quenching between Ce3+ and Eu2+, and the Eu3+ emission is observed. Meanwhile, it is worth noting that the Ce3+ emission band becomes weaker as the Eu3+ content increases. This is because the distance between the Ce3+ and Eu3+ ions becomes shorten with increasing Eu3+ content, and the MMCT quenching effect dominates. Because of the MMCT quenching effect between the Ce3+ and Eu3+ ions, the tunable emission color can be obtained in CCPO:xEu3+ samples with different Eu3+ doping concentrations, as exhibited in Figure 5b. Doping with a low concentration of Eu3+ ions, the CCPO:xEu3+ (x = 0.01 and 0.02) samples show magenta/pink emissions as a results of the blue emission from Ce3+ and the red emission from Eu3+. With a high doping concentration of Eu3+, the samples exhibit red emission only from the 5D0 → 7F2 transition of Eu3+, and the Ce3+ emission is quenched. Different from Eu3+ ions, an effective ET can occur between Ce3+ and Tb3+/Mn2+, which has been reported in many Ce3+and Tb 3 + /Mn 2 + -codoped phosphors, such as NaBa3La3Si6O20:Ce3+,Tb3+,24 Ca3Y(GaO)3(BO3)4:Ce3+,Tb3+,Mn2+,25 Y2SiO5:Ce3+,Tb3+,26 and CaSr2Al2O6:Ce3+,Mn2+.27 So, we doped Tb3+ and Mn2+ ions into the CCPO host lattice in our experiment. Figure 6 presents the typical PL excitation and emission spectra of the CCPO:Tb3+ sample. Under UV-light excitation, the emission spectrum of the CCPO:Tb3+ sample is composed of a broad emission band with a peak at 366 nm from the Ce3+ 5d → 4f transition and some sharp peaks dominated at 493 and 547 nm due to the 5D4 → 7F6, 5 transition of Tb3+. Upon monitoring with the characterized emission of Tb3+ ion at 547 nm, an excitation spectrum similar to that of the CCPO sample is observed and consists of a strong absorption band from 200 to 350 nm (f−d transition of Ce3+) and some weak peaks at 350− 500 nm (f−f transition of Tb3+). The above results suggest that

Eu3+−O2− charge-transfer (CT) transition, as well as 4f−4f characteristic transitions of Eu3+, respectively. The stronger absorptions at 396 and 467 nm indicate that the CCPO:Eu3+ sample can well match with near-UV or blue-light LED chip. Upon 396 nm excitation, the emission spectrum shows the characteristic emission lines derived from the 4f−4f transitions of Eu3+ ions, namely, 5D0 → 7F1 (593 nm) and 5D0 → 7F2 (617 nm), respectively.20 The intensity of the electric dipole transition 5D0 → 7F2 around 617 nm is much stronger than that of the magnetic dipole 5D0 → 7F1 around 593 nm, implying that the Eu3+ ions can easily occupy the crystal sites without or deviated from inversion symmetry. So, the CCPO:Eu3+ sample exhibits a red emission under 365 nm UV-lamp irradiation, as presented in the inset in Figure 4. The concentration-dependent emission intensity curve of the CCPO:xEu3+ samples shown in Figure 4b demonstrates that the optimum doping concentration of Eu3+ ions is fixed as x = 0.04. An unexpected and interesting phenomenon is observed when the CCPO:Eu3+ sample is excited with a wavelength of 254 nm, as shown in Figure 5. It is found that, under 254 nm excitation, the emission spectrum of the CCPO:Eu3+ samples simultaneously consists of the broad band emission of Ce3+ at 366 nm and the sharp peak emission of Eu3+ at 617 nm. Furthermore, Eu3+ emissions are not significantly enhanced with an increase of the Eu3+ content. According to Blasse and Grabmaier,21 the probability of ET from Ce3+ to Eu3+ is very weak because Ce has a lower fourth ionization potential and Eu has a higher third ionization potential based on the stability of fully filled and half-filled shells. The process can be expressed as Ce3+ + Eu3+ → Ce4+ + Eu2+, which is called metal−metal charge-transfer (MMCT) between Ce3+ and Eu3+.22,23 This means that it is difficult for the Ce3+ and Eu3+ ions to coexist in the same host. However, why are the blue emission from Ce3+ 6134

DOI: 10.1021/acs.inorgchem.7b00092 Inorg. Chem. 2017, 56, 6131−6140

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Figure 6. PL excitation and emission spectra of the CCPO:Tb3+ sample.

the ET from Ce3+ ions to Tb3+ ions is very effective in CCPO. As depicted in Figure 7 for CCPO:yTb3+, with 365 nm

Figure 8. (a) PL excitation and emission spectra of the CCPO:Mn2+ sample. Inset: Digital photograph of the CCPO:Mn2+ sample upon 300 nm UV-lamp box excitation. (b) PL emission spectra of the CCPO:zMn2+ samples with different Mn2+ doping concentrations (z = 0.02, 0.05, 0.07, 0.10, and 0.15).

emission of Ce3+ ions from the host; the other peak at 637 nm belongs to the typical spin- forbidden 4T1(4G) → 6A1(6S) transition of Mn2+ ions. The excitation spectrum consists of a strong absorption band centered at 321 nm and several weak bands from 350 to 500 nm. The strong absorption band corresponds to the Ce3+ 4f−5d transition, and the weak bands can be ascribed to the Mn2+ forbidden d−d transitions. It also can be seen that the emission band of Ce3+ can overlap with the Mn2+ excitation energies from 6A1(6S) to 4A1(4G), 4E(4G). Accordingly, the occurrence of efficient resonance-type ET from Ce3+ to Mn2+ in the CCPO host is possible. Figure 8b displays emission spectra for the CCPO:zMn2+ samples excited with 321 nm UV light. All emission spectra contain a blue band emission from Ce3+ and a red band emission from Mn2+, which indicates that an efficient ET has occurred between the Ce3+ and Mn2+ ions. In the CCPO:zMn2+ samples, the emission intensity of Ce3+ decreased and that for Mn2+ increased with increasing Mn2+ concentration until z = 0.07. After that, the Mn2+ emission decreased because of the concentration quenching effect. Figure 9 gives the decay curves of activator characteristic emissions in the representative CCPO:0.04Eu3+ (617 nm), CCPO:0.06Tb3+ (547 nm), and CCPO:0.07Mn2+ (637 nm) samples, respectively. The decay curves for the three samples can be well fitted with a single-exponential formula as follows: I = I0 exp(−t/τ) + A, where I refers to the fluorescent intensity, I0 represents a constant, t is the time, and τ corresponds to the decay lifetime for the exponential component, and A is the value for different fittings. The values of the lifetime are determined to be 1.63, 2.08, and 2.63 ms for Eu3+, Tb3+, and Mn2+, respectively, which are much longer than that of Ce3+ emission in the host. The lifetimes for Eu3+, Tb3+, and Mn2+ ions in all other CCPO:xEu3+/yTb3+/zMn2+ samples are listed

Figure 7. (a) Various emission spectra of CCPO with different Tb3+ doping concentrations (y). (b) Luminescence photographs of CCPO:yTb3+ samples upon 254 nm UV-lamp box excitation.

excitation, the Ce3+ emission from the host lattice decreases gradually while the Tb3+ emission increases and then decreases with increasing Tb3+ concentration. The overlap between the PL emission spectrum of the CCPO host and the PL excitation spectrum from 350 to 500 nm of Tb3+ ions further reveals the possibility of ET from Ce3+ ions to Tb3+ ions, as shown in the inset in Figure 7a. The luminescence color also can be tuned from blue to green via a gradual increase of the doping concentration of Tb3+ ions in the CCPO:yTb3+ samples due to ET from Ce3+ to Tb3+, as presented in Figure 7b. The transition-metal ion Mn2+ has a bare 3d5 electronic configuration. As a result, the Mn2+ emission spectrum is usually composed of a broad band due to the 4T1(4G) → 6 A1(6S) transition, which strongly depends on the crystal-field strength around Mn2+ ions.28 The weaker the crystal field strength, the smaller the splitting of the excited 3d energy levels. So, the Mn2+ emission will locate at higher energy in the weaker crystal field; otherwise, the Mn2+ ions can emit longerwavelength light with lower energy.21 So, red emission can be realized if the right host is chosen. Figure 8a gives the PL excitation and emission spectra of the Mn2+-doped CCPO sample. As seen, the CCPO:Mn2+ sample presents red emission under UV-light excitation as expected. The emission spectrum simultaneously consists of two emission bands: one located at 300−400 nm with a peak at 366 nm can be ascribed to the 6135

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where V is the volume of the unit cell, xc is the critical concentration (the doping concentration of Tb3+/Mn2+ ions, at which the emission intensity of the Ce3+ ions is half that of the sample in the absence of Tb3+/Mn2+ ions), and N is the number of sites that activators can occupy per unit cell, For the CCPO host, V = 3541.02 Å3, N = 57, and xc = 0.475 for CCPO:yTb3+ and 1.9 for CCPO:zMn2+; therefore, the Rc value of ET was determined to be 6.30 and 3.97 Å, respectively. Generally, there are three mechanisms for nonradiated ET including radiation reabsorption, exchange interaction, and electric multipolar interactions. The radiation reabsorption occurs noly when the emission and excitation spectra are widely overlapping with each other. The exchange interaction is predominant only when Rc is less than 5 Å.33,34 So, it can be deduced that electric multipolar interactions would explain the ET mechanism from the host (Ce3+) to Tb3+ ions, and exchange interaction may play a dominant role in the ET process between the host (Ce3+) and Mn2+ for CCPO:Mn2+ samples. On the basis of Dexter’s ET formula for multipolar and exchange interactions, the following relationship can be obtained:25,33,35

Figure 9. Decay curves of the activator characteristic emissions in CCPO:0.04Eu 3+ at 617 nm (a), CCPO:0.06Tb 3+ (b), and CCPO:0.07Mn2+ (c).

in Table S1. With increasing doping concentration of the Eu3+, Tb3+, and Mn2+ ions, the values of the corresponding lifetimes gradually decrease because of the concentration quenching effect. Furthermore, the MMCT quenching effect between Ce3+ and Eu3+ also results in a decrease of the Eu3+ lifetime. The ET efficiencies (ηT) from Ce3+ to Tb3+ and Mn2+ in the CCPO:yTb3+/zMn2+ system were calculated using the equation ηT = 1 − IS/IS0,29,30 where ηT refers to the ET efficiencies and IS0 and IS are the luminescence intensities of the CCPO host (Ce3+) with and without activators (Tb3+/Mn2+), respectively. As seen from Figure S3, the ηT values from the CCPO host (Ce3+) to the activators in the CCPO:yTb3+/zMn2+ systems are presented as a function of the Tb3+ and Mn2+ concentrations, which indicate that the ηT values monotonously increase to reach maxima at 88% and 61% for the CCPO:yTb3+ and CCPO:zMn2+ samples under 276 and 321 nm UV radiation, respectively. The result indicates that ET from the CCPO host (Ce3+) to the activators becomes more effective with increasing doping concentration of the activators. The fluorescent decay lifetime measurements for the host (Ce3+) emission (λex = 300 nm and λem = 366 nm) were conducted to further verify the occurrence of ET from the host to the activators. As illustrated earlier, triple-exponential decay behavior of the host emission (Ce3+) is observed because of the three different Ce crystallographic sites existing in the CCPO lattice. The values of the calculated decay lifetimes using the ∞ ∞ formula τ = ∫ tI(t ) dt /∫ I(t ) dt are summarized and 0 0 displayed in Table S2. We can observe that the average decay times (τ) of the host emission (Ce3+) is 43.3 ns without any doping of Tb3+ and Mn2+ ions. With increasing concentration of the dopant ions, the value of τ of the host emission (Ce3+) monotonously decreases from 43.3 to 10.1 and 9.6 ns for CCPO:Tb3+ and CCPO:Mn2+, respectively, which can well certify the existence of ET from the host (Ce3+) to Tb3+ and Mn2+ ions. In order to determine the ET mechanisms from Ce3+ to Tb3+/Mn2+ in CCPO:Tb3+/Mn2+ systems, the critical distance (Rc) between the Ce3+ and Tb3+/Mn2+ ions needs to be investigated.31 With increasing Tb3+/Mn2+ contents, ET becomes more effective and the probability of energy migration between the activators enhances. When the distance between the activators is small enough, concentration quenching occurs and energy migration is blocked. The Rc value can be obtained by the following equation:32

η0 η

∝ C n /3

and

⎛η ⎞ ln⎜ 0 ⎟ ∝ C ⎝η⎠

where η0 and η represent the luminescence quantum efficiencies of the host (Ce3+) in the absence and presence of Tb3+ or Mn2+ ions, respectively; the values of η0/η can be estimated approximately by the ratio of the related lifetime (τS0/τS), and C is the concentration of Tb3+ or Mn2+. On the other hand, ln(τS0/τS) ∝ C corresponds to the exchange interaction, and (τS0/τS) ∝ Cn/3 with n = 6, 8, and 10 corresponds to dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole interactions, respectively. The relationships between τS0/τS and Cn/3 as well as ln(τS0/τS) and C for the CCPO:yTb3+ and CCPO:zMn2+ samples are illustrated in Figures 10 and 11, respectively. It can be seen from Figure 10

Figure 10. Dependence of τS0/τS of the host emission (Ce3+) on C6/3, C8/3, and C10/3 in the CCPO:yTb3+ samples.

that the biggest R2 value of the linear fittings occurs when n = 8, corresponding to their respectively best linear behaviors. Therefore, ET from the host (Ce3+) to Tb3+ ions takes place through the dipole−quadrupole mechanisms in the CCPO:yTb3+ samples. However, the biggest R2 value of the linear fittings occurs in the relationship line of ln(τS0/τS) and C, as shown in Figure 11, implying that the exchange interaction is predominant in the ET process between the host (Ce3+) and

⎛ 3V ⎞1/3 R c = 2⎜ ⎟ ⎝ 4πxcN ⎠ 6136

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Inorganic Chemistry

Figure 11. Dependence of ln(τS0/τS) of the host emission (Ce3+) on (a) C and τS0/τS of the host emission (Ce3+) on (b) C6/3, (c) C8/3, and (d) C10/3 in the CCPO:zMn2+ samples.

Mn2+ ions in the CCPO:zMn2+ samples. The results are in good agreement with those obtained from the critical distance method. The emission intensities of the Tb3+ and Mn2+ ions increase with decreasing Ce3+−Tb3+/Mn2+ distance (namely, increasing Tb3+/Mn2+ content) until saturation is reached; furthermore, when the Tb3+/Mn2+ content continues to increase (i.e., the Ce3+−Tb3+/Mn2+ distance is shorter than Rc), the Tb3+/Mn2+ emission intensity begins to decrease, which was attributed to the occurrence of energy reabsorption among the nearest Tb3+ ions. The tunable emission color obtained in a single phase is an attractive feature that can be realized in most ET systems. So, in our study, we investigated the tunable emission colors of the Tb3+- and Mn2+-codoped CCPO samples with 316 nm UVlight excitation. As presented in Figure 12, when the Mn2+ doping concentration is fixed, the emission intensity of the host (Ce3+) at 366 nm decreases, but the Tb3+ emission intensity at 547 nm increases with increasing Tb3+ content, while an unexpected enhanced red emission of Mn2+ at 637 nm is also observed, which illustrates that ET occurs not only from the host (Ce3+) to Tb3+ and Mn2+ ions but also from Tb3+ to Mn2+ in the Tb3+- and Mn2+-codoped CCPO samples. So, the emission colors are adjusted from deep red to orange with increasing Tb3+ doping concentration. On the other hand, when the Tb3+ doping concentration is fixed (Figure 12b), the emissions of the host (Ce3+) and Tb3+ decrease with an increase of the Mn2+ content. This also indicates that ET occurs from Tb3+ to Mn2+. In particular, because of the blue emission of the host, the green emission of the Tb3+ ions, and the red

Figure 12. PL emission spectra of the representative (a) CCPO:yTb3+,Mn2+ (y = 0.005, 0.01, 0.02, 0.03, and 0.04) and (b) CCPO:Tb3+,zMn2+ (z = 0.02, 0.05, 0.07, 0.10, and 0.15) samples under 316 nm excitation. Inset: Corresponding digital photographs of the CCPO:yTb3+,Mn2+ and CCPO:Tb3+,zMn2+ samples upon 300 nm UV-lamp box excitation.

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Inorganic Chemistry emission of the Mn2+ ions, a white emission was obtained in the CCPO:Tb3+,zMn2+ samples with the right Mn2+ concentration (z = 0.02). So, with increasing Mn2+ doping concentration, the emission colors can be tuned from white to deep red via the triple ET processes between the host (Ce3+) and Tb3+, the host (Ce3+) and Mn2+, and Tb3+ and Mn2+. Figure 13 shows a simple model expressing ET from Ce3+ to Eu3+, Tb3+, and Mn2+ ions and the characteristic emission

Figure 13. Energy-level model for the ET and MMCT processes in CCPO:Eu3+/Tb3+/Mn2+ systems.

energy levels of the Eu3+, Tb3+, and Mn2+ ions in the CCPO host. In the CCPO:Tb3+/Mn2+ samples, the Ce3+ ions in the CCPO lattice absorb UV radiation and then the excitation energy is transferred from Ce3+ to the respective excited states of the Tb3+/Mn2+ ions, which causes enhancement of the characteristic emission intensities of the activators. Moreover, the excited states of Tb3+ can also transfer their energy to Mn2+ in the Tb3+- and Mn2+-codoped CCPO samples. For the CCPO:Eu3+ samples, the situation is very different. In the CCPO:Eu3+ sample, Ce3+ efficiently absorbs UV radiation, and then the energy migrates to the 5D3 energy level of the Eu3+ ions. The excited electrons in this energy level can be readily transferred to the Ce4+−Eu2+ MMCT, followed by nonradiative relaxation to the ground state of Eu3+, leading to a loss of the emission energy of Eu3+. However, because of the presence of Ce4+ and the defects in the CCPO hosts, only a small amount of energy can be contributed to Eu3+ emission. Therefore, the emission intensity of Eu3+ is much weaker than those of the Tb3+/Mn2+ ions. The presence of a variety of ET effects in CCPO:Eu3+/Tb3+/Mn2+ allows us to obtain multicolor emissions in the single-phase CCPO phosphors. Figure S4 gives the corresponding CIE coordinates for the as-prepared CCPO:Eu3+/Tb3+/Mn2+ and CCPO:Tb3+, Mn2+ samples under UV excitation. It can be found that the CCPO:Eu3+ samples can emit blue to magenta to orange-red light. The CCPO:Tb3+ samples can emit blue to cyan to bright green. The CCPO:Mn2+ samples show that the luminescence color varies from blue to red yellow. When Mn2+ and Tb3+ are codoped into the host, the tunable color can be seen from red to white. The highest QYs of the CCPO:Eu3+/Tb3+/Mn2+ samples can reach up to 20%, 71%, and 54%, respectively. The QY for the CCPO:Tb3+,Mn2+ sample with white emission is 34%. As we all know, the phosphors used in the fields of the lighting and display must have excellent PL properties at about 150 °C over the long term.36−38 Figure 14 exhibits the emission spectra of the white-emitting CCPO:0.05Tb3+ ,0.02Mn 2+ phosphor with respect to the temperature and dependence of the integrated emission intensity on the temperature. As can be seen from Figure 14a, the shape of the emission spectra with

Figure 14. (a) Representative temperature-dependent PL spectra of the CCPO:0.05Tb3+,0.02Mn2+ sample. (b) Dependence of the PL intensity of CCPO:0.05Tb3+,0.02Mn2+ on the temperature.

different temperatures is unchanged, implying that the phosphor has good color stability. The emission intensity of the CCPO:0.05Tb3+,0.02Mn2+ phosphor remains about 89% of the initial emission intensity at 150 °C, as shown in Figure 14b. This result further verifies the excellent thermal stability of the as-prepared CCPO:Tb3+,Mn2+ phosphors.

4. CONCLUSIONS A series of Eu3+/Tb3+/Mn2+-doped CCPO phosphors were prepared via conventional high-temperature solid-state reactions. The crystal structure, luminescence properties, and ET mechanism were systematically investigated. The blue emission from Ce3+ was observed in the CCPO host due to the existence of the deficiency generated during the preparation process. The Eu3+-doped CCPO phosphors can exhibit magenta to redorange emission as a result of the abnormal coexistence of Ce3+/Ce4+/Eu3+. When Tb3+/Mn2+ were doped into the hosts, the samples present various emission colors because of ET from the host (Ce3+) to the activators with increasing activator concentrations upon 300 nm UV excitation. The emission colors of the CCPO:Tb3+ phosphors can be tuned from blue to green, and the CCPO:Mn2+ phosphors can emit red emission. The ET mechanisms from the host (Ce3+) to Tb3+/Mn2+ have been demonstrated to be dipole−quadrapole and exchange interactions in CCPO:Tb3+/Mn2+. Abundant emission colors containing white emission were obtained in the Tb3+- and Mn2+-codoped CCPO phosphors through control of the levels of doped Tb3+ and Mn2+ ions. The white-emitted CCPO:Tb3+/ Mn2+ phosphor exhibited excellent thermal stability. The PL properties have shown that these materials might have the potential for a UV-pumped WLED field. 6138

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Inorganic Chemistry

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00092. Decay lifetimes, PL emission spectra, XPS spectra, ET efficiencies, and a CIE chromaticity coordinates diagram (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.L.). ORCID

Jun Lin: 0000-0001-9572-2134 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grants 51472234, 91433110, 51672265, 51672266, and 51572257), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (Grant U1301242), the National Basic Research Program of China (Grant 2014CB643803), the Scientific and Technological Department of Jilin Province (Grant 20150520029JH, 20170414003GH), and the Young Elite Scientist Sponsorship Program by CAST (YESS).

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